feat(nwsync): emit blobs in parallel with a bounded worker pool (#80)

Closes #79.

Emit is latency-bound, not CPU-bound. Every blob costs two serial round-trips to the zone — a `ProbeKey` HEAD, then a `PutReader` PUT — so a hak with a few thousand resources pays a few thousand serialised latencies. Measured on the live sow-assets-manifest backfill: 26 s of CPU across 9.5 minutes of wall clock, on a 4-core host with 5 GB free and peak RSS of 51 MB.

`emit` now hashes, compresses and stores `--jobs N` resources at once, default 16 — matching `DEPOT_JOBS` and the transport's `MaxIdleConnsPerHost`, so a worker per connection needs no fresh TLS handshake. `--jobs 1` is exactly the old behaviour.

### Three properties had to survive

Each has a test in `internal/nwsync/jobs_test.go`:

- **Deterministic manifest bytes.** `emitterVersion` promises a manifest is a function of its artifact, so `entries` is index-addressed rather than appended to — a worker owns `entries[i]` alone and the slice comes back in artifact order whatever order uploads finish in. `TestEmitProducesTheSameIndexAtEveryJobCount` diffs the `.nsym` and its sidecar between `-jobs 1` and `-jobs 16`.
- **Index still lands last.** Any worker's failure aborts before a manifest is written. `TestEmitLeavesNoIndexWhenAParallelUploadFails` fails every blob PUT with 16 workers in flight and asserts no `.nsym` appears. Under `-race` it also covers the shared counters.
- **Identical content still shares one blob.** This one bit during development and is the reason to read the diff carefully: serially, the sink's existence check absorbed two resrefs with identical bytes. In parallel both workers probe, both miss, and both upload — `TestEmitWritesBlobsAndManifest` caught it as "wrote 2 blobs, want 1". Claiming the sha1 in-process restores the dedupe and skips a probe round-trip as well.

### Memory

Peak now tracks the resources in flight rather than one resource. The ceiling is `N` × the 15 MB `fileSizeLimit` plus its compressed copy — bounded by a constant this package enforces itself, and still not tracking the archive. `TestEmitPeakMemoryIsBoundedByJobCount` re-runs the #76 regression check at `-jobs 8`: a hak 8× bigger still costs the same.

This is only cheap because of #78. Before streaming emit, N workers would have meant N whole archives resident.

### Not done

Skipping the `ProbeKey` HEAD on a first-time emit would halve round-trips, but doubles uploaded bytes on a re-run — which is exactly what a backfill is. Noted in #79 so it is not rediscovered; parallelism is the better lever and this PR takes it.

Worker compression still serialises on `blobEncoder`, which is `WithEncoderConcurrency(1)` for the memory reason in `compressedbuf.go`. At 26 s of CPU per hak that is not worth trading memory for, but it is where to look if the numbers ever say otherwise.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #80

Co-authored-by: vickydotbat <vickydotbat@tutamail.com>

docs/command-surface.md

@@ -43,10 +43,70 @@ flags are mutually exclusive.
 
 `nwsync emit` runs where an artifact is born (a `.hak`/`.erf`, or a loose file
 such as the TLK); `nwsync assemble` runs at module release and reads only the
-small per-artifact manifests. `--order` lists artifact names highest priority
-first: a resref in more than one artifact resolves to the earliest one, the way
-the game resolves it. `--group-id` is per channel — 1 is current, 2 is testing,
-and 0 leaves the field out of the sidecar.
+small per-artifact indexes. Both take **depot keys**: an artifact's index lives
+beside the artifact itself with the extension replaced, so `emit` and
+`assemble` agree on where it is without being told.
+
+```
+nwsync emit     [--as NAME] [--out DIR] [--jobs N] <artifact-key> <file>
+nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
+```
+
+`emit` is latency-bound, not CPU-bound: every blob costs an existence probe
+plus an upload, and a measured backfill spent 26 seconds of CPU across 9.5
+minutes of wall clock. `--jobs N` (default 16) sets how many resources are in
+flight at once. The manifest is byte-identical at any value — the number of
+workers is never observable in the output. Peak memory is `N` times the
+per-resource limit of 15 MB plus its compressed copy, so raising `N` far past
+the default costs real memory for little gain: the transport keeps 16 idle
+connections per host, and past that a worker pays a fresh TLS handshake.
+
+Both verbs upload by default; nothing bulky is ever written to the runner's
+disk. `--out DIR` writes a local repository tree instead, which is the
+conformance path against upstream `nwn_nwsync_write`. The zone comes from
+`NWSYNC_STORAGE_ZONE` and `NWSYNC_STORAGE_PASSWORD`, with the host from
+`BUNNY_STORAGE_HOST` — NWSync data is a separate zone from the asset depot.
+
+`assemble`'s artifact keys are in `Mod_HakList` order, highest priority first: a
+resref in more than one artifact resolves to the earliest one, the way the game
+resolves it. `--tlk-key` has its own slot because the TLK shadows nothing.
+`--group-id` is per channel — 1 is current, 2 is testing, and 0 leaves the field
+out of the sidecar.
+
+`emit` uploads blobs first and the index last, so the presence of an index is
+the publication marker: an artifact whose emit died halfway leaves real blobs in
+the zone and no index. Blob names are content hashes, so re-running skips
+whatever already landed, and `assemble` fails closed on an artifact with no
+index rather than publishing a manifest that is missing a hak.
+
+### What an emitted tree looks like
+
+`--out DIR` produces the same tree `emit` would upload, which makes it the way
+to check a zone by hand without touching one:
+
+```
+<artifact-sha>.nsym          binary index
+<artifact-sha>.nsym.json     the same index, readable
+data/sha1/a7/4a/a74aa84a...  one blob per resource, two-level fanout
+```
+
+A blob's name is the SHA-1 of the resource's **original** bytes, but the file on
+disk is not those bytes: each blob is wrapped in NWCompressedBuffer framing, a
+24-byte `NSYC` header followed by a zstd frame. Hashing the file directly will
+not match its name, which is the obvious
+first thing to try and the obvious first thing to be confused by. Strip the
+header first:
+
+```
+tail -c +25 <blob> | zstd -dc | sha1sum   # == the blob's filename
+```
+
+The header carries the uncompressed length as a little-endian `uint32` at offset
+12, so the decompressed size is checkable without decompressing. Compression is
+worth roughly a 4:1
+saving on hak content: a 250 MB hak emitted 2296 blobs totalling 59 MB on disk
+against 249 MB of resources, as recorded in the sidecar's `on_disk_bytes` and
+`total_bytes`.
 
 ## Hidden compatibility aliases
 

internal/depot/keystore.go

@@ -0,0 +1,121 @@
+package depot
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"strings"
+)
+
+// KeyStore is the zone addressed by object key rather than by depot sha. The
+// depot names every object after the sha256 of its contents; NWSync does not —
+// a blob is named after the sha1 of its *uncompressed* bytes while the body
+// uploaded is the compressed form, and a per-artifact index is named after its
+// artifact. Both addressing modes want the same transport, retry and probe
+// discipline, so the sha-addressed Backend rides on this rather than the other
+// way round.
+type KeyStore interface {
+	// ProbeKey returns the existence state of one key. transient=true means a
+	// retry might change the answer — never read it as "missing, re-upload".
+	ProbeKey(ctx context.Context, key string) (state ProbeState, transient bool, err error)
+	// PutReader uploads size bytes read from r to key. checksum is the
+	// uppercase hex sha256 of those bytes, which Bunny verifies server-side.
+	PutReader(ctx context.Context, key string, r io.Reader, size int64, checksum string) error
+	// GetKey fetches the whole object at key. Small objects only — it holds
+	// the body in memory and does no hash check, because a key is not always
+	// a content hash.
+	GetKey(ctx context.Context, key string) ([]byte, error)
+}
+
+// NewKeyStore returns a KeyStore for cfg's storage zone. Fails closed on a
+// missing host or read key, matching NewBackend.
+func NewKeyStore(cfg Config) (KeyStore, error) {
+	if cfg.StorageHost == "" {
+		return nil, errors.New("storage backend requires a storage host")
+	}
+	if cfg.StorageZone == "" {
+		return nil, errors.New("storage backend requires a storage zone")
+	}
+	if cfg.ReadKey == "" {
+		return nil, errors.New("storage backend requires a read key")
+	}
+	return &httpBackend{name: "bunny", client: newHTTPClient(cfg), cfg: cfg}, nil
+}
+
+// keyURL is the storage URL of one object key.
+func (b *httpBackend) keyURL(key string) string {
+	host := b.cfg.StorageHost
+	// StorageHost is normally a bare host ("storage.bunnycdn.com"); allow a
+	// full scheme (used by tests against httptest.NewServer) to pass through
+	// unchanged.
+	if !strings.Contains(host, "://") {
+		host = "https://" + host
+	}
+	return fmt.Sprintf("%s/%s/%s", strings.TrimSuffix(host, "/"), b.cfg.StorageZone, key)
+}
+
+func (b *httpBackend) ProbeKey(ctx context.Context, key string) (ProbeState, bool, error) {
+	return b.rangeProbe(ctx, b.keyURL(key), map[string]string{"AccessKey": b.cfg.ReadKey})
+}
+
+func (b *httpBackend) PutReader(ctx context.Context, key string, r io.Reader, size int64, checksum string) error {
+	if b.name == "cdn" {
+		return errors.New("cdn backend is read-only")
+	}
+	if b.cfg.WriteKey == "" {
+		return errors.New("storage backend requires a write key to write")
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPut, b.keyURL(key), r)
+	if err != nil {
+		return err
+	}
+	req.ContentLength = size
+	req.Header.Set("AccessKey", b.cfg.WriteKey)
+	// Bunny defines Checksum as sha256 of the body and rejects a mismatch, so
+	// this is server-side integrity checking, not decoration.
+	req.Header.Set("Checksum", strings.ToUpper(checksum))
+
+	resp, err := b.client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	_, _ = io.Copy(io.Discard, resp.Body)
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return fmt.Errorf("put %s: unexpected status %d", key, resp.StatusCode)
+	}
+	return nil
+}
+
+func (b *httpBackend) GetKey(ctx context.Context, key string) ([]byte, error) {
+	resp, err := b.get(ctx, b.keyURL(key), map[string]string{"AccessKey": b.cfg.ReadKey})
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		_, _ = io.Copy(io.Discard, resp.Body)
+		return nil, fmt.Errorf("get %s: unexpected status %d", key, resp.StatusCode)
+	}
+	return io.ReadAll(resp.Body)
+}
+
+// putFile uploads the file at src to key, streaming it. checksum is the
+// uppercase hex sha256 of the file's bytes.
+func (b *httpBackend) putFile(ctx context.Context, key, src, checksum string) error {
+	f, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	info, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	return b.PutReader(ctx, key, f, info.Size(), checksum)
+}

internal/depot/remote.go

@@ -10,7 +10,6 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
-	"strings"
 	"time"
 )
 
@@ -71,16 +70,7 @@ type httpBackend struct {
 
 func (b *httpBackend) Name() string { return b.name }
 
-func (b *httpBackend) storageURL(sha string) string {
-	host := b.cfg.StorageHost
-	// StorageHost is normally a bare host ("storage.bunnycdn.com"); allow a
-	// full scheme (used by tests against httptest.NewServer) to pass through
-	// unchanged.
-	if strings.Contains(host, "://") {
-		return fmt.Sprintf("%s/%s/%s", strings.TrimSuffix(host, "/"), b.cfg.StorageZone, BlobKey(sha))
-	}
-	return fmt.Sprintf("https://%s/%s/%s", host, b.cfg.StorageZone, BlobKey(sha))
-}
+func (b *httpBackend) storageURL(sha string) string { return b.keyURL(BlobKey(sha)) }
 
 func (b *httpBackend) cdnURL(sha string) string {
 	return fmt.Sprintf("%s/%s", b.cfg.CDNBase, BlobKey(sha))
@@ -140,7 +130,8 @@ func (b *httpBackend) Probe(ctx context.Context, sha string) (ProbeState, bool,
 	return b.rangeProbe(ctx, b.storageURL(sha), map[string]string{"AccessKey": b.cfg.ReadKey})
 }
 
-// Put uploads src for sha. cdn is read-only.
+// Put uploads src for sha. cdn is read-only. A depot object is named after the
+// sha256 of its own bytes, so the key's sha doubles as the Checksum header.
 func (b *httpBackend) Put(ctx context.Context, sha, src string) error {
 	if b.name == "cdn" {
 		return errors.New("cdn backend is read-only")
@@ -157,30 +148,7 @@ func (b *httpBackend) Put(ctx context.Context, sha, src string) error {
 		return nil
 	}
 
-	f, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	req, err := http.NewRequestWithContext(ctx, http.MethodPut, b.storageURL(sha), f)
-	if err != nil {
-		return err
-	}
-	req.Header.Set("AccessKey", b.cfg.WriteKey)
-	req.Header.Set("Checksum", strings.ToUpper(sha))
-
-	resp, err := b.client.Do(req)
-	if err != nil {
-		return err
-	}
-	defer resp.Body.Close()
-	_, _ = io.Copy(io.Discard, resp.Body)
-
-	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		return fmt.Errorf("bunny put %s: unexpected status %d", sha, resp.StatusCode)
-	}
-	return nil
+	return b.putFile(ctx, BlobKey(sha), src, sha)
 }
 
 // Get fetches sha into dest via temp file + rename, re-hashing and deleting

internal/erf/erf.go

@@ -311,63 +311,110 @@ func Write(w io.Writer, archive Archive) error {
 	return nil
 }
 
+// IndexEntry locates one resource inside an archive without holding its
+// payload. Streaming callers read one payload at a time from these, so peak
+// memory tracks the largest resource instead of the whole archive.
+type IndexEntry struct {
+	Name   string
+	Type   uint16
+	Offset int64
+	Size   int64
+}
+
+// Index is the header plus the resource table of an ERF: everything except the
+// payloads.
+type Index struct {
+	FileType string
+	Version  string
+	Entries  []IndexEntry
+}
+
+// ReadIndex parses the tables of an ERF of the given size, reading only the
+// header, the key list and the resource list.
+func ReadIndex(r io.ReaderAt, size int64) (Index, error) {
+	if size < headerSize {
+		return Index{}, fmt.Errorf("erf file too small: %d bytes", size)
+	}
+
+	var hdr header
+	if err := binary.Read(io.NewSectionReader(r, 0, headerSize), binary.LittleEndian, &hdr); err != nil {
+		return Index{}, fmt.Errorf("decode erf header: %w", err)
+	}
+
+	if int64(hdr.KeyListOffset)+int64(hdr.EntryCount)*24 > size {
+		return Index{}, fmt.Errorf("erf key list exceeds file bounds")
+	}
+	keys := make([]keyEntry, hdr.EntryCount)
+	keyReader := io.NewSectionReader(r, int64(hdr.KeyListOffset), int64(hdr.EntryCount)*24)
+	if err := binary.Read(keyReader, binary.LittleEndian, &keys); err != nil {
+		return Index{}, fmt.Errorf("decode key list: %w", err)
+	}
+
+	if int64(hdr.ResourceListOffset)+int64(hdr.EntryCount)*8 > size {
+		return Index{}, fmt.Errorf("erf resource list exceeds file bounds")
+	}
+	entries := make([]resourceEntry, hdr.EntryCount)
+	entryReader := io.NewSectionReader(r, int64(hdr.ResourceListOffset), int64(hdr.EntryCount)*8)
+	if err := binary.Read(entryReader, binary.LittleEndian, &entries); err != nil {
+		return Index{}, fmt.Errorf("decode resource list: %w", err)
+	}
+
+	index := Index{
+		FileType: string(hdr.FileType[:]),
+		Version:  string(hdr.Version[:]),
+		Entries:  make([]IndexEntry, 0, hdr.EntryCount),
+	}
+	for position, key := range keys {
+		entry := entries[position]
+		if int64(entry.Offset)+int64(entry.Size) > size {
+			return Index{}, fmt.Errorf("resource %d exceeds file bounds", position)
+		}
+		index.Entries = append(index.Entries, IndexEntry{
+			Name:   string(bytes.TrimRight(key.ResRef[:], "\x00")),
+			Type:   key.ResourceType,
+			Offset: int64(entry.Offset),
+			Size:   int64(entry.Size),
+		})
+	}
+	return index, nil
+}
+
+// ReadPayload returns one resource's bytes.
+func ReadPayload(r io.ReaderAt, entry IndexEntry) ([]byte, error) {
+	payload := make([]byte, entry.Size)
+	if _, err := r.ReadAt(payload, entry.Offset); err != nil {
+		return nil, fmt.Errorf("read resource %q: %w", entry.Name, err)
+	}
+	return payload, nil
+}
+
+// Read materialises a whole archive. Payloads are subslices of the buffer the
+// archive was read into, so nothing is copied twice: a caller must not mutate
+// Data. Callers that only need one resource at a time should use ReadIndex
+// instead, which never holds the archive at all.
 func Read(r io.Reader) (Archive, error) {
 	data, err := io.ReadAll(r)
 	if err != nil {
 		return Archive{}, fmt.Errorf("read erf: %w", err)
 	}
-	if len(data) < headerSize {
-		return Archive{}, fmt.Errorf("erf file too small: %d bytes", len(data))
+	index, err := ReadIndex(bytes.NewReader(data), int64(len(data)))
+	if err != nil {
+		return Archive{}, err
 	}
 
-	var hdr header
-	if err := binary.Read(bytes.NewReader(data[:headerSize]), binary.LittleEndian, &hdr); err != nil {
-		return Archive{}, fmt.Errorf("decode erf header: %w", err)
-	}
-
-	keyStart := int(hdr.KeyListOffset)
-	keyEnd := keyStart + int(hdr.EntryCount)*24
-	if keyEnd > len(data) {
-		return Archive{}, fmt.Errorf("erf key list exceeds file bounds")
-	}
-	keys := make([]keyEntry, hdr.EntryCount)
-	if err := binary.Read(bytes.NewReader(data[keyStart:keyEnd]), binary.LittleEndian, &keys); err != nil {
-		return Archive{}, fmt.Errorf("decode key list: %w", err)
-	}
-
-	resourceStart := int(hdr.ResourceListOffset)
-	resourceEnd := resourceStart + int(hdr.EntryCount)*8
-	if resourceEnd > len(data) {
-		return Archive{}, fmt.Errorf("erf resource list exceeds file bounds")
-	}
-	entries := make([]resourceEntry, hdr.EntryCount)
-	if err := binary.Read(bytes.NewReader(data[resourceStart:resourceEnd]), binary.LittleEndian, &entries); err != nil {
-		return Archive{}, fmt.Errorf("decode resource list: %w", err)
-	}
-
-	resources := make([]Resource, 0, hdr.EntryCount)
-	for index, key := range keys {
-		entry := entries[index]
-		start := int(entry.Offset)
-		end := start + int(entry.Size)
-		if end > len(data) {
-			return Archive{}, fmt.Errorf("resource %d exceeds file bounds", index)
-		}
-
-		resref := string(bytes.TrimRight(key.ResRef[:], "\x00"))
-		payload := make([]byte, entry.Size)
-		copy(payload, data[start:end])
+	resources := make([]Resource, 0, len(index.Entries))
+	for _, entry := range index.Entries {
 		resources = append(resources, Resource{
-			Name: resref,
-			Type: key.ResourceType,
-			Data: payload,
-			Size: int64(entry.Size),
+			Name: entry.Name,
+			Type: entry.Type,
+			Data: data[entry.Offset : entry.Offset+entry.Size],
+			Size: entry.Size,
 		})
 	}
 
 	return Archive{
-		FileType:  string(hdr.FileType[:]),
-		Version:   string(hdr.Version[:]),
+		FileType:  index.FileType,
+		Version:   index.Version,
 		Resources: resources,
 	}, nil
 }

internal/nwsync/assemble.go

@@ -4,18 +4,21 @@ import (
 	"crypto/sha1"
 	"encoding/json"
 	"fmt"
-	"os"
-	"path/filepath"
+	"path"
 )
 
 // AssembleOptions describes one merged manifest.
 type AssembleOptions struct {
-	Order       []string // artifact names, highest priority first
-	EntriesDir  string   // directory holding <name>.nsym and <name>.nsym.json
-	OutDir      string   // repository root; the manifest lands in <out>/manifests
+	// ArtifactKeys are the depot keys of the artifacts to merge, in
+	// Mod_HakList order — highest priority first. Each one's index is read
+	// from the key beside it.
+	ArtifactKeys []string
+	TLKKey       string // the TLK's key, if the manifest carries one
+	OutDir       string // write locally instead of uploading — the conformance path
 	GroupID      int    // 1 = current, 2 = testing; 0 means absent
 	ModuleName   string
 	Description  string
+	Sink         sink // test seam; nil means OutDir or the zone
 }
 
 // AssembleResult reports what one assemble run produced.
@@ -33,25 +36,43 @@ type AssembleResult struct {
 // the game resolves it (upstream's resman adds haks in reverse and lets the
 // last one win). Get this backwards and the wrong texture ships silently.
 func Assemble(options AssembleOptions) (AssembleResult, error) {
-	if len(options.Order) == 0 {
-		return AssembleResult{}, fmt.Errorf("assemble: --order names no artifacts")
+	if len(options.ArtifactKeys) == 0 {
+		return AssembleResult{}, fmt.Errorf("assemble: no artifact keys given")
+	}
+
+	target, err := openSink(options.OutDir, options.Sink)
+	if err != nil {
+		return AssembleResult{}, err
+	}
+
+	// The TLK carries no precedence — it is not a hak and shadows nothing —
+	// so it merges last, after every hak has had its say.
+	keys := append([]string{}, options.ArtifactKeys...)
+	if options.TLKKey != "" {
+		keys = append(keys, options.TLKKey)
 	}
 
 	merged := make([]Entry, 0, 1024)
 	winner := make(map[Identity]bool, 1024)
 	var onDiskBytes int64
 
-	for _, name := range options.Order {
-		manifestPath := filepath.Join(options.EntriesDir, name+".nsym")
-		data, err := os.ReadFile(manifestPath)
+	for _, artifactKey := range keys {
+		key, err := resolveIndexKey(artifactKey, options.OutDir)
 		if err != nil {
-			return AssembleResult{}, fmt.Errorf("assemble: no index for %q: %w", name, err)
+			return AssembleResult{}, err
+		}
+		data, sidecarBody, err := target.getIndex(key)
+		if err != nil {
+			// An artifact with no index is an artifact whose emit never
+			// finished. Publishing a manifest without it would ship a release
+			// missing a hak, so this fails closed.
+			return AssembleResult{}, fmt.Errorf("assemble: no index for %s: %w", artifactKey, err)
 		}
 		entries, err := readManifest(data)
 		if err != nil {
-			return AssembleResult{}, fmt.Errorf("%s: %w", manifestPath, err)
+			return AssembleResult{}, fmt.Errorf("%s: %w", target.describe(key), err)
 		}
-		sidecar, err := readSidecar(manifestPath + ".json")
+		sidecar, err := parseSidecar(target.describe(key), sidecarBody)
 		if err != nil {
 			return AssembleResult{}, err
 		}
@@ -61,7 +82,7 @@ func Assemble(options AssembleOptions) (AssembleResult, error) {
 		if sidecar.EmitterVersion != emitterVersion {
 			return AssembleResult{}, fmt.Errorf(
 				"assemble: emitter version mismatch: %s was emitted by emitter %q, this is emitter %q",
-				name, sidecar.EmitterVersion, emitterVersion)
+				artifactKey, sidecar.EmitterVersion, emitterVersion)
 		}
 		// on_disk_bytes overcounts by the handful of cross-artifact
 		// duplicates. It is a display statistic; no dedupe pass for it.
@@ -86,26 +107,22 @@ func Assemble(options AssembleOptions) (AssembleResult, error) {
 		return AssembleResult{}, err
 	}
 	sha1Hex := fmt.Sprintf("%x", sha1.Sum(data))
-	manifestPath := filepath.Join(options.OutDir, "manifests", sha1Hex)
+	manifestKey := path.Join("manifests", sha1Hex)
 	sidecar := Sidecar{
 		ModuleName:  options.ModuleName,
 		Description: options.Description,
 		GroupID:     options.GroupID,
 	}
-	if err := writeManifestPair(manifestPath, data, merged, onDiskBytes, sidecar); err != nil {
+	if err := putManifestPair(target, manifestKey, data, merged, onDiskBytes, sidecar); err != nil {
 		return AssembleResult{}, err
 	}
-	return AssembleResult{SHA1: sha1Hex, ManifestPath: manifestPath, Entries: len(merged)}, nil
+	return AssembleResult{SHA1: sha1Hex, ManifestPath: target.describe(manifestKey), Entries: len(merged)}, nil
 }
 
-func readSidecar(path string) (Sidecar, error) {
-	data, err := os.ReadFile(path)
-	if err != nil {
-		return Sidecar{}, fmt.Errorf("assemble: missing sidecar: %w", err)
-	}
+func parseSidecar(where string, data []byte) (Sidecar, error) {
 	var sidecar Sidecar
 	if err := json.Unmarshal(data, &sidecar); err != nil {
-		return Sidecar{}, fmt.Errorf("%s: %w", path, err)
+		return Sidecar{}, fmt.Errorf("%s: %w", where, err)
 	}
 	return sidecar, nil
 }

internal/nwsync/compressedbuf.go

@@ -22,9 +22,13 @@ const (
 	blobHeaderBytes = 24
 )
 
+// EncodeAll/DecodeAll are single-threaded per call, so the default pool of one
+// encoder per CPU only buys idle memory: each holds a window-sized history, so
+// on a 24-core runner that is ~200 MB of live heap doing nothing. Concurrency 1
+// produces byte-identical output.
 var (
-	blobEncoder, _ = zstd.NewWriter(nil)
-	blobDecoder, _ = zstd.NewReader(nil)
+	blobEncoder, _ = zstd.NewWriter(nil, zstd.WithEncoderConcurrency(1))
+	blobDecoder, _ = zstd.NewReader(nil, zstd.WithDecoderConcurrency(1))
 )
 
 // compressBlob wraps data in NWCompressedBuffer framing.

internal/nwsync/emit.go

@@ -1,15 +1,18 @@
 package nwsync
 
 import (
-	"bytes"
+	"context"
 	"crypto/sha1"
 	"fmt"
+	"io"
 	"os"
+	"path"
 	"path/filepath"
 	"slices"
 	"sort"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"git.westgate.pw/ShadowsOverWestgate/sow-tools/internal/buildinfo"
@@ -60,119 +63,259 @@ type EmitResult struct {
 	BlobsWritten int
 }
 
-// Emit explodes one artifact — a .hak/.erf/.mod or a loose file such as the
-// TLK — into NWSync blobs plus a NSYM manifest describing only that artifact.
-func Emit(artifactPath, outDir string) (EmitResult, error) {
-	resources, err := readArtifact(artifactPath)
+// defaultEmitJobs is how many resources are hashed, compressed and stored at
+// once. Emit is latency-bound, not CPU-bound: a blob costs a probe round-trip
+// plus an upload round-trip, and a measured backfill spent 26 s of CPU across
+// 9.5 minutes of wall clock. The figure matches depot's DEPOT_JOBS default and
+// the transport's MaxIdleConnsPerHost, so a worker per connection needs no new
+// TLS handshake.
+const defaultEmitJobs = 16
+
+// EmitOptions describes one emit run.
+type EmitOptions struct {
+	ArtifactKey  string // depot key of the artifact; the NSYM key is derived from it
+	ArtifactPath string // the file on disk
+	As           string // name override, for a TLK whose filename is not its published name
+	OutDir       string // write locally instead of uploading — the conformance path
+	Jobs         int    // resources in flight at once; 0 means defaultEmitJobs
+	Sink         sink   // test seam; nil means OutDir or the zone
+}
+
+// Emit explodes one artifact — a .hak/.erf or a loose file such as the TLK —
+// into NWSync blobs plus a NSYM manifest describing only that artifact.
+//
+// Blobs go up as they are produced and the index lands last, so the presence of
+// an index is the publication marker: an artifact whose emit died halfway has
+// real blobs in the zone and no index, which is unambiguous. Blob names are
+// content hashes, so re-running skips whatever already landed.
+func Emit(options EmitOptions) (EmitResult, error) {
+	artifact, err := os.Open(options.ArtifactPath)
+	if err != nil {
+		return EmitResult{}, fmt.Errorf("read artifact: %w", err)
+	}
+	defer artifact.Close()
+	info, err := artifact.Stat()
+	if err != nil {
+		return EmitResult{}, fmt.Errorf("read artifact: %w", err)
+	}
+	// A section reader, not the file itself: hashing must not move the file
+	// offset out from under everything that reads the artifact afterwards.
+	if err := checkArtifactKey(options.ArtifactKey, io.NewSectionReader(artifact, 0, info.Size())); err != nil {
+		return EmitResult{}, err
+	}
+	name := options.As
+	if name == "" {
+		name = path.Base(options.ArtifactKey)
+	}
+	extension := path.Ext(name)
+	name = strings.TrimSuffix(name, extension)
+
+	key, err := resolveIndexKey(options.ArtifactKey, options.OutDir)
 	if err != nil {
 		return EmitResult{}, err
 	}
-	name := strings.TrimSuffix(filepath.Base(artifactPath), filepath.Ext(artifactPath))
 
-	entries, blobs, onDiskBytes, err := emitResources(resources, outDir)
+	index, err := readArtifactIndex(options.ArtifactPath, artifact, info.Size(), name)
+	if err != nil {
+		return EmitResult{}, err
+	}
+
+	target, err := openSink(options.OutDir, options.Sink)
+	if err != nil {
+		return EmitResult{}, err
+	}
+
+	jobs := options.Jobs
+	if jobs < 1 {
+		jobs = defaultEmitJobs
+	}
+	entries, blobs, onDiskBytes, err := emitResources(artifact, index, target, jobs)
 	if err != nil {
 		return EmitResult{}, err
 	}
 	if len(entries) == 0 {
-		return EmitResult{}, fmt.Errorf("%s: nothing to index (no publishable resources)", artifactPath)
+		return EmitResult{}, fmt.Errorf("%s: nothing to index (no publishable resources)", options.ArtifactPath)
 	}
 
 	data, err := writeManifest(entries)
 	if err != nil {
 		return EmitResult{}, err
 	}
-	manifestPath := filepath.Join(outDir, name+".nsym")
-	if err := writeManifestPair(manifestPath, data, entries, onDiskBytes, Sidecar{ModuleName: name}); err != nil {
+	if err := putManifestPair(target, key, data, entries, onDiskBytes, Sidecar{ModuleName: name}); err != nil {
 		return EmitResult{}, err
 	}
-	return EmitResult{Name: name, ManifestPath: manifestPath, Entries: len(entries), BlobsWritten: blobs}, nil
+	return EmitResult{Name: name, ManifestPath: target.describe(key), Entries: len(entries), BlobsWritten: blobs}, nil
 }
 
-// readArtifact returns the resources of an ERF/HAK/MOD, or the single resource
-// a loose file represents. Upstream's resman does the same dispatch on the
-// file's first three bytes.
-func readArtifact(path string) ([]erf.Resource, error) {
-	data, err := os.ReadFile(path)
-	if err != nil {
-		return nil, fmt.Errorf("read artifact: %w", err)
+// openSink returns the zone sink, or a local directory when outDir is set.
+func openSink(outDir string, injected sink) (sink, error) {
+	if injected != nil {
+		return injected, nil
 	}
-	if len(data) >= 3 {
-		switch string(data[:3]) {
+	if outDir != "" {
+		return dirSink{root: outDir}, nil
+	}
+	return newZoneSink(context.Background(), os.Getenv)
+}
+
+// readArtifactIndex locates the resources of an ERF/HAK/MOD, or the single
+// resource a loose file represents, without reading any payload. Upstream's
+// resman does the same dispatch on the file's first three bytes. name is the
+// artifact's published name, which for a loose file is also its resref.
+func readArtifactIndex(path string, artifact io.ReaderAt, size int64, name string) ([]erf.IndexEntry, error) {
+	magic := make([]byte, 3)
+	if size >= 3 {
+		if _, err := artifact.ReadAt(magic, 0); err != nil {
+			return nil, fmt.Errorf("%s: %w", path, err)
+		}
+	}
+	switch string(magic) {
 	case "ERF", "HAK":
-			archive, err := erf.Read(bytes.NewReader(data))
+		index, err := erf.ReadIndex(artifact, size)
 		if err != nil {
 			return nil, fmt.Errorf("%s: %w", path, err)
 		}
-			return archive.Resources, nil
+		return index.Entries, nil
 	case "MOD":
 		// A persistent world never publishes module contents, so the .mod
 		// contributes no bytes to a manifest — it only says which haks and
 		// which TLK the manifest covers.
 		return nil, fmt.Errorf("%s: a module is never emitted; a manifest is haks plus the TLK", path)
 	}
-	}
-	base := filepath.Base(path)
-	extension := filepath.Ext(base)
+	extension := filepath.Ext(filepath.Base(path))
 	restype, ok := erf.ResourceTypeForExtension(extension)
 	if !ok {
 		return nil, fmt.Errorf("%s: unknown resource type %q", path, extension)
 	}
-	return []erf.Resource{{
-		Name: strings.TrimSuffix(base, extension),
-		Type: restype,
-		Data: data,
-	}}, nil
+	return []erf.IndexEntry{{Name: name, Type: restype, Offset: 0, Size: size}}, nil
 }
 
-func emitResources(resources []erf.Resource, outDir string) ([]Entry, int, int64, error) {
+// emitResources hashes, compresses and stores resources, reading each payload
+// from the artifact only when its turn comes. Peak memory tracks the resources
+// in flight, not the archive: a 2 GB hak must emit inside a runner's few spare
+// GB. jobs of them are in flight at once, so the ceiling is jobs multiplied by
+// fileSizeLimit and its compressed copy — bounded, and bounded by a constant
+// this package enforces itself.
+//
+// The returned entries are in artifact order whatever order the workers finish
+// in, because a manifest's bytes are promised deterministic by emitterVersion.
+func emitResources(artifact io.ReaderAt, index []erf.IndexEntry, target sink, jobs int) ([]Entry, int, int64, error) {
 	// A resref appearing twice inside one artifact resolves to the last one,
 	// the way resman lets the last container added win.
-	order := make([]Identity, 0, len(resources))
-	latest := make(map[Identity]erf.Resource, len(resources))
+	order := make([]Identity, 0, len(index))
+	latest := make(map[Identity]erf.IndexEntry, len(index))
 	var tooBig []string
-	for _, resource := range resources {
-		if _, ok := erf.ExtensionForResourceType(resource.Type); !ok {
-			return nil, 0, 0, fmt.Errorf("resref %s is not resolvable (unknown restype %d)", resource.Name, resource.Type)
+	for _, entry := range index {
+		if _, ok := erf.ExtensionForResourceType(entry.Type); !ok {
+			return nil, 0, 0, fmt.Errorf("resref %s is not resolvable (unknown restype %d)", entry.Name, entry.Type)
 		}
-		if slices.Contains(skippedTypes, resource.Type) {
+		if slices.Contains(skippedTypes, entry.Type) {
 			continue
 		}
-		if len(resource.Data) > fileSizeLimit {
-			tooBig = append(tooBig, fmt.Sprintf("%s: %d bytes > %d", resource.Name, len(resource.Data), fileSizeLimit))
+		if entry.Size > fileSizeLimit {
+			tooBig = append(tooBig, fmt.Sprintf("%s: %d bytes > %d", entry.Name, entry.Size, fileSizeLimit))
 			continue
 		}
-		identity := Identity{ResRef: strings.ToLower(resource.Name), ResType: resource.Type}
+		identity := Identity{ResRef: strings.ToLower(entry.Name), ResType: entry.Type}
 		if _, seen := latest[identity]; !seen {
 			order = append(order, identity)
 		}
-		latest[identity] = resource
+		latest[identity] = entry
 	}
 	if len(tooBig) > 0 {
 		sort.Strings(tooBig)
 		return nil, 0, 0, fmt.Errorf("resources exceed the file size limit:\n  %s", strings.Join(tooBig, "\n  "))
 	}
 
-	entries := make([]Entry, 0, len(order))
+	// Index-addressed, never appended to: a worker owns entries[i] alone, so
+	// the slice comes back in artifact order and needs no lock.
+	entries := make([]Entry, len(order))
 	var blobs int
 	var onDiskBytes int64
-	for _, identity := range order {
-		resource := latest[identity]
-		sum := sha1.Sum(resource.Data)
-		entries = append(entries, Entry{
+	var mu sync.Mutex
+	var firstErr error
+	// Two resrefs in one artifact can hold identical bytes, and therefore one
+	// blob. Serially the sink's existence check absorbed that; in parallel both
+	// workers would probe, both miss, and both upload. Claiming the sha1 here
+	// restores the dedupe and skips the probe round-trip as well.
+	claimed := make(map[[20]byte]bool, len(order))
+
+	failed := func() bool {
+		mu.Lock()
+		defer mu.Unlock()
+		return firstErr != nil
+	}
+
+	store := func(i int) {
+		identity := order[i]
+		payload, err := erf.ReadPayload(artifact, latest[identity])
+		if err != nil {
+			mu.Lock()
+			if firstErr == nil {
+				firstErr = err
+			}
+			mu.Unlock()
+			return
+		}
+		sum := sha1.Sum(payload)
+		entries[i] = Entry{
 			SHA1:    sum,
-			Size:    uint32(len(resource.Data)),
+			Size:    uint32(len(payload)),
 			ResRef:  identity.ResRef,
 			ResType: identity.ResType,
-		})
-		written, err := writeBlob(outDir, sum, resource.Data)
+		}
+		mu.Lock()
+		duplicate := claimed[sum]
+		claimed[sum] = true
+		mu.Unlock()
+		if duplicate {
+			return
+		}
+		written, err := target.putBlob(fmt.Sprintf("%x", sum), func() []byte { return compressBlob(payload) })
+		mu.Lock()
+		defer mu.Unlock()
 		if err != nil {
-			return nil, 0, 0, err
+			if firstErr == nil {
+				firstErr = err
+			}
+			return
 		}
 		if written > 0 {
 			blobs++
 			onDiskBytes += written
 		}
 	}
+
+	if jobs < 1 {
+		jobs = 1
+	}
+	work := make(chan int)
+	var wg sync.WaitGroup
+	for range jobs {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := range work {
+				// After a failure the run is over — the caller discards
+				// everything and no index is written. Draining the rest of the
+				// channel cheaply, rather than returning, keeps the feeder from
+				// blocking on workers that have gone away.
+				if failed() {
+					continue
+				}
+				store(i)
+			}
+		}()
+	}
+	for i := range order {
+		work <- i
+	}
+	close(work)
+	wg.Wait()
+
+	if firstErr != nil {
+		return nil, 0, 0, firstErr
+	}
 	return entries, blobs, onDiskBytes, nil
 }
 
@@ -187,35 +330,10 @@ func created() int64 {
 	return time.Now().Unix()
 }
 
-// writeBlob writes one NWCompressedBuffer blob and returns its size on disk,
-// or 0 if the blob already existed. Blob names are content hashes, so an
-// existing name is existing content.
-func writeBlob(outDir string, sum [20]byte, data []byte) (int64, error) {
-	path := blobPath(outDir, fmt.Sprintf("%x", sum))
-	if _, err := os.Stat(path); err == nil {
-		return 0, nil
-	}
-	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
-		return 0, fmt.Errorf("create blob directory: %w", err)
-	}
-	blob := compressBlob(data)
-	if err := os.WriteFile(path, blob, 0o644); err != nil {
-		return 0, fmt.Errorf("write blob: %w", err)
-	}
-	return int64(len(blob)), nil
-}
-
-// writeManifestPair writes a NSYM manifest and its .json sidecar. The caller
-// supplies the sidecar fields it knows; the rest are derived from the entries.
-// data must be the serialised form of entries.
-func writeManifestPair(manifestPath string, data []byte, entries []Entry, onDiskBytes int64, sidecar Sidecar) error {
-	if err := os.MkdirAll(filepath.Dir(manifestPath), 0o755); err != nil {
-		return fmt.Errorf("create manifest directory: %w", err)
-	}
-	if err := os.WriteFile(manifestPath, data, 0o644); err != nil {
-		return fmt.Errorf("write manifest: %w", err)
-	}
-
+// putManifestPair stores a NSYM manifest and its .json sidecar at key. The
+// caller supplies the sidecar fields it knows; the rest are derived from the
+// entries. data must be the serialised form of entries.
+func putManifestPair(target sink, key string, data []byte, entries []Entry, onDiskBytes int64, sidecar Sidecar) error {
 	var totalBytes int64
 	clientContents := false
 	for _, entry := range entries {
@@ -240,8 +358,5 @@ func writeManifestPair(manifestPath string, data []byte, entries []Entry, onDisk
 	if err != nil {
 		return err
 	}
-	if err := os.WriteFile(manifestPath+".json", body, 0o644); err != nil {
-		return fmt.Errorf("write sidecar: %w", err)
-	}
-	return nil
+	return target.putIndex(key, data, body)
 }

internal/nwsync/jobs_test.go

@@ -0,0 +1,134 @@
+package nwsync
+
+import (
+	"bytes"
+	"fmt"
+	"path/filepath"
+	"runtime/debug"
+	"strings"
+	"testing"
+)
+
+// manyResources is a hak body with enough distinct resources that a worker pool
+// actually interleaves. Payloads differ so nothing is deduplicated away.
+func manyResources(count int) map[string][]byte {
+	contents := make(map[string][]byte, count)
+	for i := range count {
+		contents[fmt.Sprintf("res%05d.tga", i)] = []byte(fmt.Sprintf("payload %d", i))
+	}
+	return contents
+}
+
+// TestEmitProducesTheSameIndexAtEveryJobCount is the contract that lets emit be
+// parallel at all: emitterVersion promises a manifest's bytes are a function of
+// its artifact, so the number of workers must not be observable in the output.
+func TestEmitProducesTheSameIndexAtEveryJobCount(t *testing.T) {
+	// The sidecar stamps a wall-clock time unless this is set, which would make
+	// two runs differ for a reason that has nothing to do with job count.
+	t.Setenv("SOURCE_DATE_EPOCH", "1700000000")
+
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	writeHak(t, hak, manyResources(64))
+	key := artifactKey(t, hak)
+
+	emit := func(jobs int) (manifest, sidecar []byte, result EmitResult) {
+		out := filepath.Join(t.TempDir(), "out")
+		result, err := Emit(EmitOptions{
+			ArtifactKey:  key,
+			ArtifactPath: hak,
+			OutDir:       out,
+			Jobs:         jobs,
+		})
+		if err != nil {
+			t.Fatalf("emit at -jobs %d: %v", jobs, err)
+		}
+		manifest, sidecar, err = dirSink{root: out}.getIndex(filepath.Base(result.ManifestPath))
+		if err != nil {
+			t.Fatalf("read index at -jobs %d: %v", jobs, err)
+		}
+		return manifest, sidecar, result
+	}
+
+	serialManifest, serialSidecar, serial := emit(1)
+	parallelManifest, parallelSidecar, parallel := emit(16)
+
+	if !bytes.Equal(serialManifest, parallelManifest) {
+		t.Errorf("manifest bytes differ between -jobs 1 and -jobs 16")
+	}
+	if !bytes.Equal(serialSidecar, parallelSidecar) {
+		t.Errorf("sidecar bytes differ between -jobs 1 and -jobs 16:\n %s\n %s", serialSidecar, parallelSidecar)
+	}
+	if serial.Entries != parallel.Entries || serial.BlobsWritten != parallel.BlobsWritten {
+		t.Errorf("-jobs 1 wrote %d entries/%d blobs, -jobs 16 wrote %d/%d",
+			serial.Entries, serial.BlobsWritten, parallel.Entries, parallel.BlobsWritten)
+	}
+}
+
+// TestEmitLeavesNoIndexWhenAParallelUploadFails is the fail-closed check with
+// workers in flight: several uploads are in the air when the first one fails,
+// and the index must still never appear. Run under -race this also covers the
+// shared counters.
+func TestEmitLeavesNoIndexWhenAParallelUploadFails(t *testing.T) {
+	fixture := newZoneFixture(t)
+	fixture.zone.failOn = func(key string) bool { return strings.HasPrefix(key, "data/sha1/") }
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	writeHak(t, hak, manyResources(64))
+
+	if _, err := Emit(EmitOptions{
+		ArtifactKey:  artifactKey(t, hak),
+		ArtifactPath: hak,
+		Sink:         fixture.sink,
+		Jobs:         16,
+	}); err == nil {
+		t.Fatal("emit reported success after an upload failed")
+	}
+	fixture.zone.mu.Lock()
+	defer fixture.zone.mu.Unlock()
+	for key := range fixture.zone.objects {
+		if strings.HasSuffix(key, ".nsym") {
+			t.Errorf("a half-emitted artifact published an index: %s", key)
+		}
+	}
+}
+
+// TestEmitPeakMemoryIsBoundedByJobCount pins the ceiling the parallel emit
+// rests on. Peak still must not track the archive — it tracks the resources in
+// flight, so a bigger hak at the same job count costs the same.
+func TestEmitPeakMemoryIsBoundedByJobCount(t *testing.T) {
+	if testing.Short() {
+		t.Skip("writes a 64 MB fixture")
+	}
+	defer debug.SetGCPercent(debug.SetGCPercent(10))
+
+	measure := func(count, jobs int) uint64 {
+		dir := t.TempDir()
+		hak := filepath.Join(dir, "big.hak")
+		writeStreamedHak(t, hak, count)
+		options := EmitOptions{
+			ArtifactKey:  artifactKey(t, hak),
+			ArtifactPath: hak,
+			As:           filepath.Base(hak),
+			OutDir:       filepath.Join(dir, "out"),
+			Jobs:         jobs,
+		}
+		return peakHeapDuring(func() {
+			if _, err := Emit(options); err != nil {
+				t.Fatalf("emit %d resources at -jobs %d: %v", count, jobs, err)
+			}
+		})
+	}
+
+	const jobs = 8
+	small := measure(8, jobs)  // 8 MB
+	large := measure(64, jobs) // 64 MB
+	// Each worker may hold one resourceSize payload plus its compressed copy,
+	// so the pool itself is the slack — not the archive.
+	const slack = 24 << 20
+
+	t.Logf("peak heap at -jobs %d: 8 MB hak %d bytes, 64 MB hak %d bytes", jobs, small, large)
+	if large > small+slack {
+		t.Fatalf("peak heap scaled with artifact size at -jobs %d: 8 MB hak peaked at %d bytes, 64 MB hak at %d", jobs, small, large)
+	}
+}

internal/nwsync/memory_test.go

@@ -0,0 +1,150 @@
+package nwsync
+
+import (
+	"bytes"
+	"fmt"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"runtime"
+	"runtime/debug"
+	"testing"
+	"time"
+
+	"github.com/klauspost/compress/zstd"
+
+	"git.westgate.pw/ShadowsOverWestgate/sow-tools/internal/erf"
+)
+
+// TestSingleThreadedEncoderMatchesDefault pins the claim the blob encoder's
+// concurrency setting rests on: it saves memory only, and a published blob is
+// the same bytes either way.
+func TestSingleThreadedEncoderMatchesDefault(t *testing.T) {
+	standard, err := zstd.NewWriter(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer standard.Close()
+
+	body := make([]byte, 4<<20)
+	random := rand.New(rand.NewSource(1))
+	random.Read(body[:len(body)/2])
+	for _, size := range []int{0, 1, 4 << 10, len(body)} {
+		if !bytes.Equal(blobEncoder.EncodeAll(body[:size], nil), standard.EncodeAll(body[:size], nil)) {
+			t.Fatalf("%d bytes compress differently at concurrency 1", size)
+		}
+	}
+}
+
+// resourceSize is one payload in the memory fixtures. Real haks hold a few MB
+// per resource, and peak memory is meant to track that, not the archive.
+const resourceSize = 1 << 20
+
+// writeStreamedHak builds a hak of count resources without ever holding the
+// archive in memory, so the fixture itself does not decide the measurement.
+// Payloads are distinct, so no blob is deduplicated away.
+func writeStreamedHak(t *testing.T, path string, count int) {
+	t.Helper()
+	payload := filepath.Join(t.TempDir(), "payload.bin")
+	body := make([]byte, resourceSize)
+	for index := range body {
+		body[index] = byte(index)
+	}
+
+	resources := make([]erf.Resource, 0, count)
+	for index := range count {
+		// A distinct first byte per resource is enough to give every payload
+		// its own sha1 while still streaming from one file per resource.
+		unique := filepath.Join(filepath.Dir(payload), fmt.Sprintf("p%d.bin", index))
+		body[0] = byte(index)
+		body[1] = byte(index >> 8)
+		if err := os.WriteFile(unique, body, 0o644); err != nil {
+			t.Fatal(err)
+		}
+		resources = append(resources, erf.Resource{
+			Name:       fmt.Sprintf("res%05d", index),
+			Type:       restype(t, "tga"),
+			SourcePath: unique,
+			Size:       resourceSize,
+		})
+	}
+
+	file, err := os.Create(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file.Close()
+	if err := erf.Write(file, erf.New("HAK", resources)); err != nil {
+		t.Fatalf("write hak: %v", err)
+	}
+}
+
+// peakHeapDuring runs work while sampling the heap, and returns the largest
+// live heap it saw.
+func peakHeapDuring(work func()) uint64 {
+	runtime.GC()
+	done := make(chan struct{})
+	peak := make(chan uint64, 1)
+	go func() {
+		var highest uint64
+		var stats runtime.MemStats
+		for {
+			select {
+			case <-done:
+				peak <- highest
+				return
+			default:
+			}
+			runtime.ReadMemStats(&stats)
+			if stats.HeapAlloc > highest {
+				highest = stats.HeapAlloc
+			}
+			time.Sleep(time.Millisecond)
+		}
+	}()
+	work()
+	close(done)
+	return <-peak
+}
+
+// TestEmitPeakMemoryDoesNotScaleWithArtifactSize is the regression check for
+// the OOM kills on large haks: emit used to hold the whole archive (twice), so
+// a 2 GB hak needed about 10 GB. Emitting an archive 8× bigger must not cost
+// meaningfully more memory.
+func TestEmitPeakMemoryDoesNotScaleWithArtifactSize(t *testing.T) {
+	if testing.Short() {
+		t.Skip("writes a 64 MB fixture")
+	}
+	// A lazy GC lets garbage pile up in proportion to the live heap, which
+	// hides the thing under test. Collecting eagerly makes the sampled heap
+	// track what emit actually holds.
+	defer debug.SetGCPercent(debug.SetGCPercent(10))
+
+	measure := func(count int) uint64 {
+		dir := t.TempDir()
+		hak := filepath.Join(dir, "big.hak")
+		writeStreamedHak(t, hak, count)
+		// The key is computed outside the measurement: the test helper reads
+		// the whole file to hash it, which emit itself no longer does.
+		options := EmitOptions{
+			ArtifactKey:  artifactKey(t, hak),
+			ArtifactPath: hak,
+			As:           filepath.Base(hak),
+			OutDir:       filepath.Join(dir, "out"),
+		}
+		return peakHeapDuring(func() {
+			if _, err := Emit(options); err != nil {
+				t.Fatalf("emit %d resources: %v", count, err)
+			}
+		})
+	}
+
+	small := measure(8)  // 8 MB
+	large := measure(64) // 64 MB
+	const slack = 24 << 20
+
+	t.Logf("peak heap: 8 MB hak %d bytes, 64 MB hak %d bytes", small, large)
+	if large > small+slack {
+		t.Fatalf("peak heap scaled with artifact size: 8 MB hak peaked at %d bytes, 64 MB hak at %d", small, large)
+	}
+}

internal/nwsync/nwsync_test.go

@@ -3,6 +3,7 @@ package nwsync
 import (
 	"bytes"
 	"crypto/sha1"
+	"crypto/sha256"
 	"encoding/binary"
 	"encoding/hex"
 	"encoding/json"
@@ -45,6 +46,30 @@ func writeHak(t *testing.T, path string, contents map[string][]byte) {
 	}
 }
 
+// artifactKey is the depot key a file would be published under: the sha256 of
+// its bytes, hash-tree depth 2, keeping the extension.
+func artifactKey(t *testing.T, path string) string {
+	t.Helper()
+	body, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sum := sha256.Sum256(body)
+	digest := hex.EncodeToString(sum[:])
+	return "artifacts/haks/sha256/" + digest[0:2] + "/" + digest[2:4] + "/" + digest + filepath.Ext(path)
+}
+
+// emitLocal emits one artifact into a local tree, the conformance path.
+func emitLocal(t *testing.T, path, out string) (EmitResult, error) {
+	t.Helper()
+	return Emit(EmitOptions{
+		ArtifactKey:  artifactKey(t, path),
+		ArtifactPath: path,
+		As:           filepath.Base(path),
+		OutDir:       out,
+	})
+}
+
 func TestBlobFramingRoundTrips(t *testing.T) {
 	data := []byte("the quick brown fox jumps over the lazy dog, repeatedly and at length")
 	blob := compressBlob(data)
@@ -143,7 +168,7 @@ func TestEmitWritesBlobsAndManifest(t *testing.T) {
 	})
 
 	out := filepath.Join(dir, "out")
-	result, err := Emit(hak, out)
+	result, err := emitLocal(t, hak, out)
 	if err != nil {
 		t.Fatalf("emit: %v", err)
 	}
@@ -171,7 +196,7 @@ func TestEmitWritesBlobsAndManifest(t *testing.T) {
 		t.Errorf("blob decompressed to %q, want %q", got, body)
 	}
 
-	entries := readEmitted(t, out, "sow_test_01")
+	entries := readEmitted(t, result.ManifestPath)
 	for _, entry := range entries {
 		if entry.ResType == restype(t, "nss") || entry.ResType == restype(t, "ndb") || entry.ResType == restype(t, "gic") {
 			t.Errorf("skipped restype leaked into the manifest: %+v", entry)
@@ -200,9 +225,9 @@ func TestEmitWritesBlobsAndManifest(t *testing.T) {
 	}
 }
 
-func readEmitted(t *testing.T, dir, name string) []Entry {
+func readEmitted(t *testing.T, indexPath string) []Entry {
 	t.Helper()
-	data, err := os.ReadFile(filepath.Join(dir, name+".nsym"))
+	data, err := os.ReadFile(indexPath)
 	if err != nil {
 		t.Fatalf("read emitted manifest: %v", err)
 	}
@@ -218,7 +243,7 @@ func TestEmitFailsClosedOnOversizeResource(t *testing.T) {
 	hak := filepath.Join(dir, "big.hak")
 	writeHak(t, hak, map[string][]byte{"huge1.tga": make([]byte, fileSizeLimit+1)})
 
-	if _, err := Emit(hak, filepath.Join(dir, "out")); err == nil {
+	if _, err := emitLocal(t, hak, filepath.Join(dir, "out")); err == nil {
 		t.Fatal("emit accepted a resource over the 15 MB limit")
 	}
 }
@@ -230,11 +255,11 @@ func TestEmitLooseFile(t *testing.T) {
 		t.Fatal(err)
 	}
 	out := filepath.Join(dir, "out")
-	result, err := Emit(tlk, out)
+	result, err := emitLocal(t, tlk, out)
 	if err != nil {
 		t.Fatalf("emit tlk: %v", err)
 	}
-	entries := readEmitted(t, out, "sow_tlk")
+	entries := readEmitted(t, result.ManifestPath)
 	if len(entries) != 1 || entries[0].ResRef != "sow_tlk" || entries[0].ResType != restype(t, "tlk") {
 		t.Fatalf("tlk emitted as %+v", entries)
 	}
@@ -245,33 +270,34 @@ func TestEmitLooseFile(t *testing.T) {
 
 // emitFixture emits two haks that share a resref, so the merge rule is
 // observable: "top" holds the winning body, "assets" the shadowed one.
-func emitFixture(t *testing.T) (string, []byte, []byte) {
+func emitFixture(t *testing.T) (out string, keys map[string]string, topBody, assetBody []byte) {
 	t.Helper()
 	dir := t.TempDir()
-	topBody := []byte("2da from sow_top")
-	assetBody := []byte("2da from the asset hak")
+	topBody = []byte("2da from sow_top")
+	assetBody = []byte("2da from the asset hak")
 	writeHak(t, filepath.Join(dir, "sow_top.hak"), map[string][]byte{"appearance.2da": topBody})
 	writeHak(t, filepath.Join(dir, "sow_core_01.hak"), map[string][]byte{
 		"appearance.2da":  assetBody,
 		"bloodstain1.tga": []byte("blood"),
 	})
-	out := filepath.Join(dir, "out")
+	out = filepath.Join(dir, "out")
+	keys = map[string]string{}
 	for _, name := range []string{"sow_top", "sow_core_01"} {
-		if _, err := Emit(filepath.Join(dir, name+".hak"), out); err != nil {
+		path := filepath.Join(dir, name+".hak")
+		keys[name] = artifactKey(t, path)
+		if _, err := emitLocal(t, path, out); err != nil {
 			t.Fatalf("emit %s: %v", name, err)
 		}
 	}
-	return out, topBody, assetBody
+	return out, keys, topBody, assetBody
 }
 
 func TestAssembleShadowsByOrder(t *testing.T) {
-	entriesDir, topBody, assetBody := emitFixture(t)
-	out := t.TempDir()
+	entriesDir, keys, topBody, assetBody := emitFixture(t)
 
 	result, err := Assemble(AssembleOptions{
-		Order:      []string{"sow_top", "sow_core_01"},
-		EntriesDir: entriesDir,
-		OutDir:     out,
+		ArtifactKeys: []string{keys["sow_top"], keys["sow_core_01"]},
+		OutDir:       entriesDir,
 		GroupID:      2,
 	})
 	if err != nil {
@@ -325,13 +351,11 @@ func TestAssembleShadowsByOrder(t *testing.T) {
 }
 
 func TestAssembleReversedOrderPicksTheOtherHak(t *testing.T) {
-	entriesDir, topBody, assetBody := emitFixture(t)
-	out := t.TempDir()
+	entriesDir, keys, topBody, assetBody := emitFixture(t)
 
 	result, err := Assemble(AssembleOptions{
-		Order:      []string{"sow_core_01", "sow_top"},
-		EntriesDir: entriesDir,
-		OutDir:     out,
+		ArtifactKeys: []string{keys["sow_core_01"], keys["sow_top"]},
+		OutDir:       entriesDir,
 	})
 	if err != nil {
 		t.Fatalf("assemble: %v", err)
@@ -350,9 +374,13 @@ func TestAssembleReversedOrderPicksTheOtherHak(t *testing.T) {
 }
 
 func TestAssembleRefusesMismatchedEmitterVersions(t *testing.T) {
-	entriesDir, _, _ := emitFixture(t)
+	entriesDir, keys, _, _ := emitFixture(t)
 
-	path := filepath.Join(entriesDir, "sow_core_01.nsym.json")
+	index, err := resolveIndexKey(keys["sow_core_01"], entriesDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := filepath.Join(entriesDir, index+".json")
 	var sidecar Sidecar
 	body, err := os.ReadFile(path)
 	if err != nil {
@@ -371,9 +399,8 @@ func TestAssembleRefusesMismatchedEmitterVersions(t *testing.T) {
 	}
 
 	_, err = Assemble(AssembleOptions{
-		Order:      []string{"sow_top", "sow_core_01"},
-		EntriesDir: entriesDir,
-		OutDir:     t.TempDir(),
+		ArtifactKeys: []string{keys["sow_top"], keys["sow_core_01"]},
+		OutDir:       entriesDir,
 	})
 	if err == nil || !strings.Contains(err.Error(), "emitter version mismatch") {
 		t.Fatalf("assemble merged across emitter versions: %v", err)
@@ -385,7 +412,7 @@ func TestEmitHonoursSourceDateEpoch(t *testing.T) {
 	dir := t.TempDir()
 	hak := filepath.Join(dir, "pinned.hak")
 	writeHak(t, hak, map[string][]byte{"one1.tga": []byte("body")})
-	result, err := Emit(hak, filepath.Join(dir, "out"))
+	result, err := emitLocal(t, hak, filepath.Join(dir, "out"))
 	if err != nil {
 		t.Fatalf("emit: %v", err)
 	}
@@ -414,19 +441,19 @@ func TestEmitRejectsAModule(t *testing.T) {
 	if err := os.WriteFile(path, out.Bytes(), 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := Emit(path, filepath.Join(dir, "out")); err == nil {
+	if _, err := emitLocal(t, path, filepath.Join(dir, "out")); err == nil {
 		t.Fatal("emit accepted a .mod; a manifest never carries module contents")
 	}
 }
 
 func TestAssembleFailsClosedOnMissingIndex(t *testing.T) {
-	entriesDir, _, _ := emitFixture(t)
+	entriesDir, keys, _, _ := emitFixture(t)
+	missing := "artifacts/haks/sha256/00/11/" + strings.Repeat("0", 64) + ".hak"
 	_, err := Assemble(AssembleOptions{
-		Order:      []string{"sow_top", "sow_never_published"},
-		EntriesDir: entriesDir,
-		OutDir:     t.TempDir(),
+		ArtifactKeys: []string{keys["sow_top"], missing},
+		OutDir:       entriesDir,
 	})
-	if err == nil || !strings.Contains(err.Error(), "sow_never_published") {
+	if err == nil || !strings.Contains(err.Error(), missing) {
 		t.Fatalf("assemble did not fail closed and name the missing artifact: %v", err)
 	}
 }
@@ -436,10 +463,9 @@ func TestRunUsageErrors(t *testing.T) {
 		nil,
 		{"nope"},
 		{"emit"},
-		{"emit", "artifact.hak"},
-		{"assemble", "--entries", "x", "--out", "y"},
-		{"assemble", "--order", "a", "--out", "y"},
-		{"assemble", "--order", "a", "--entries", "x"},
+		{"emit", "artifact-key.hak"},
+		{"emit", "a", "b", "c"},
+		{"assemble", "--out", "y"},
 	}
 	for _, args := range cases {
 		var out, errw bytes.Buffer

internal/nwsync/run.go

@@ -12,7 +12,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"strings"
 )
 
 const (
@@ -45,37 +44,69 @@ func Run(args []string, stdout, stderr io.Writer) int {
 
 func printRunUsage(w io.Writer) {
 	fmt.Fprint(w, `usage:
-  nwsync emit     <artifact> --out DIR
-  nwsync assemble --order NAMES --entries DIR --out DIR [--group-id N]
+  nwsync emit     [--as NAME] [--out DIR] <artifact-key> <file>
+  nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
 
 emit explodes one .hak/.erf or one loose file (the TLK) into NWSync blobs plus
-a NSYM manifest covering only that artifact. assemble merges those per-artifact
-manifests into one, reading no bulk data.
+a NSYM index covering only that artifact, and uploads both. assemble merges
+those indexes into one manifest, reading no bulk data. Artifact keys are depot
+keys; an index lives beside its artifact, with the extension replaced.
+
+--out DIR writes to a local repository tree instead of uploading, which is the
+conformance path against upstream nwn_nwsync_write. Without it, the zone comes
+from NWSYNC_STORAGE_ZONE, NWSYNC_STORAGE_PASSWORD and BUNNY_STORAGE_HOST.
 `)
 }
 
+// parseArgs parses flags that may appear before, after or between positionals.
+// Go's flag package stops at the first non-flag argument, which turns
+// `emit <key> <file> --out DIR` into a confusing arity error.
+func parseArgs(fs *flag.FlagSet, args []string) ([]string, error) {
+	var positional []string
+	for {
+		if err := fs.Parse(args); err != nil {
+			return nil, err
+		}
+		rest := fs.Args()
+		if len(rest) == 0 {
+			return positional, nil
+		}
+		positional = append(positional, rest[0])
+		args = rest[1:]
+	}
+}
+
 func runEmit(args []string, stdout, stderr io.Writer) int {
 	fs := flag.NewFlagSet("emit", flag.ContinueOnError)
 	fs.SetOutput(stderr)
-	out := fs.String("out", "", "output directory (blobs plus the per-artifact manifest)")
-	if err := fs.Parse(args); err != nil {
+	as := fs.String("as", "", "published name of the artifact, when it differs from the key")
+	out := fs.String("out", "", "write to a local repository tree instead of uploading")
+	jobs := fs.Int("jobs", defaultEmitJobs, "resources to hash, compress and store at once")
+	positional, err := parseArgs(fs, args)
+	if err != nil {
 		return exitUsage
 	}
-	if fs.NArg() != 1 {
-		fmt.Fprintf(stderr, "nwsync emit: exactly one artifact is required\n")
+	if len(positional) != 2 {
+		fmt.Fprintf(stderr, "nwsync emit: <artifact-key> and <file> are both required\n")
 		return exitUsage
 	}
-	if *out == "" {
-		fmt.Fprintf(stderr, "nwsync emit: --out is required\n")
+	if *jobs < 1 {
+		fmt.Fprintf(stderr, "nwsync emit: -jobs must be at least 1, got %d\n", *jobs)
 		return exitUsage
 	}
 
-	result, err := Emit(fs.Arg(0), *out)
+	result, err := Emit(EmitOptions{
+		ArtifactKey:  positional[0],
+		ArtifactPath: positional[1],
+		As:           *as,
+		OutDir:       *out,
+		Jobs:         *jobs,
+	})
 	if err != nil {
 		fmt.Fprintf(stderr, "nwsync emit: %v\n", err)
 		return exitInternal
 	}
-	fmt.Fprintf(stdout, "emitted %s: %d resources, %d new blobs, manifest %s\n",
+	fmt.Fprintf(stdout, "emitted %s: %d resources, %d new blobs, index %s\n",
 		result.Name, result.Entries, result.BlobsWritten, result.ManifestPath)
 	return exitOK
 }
@@ -83,37 +114,23 @@ func runEmit(args []string, stdout, stderr io.Writer) int {
 func runAssemble(args []string, stdout, stderr io.Writer) int {
 	fs := flag.NewFlagSet("assemble", flag.ContinueOnError)
 	fs.SetOutput(stderr)
-	order := fs.String("order", "", "comma-separated artifact names, highest priority first")
-	entries := fs.String("entries", "", "directory holding the per-artifact .nsym files")
-	out := fs.String("out", "", "repository root; the manifest lands in <out>/manifests")
+	tlkKey := fs.String("tlk-key", "", "depot key of the TLK, which shadows nothing and merges last")
+	out := fs.String("out", "", "write to a local repository tree instead of uploading")
 	groupID := fs.Int("group-id", 0, "NWSync group id (1 = current, 2 = testing; 0 omits it)")
 	moduleName := fs.String("module-name", "", "module name recorded in the sidecar")
 	description := fs.String("description", "", "description recorded in the sidecar")
-	if err := fs.Parse(args); err != nil {
+	positional, err := parseArgs(fs, args)
+	if err != nil {
 		return exitUsage
 	}
-	switch {
-	case *order == "":
-		fmt.Fprintf(stderr, "nwsync assemble: --order is required\n")
+	if len(positional) == 0 {
+		fmt.Fprintf(stderr, "nwsync assemble: at least one artifact key is required\n")
 		return exitUsage
-	case *entries == "":
-		fmt.Fprintf(stderr, "nwsync assemble: --entries is required\n")
-		return exitUsage
-	case *out == "":
-		fmt.Fprintf(stderr, "nwsync assemble: --out is required\n")
-		return exitUsage
-	}
-
-	names := make([]string, 0, 8)
-	for _, name := range strings.Split(*order, ",") {
-		if name = strings.TrimSpace(name); name != "" {
-			names = append(names, name)
-		}
 	}
 
 	result, err := Assemble(AssembleOptions{
-		Order:       names,
-		EntriesDir:  *entries,
+		ArtifactKeys: positional,
+		TLKKey:       *tlkKey,
 		OutDir:       *out,
 		GroupID:      *groupID,
 		ModuleName:   *moduleName,
@@ -123,7 +140,7 @@ func runAssemble(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintf(stderr, "nwsync assemble: %v\n", err)
 		return exitInternal
 	}
-	fmt.Fprintf(stdout, "assembled %s: %d resources from %d artifacts\n",
-		result.SHA1, result.Entries, len(names))
+	fmt.Fprintf(stdout, "assembled manifest %s: %d resources, %s\n",
+		result.SHA1, result.Entries, result.ManifestPath)
 	return exitOK
 }

internal/nwsync/sink.go

@@ -0,0 +1,212 @@
+package nwsync
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"git.westgate.pw/ShadowsOverWestgate/sow-tools/internal/depot"
+)
+
+// sink is where an emit or assemble run puts what it produces. The zone is the
+// production sink; a local directory exists only as the conformance path, so
+// upstream's output and ours can be diffed on a developer machine.
+type sink interface {
+	// putBlob stores one NWCompressedBuffer blob under its sha1 name and
+	// returns the bytes stored, or 0 if the blob was already there. Blob names
+	// are content hashes, so an existing name is existing content — which is
+	// why body is a thunk: compression is the expensive part of emit and a
+	// blob that is already stored must not pay for it.
+	putBlob(sha1Hex string, body func() []byte) (int64, error)
+	// putIndex stores a NSYM manifest and its sidecar under key, which is
+	// either an artifact-derived object key or a local path.
+	putIndex(key string, manifest, sidecar []byte) error
+	// getIndex reads back a NSYM manifest and its sidecar.
+	getIndex(key string) (manifest, sidecar []byte, err error)
+	// describe names the sink for messages.
+	describe(key string) string
+}
+
+// dirSink writes a local NWSync repository tree.
+type dirSink struct{ root string }
+
+func (s dirSink) putBlob(sha1Hex string, body func() []byte) (int64, error) {
+	blob := blobPath(s.root, sha1Hex)
+	if _, err := os.Stat(blob); err == nil {
+		return 0, nil
+	}
+	if err := os.MkdirAll(filepath.Dir(blob), 0o755); err != nil {
+		return 0, fmt.Errorf("create blob directory: %w", err)
+	}
+	data := body()
+	if err := os.WriteFile(blob, data, 0o644); err != nil {
+		return 0, fmt.Errorf("write blob: %w", err)
+	}
+	return int64(len(data)), nil
+}
+
+func (s dirSink) putIndex(key string, manifest, sidecar []byte) error {
+	target := filepath.Join(s.root, filepath.FromSlash(key))
+	if err := os.MkdirAll(filepath.Dir(target), 0o755); err != nil {
+		return fmt.Errorf("create manifest directory: %w", err)
+	}
+	if err := os.WriteFile(target, manifest, 0o644); err != nil {
+		return fmt.Errorf("write manifest: %w", err)
+	}
+	if err := os.WriteFile(target+".json", sidecar, 0o644); err != nil {
+		return fmt.Errorf("write sidecar: %w", err)
+	}
+	return nil
+}
+
+func (s dirSink) getIndex(key string) ([]byte, []byte, error) {
+	target := filepath.Join(s.root, filepath.FromSlash(key))
+	manifest, err := os.ReadFile(target)
+	if err != nil {
+		return nil, nil, fmt.Errorf("read index: %w", err)
+	}
+	sidecar, err := os.ReadFile(target + ".json")
+	if err != nil {
+		return nil, nil, fmt.Errorf("read sidecar: %w", err)
+	}
+	return manifest, sidecar, nil
+}
+
+func (s dirSink) describe(key string) string {
+	return filepath.Join(s.root, filepath.FromSlash(key))
+}
+
+// zoneSink uploads straight to the NWSync storage zone. Nothing bulky is ever
+// written to the runner's disk: the working set is one resource at a time.
+type zoneSink struct {
+	store depot.KeyStore
+	ctx   context.Context
+	zone  string
+}
+
+func (s zoneSink) putBlob(sha1Hex string, body func() []byte) (int64, error) {
+	key := path.Join("data", "sha1", sha1Hex[0:2], sha1Hex[2:4], sha1Hex)
+	// A throttled probe must never be read as "missing, re-upload" or as
+	// "present, skip", so only a confirmed Present skips the upload.
+	state, _, err := s.store.ProbeKey(s.ctx, key)
+	if err != nil {
+		return 0, fmt.Errorf("probe blob %s: %w", sha1Hex, err)
+	}
+	if state == depot.Present {
+		return 0, nil
+	}
+	data := body()
+	if err := s.put(key, data); err != nil {
+		return 0, fmt.Errorf("upload blob %s: %w", sha1Hex, err)
+	}
+	return int64(len(data)), nil
+}
+
+func (s zoneSink) putIndex(key string, manifest, sidecar []byte) error {
+	// The manifest lands last: its presence is the publication marker, so it
+	// must never appear before the blobs it names.
+	if err := s.put(key+".json", sidecar); err != nil {
+		return fmt.Errorf("upload sidecar %s: %w", key, err)
+	}
+	if err := s.put(key, manifest); err != nil {
+		return fmt.Errorf("upload index %s: %w", key, err)
+	}
+	return nil
+}
+
+func (s zoneSink) getIndex(key string) ([]byte, []byte, error) {
+	manifest, err := s.store.GetKey(s.ctx, key)
+	if err != nil {
+		return nil, nil, fmt.Errorf("read index: %w", err)
+	}
+	sidecar, err := s.store.GetKey(s.ctx, key+".json")
+	if err != nil {
+		return nil, nil, fmt.Errorf("read sidecar: %w", err)
+	}
+	return manifest, sidecar, nil
+}
+
+func (s zoneSink) describe(key string) string { return s.zone + "/" + key }
+
+func (s zoneSink) put(key string, body []byte) error {
+	sum := sha256.Sum256(body)
+	return s.store.PutReader(s.ctx, key, bytes.NewReader(body), int64(len(body)), hex.EncodeToString(sum[:]))
+}
+
+// newZoneSink builds the upload sink from the environment. NWSync data lives
+// in its own zone, separate from the asset depot, so it has its own zone and
+// credential; only the host is shared, and Crucible has no default host.
+func newZoneSink(ctx context.Context, getenv func(string) string) (sink, error) {
+	cfg := depot.LoadConfig(getenv)
+	cfg.StorageZone = getenv("NWSYNC_STORAGE_ZONE")
+	cfg.WriteKey = getenv("NWSYNC_STORAGE_PASSWORD")
+	cfg.ReadKey = cfg.WriteKey
+	if cfg.StorageZone == "" {
+		return nil, fmt.Errorf("NWSYNC_STORAGE_ZONE is unset (or pass --out DIR to write locally)")
+	}
+	if cfg.WriteKey == "" {
+		return nil, fmt.Errorf("NWSYNC_STORAGE_PASSWORD is unset (or pass --out DIR to write locally)")
+	}
+	if cfg.StorageHost == "" {
+		return nil, fmt.Errorf("BUNNY_STORAGE_HOST is unset")
+	}
+	store, err := depot.NewKeyStore(cfg)
+	if err != nil {
+		return nil, err
+	}
+	return zoneSink{store: store, ctx: ctx, zone: cfg.StorageZone}, nil
+}
+
+// indexKey is where an artifact's NSYM lives: beside the artifact itself, with
+// the final extension replaced. emit and assemble must agree on this one rule,
+// so it lives here and nowhere else.
+//
+// artifacts/haks/sha256/30/46/3046….hak -> artifacts/haks/sha256/30/46/3046….nsym
+func indexKey(artifactKey string) (string, error) {
+	extension := path.Ext(artifactKey)
+	if extension == "" {
+		return "", fmt.Errorf("artifact key %q has no extension", artifactKey)
+	}
+	return strings.TrimSuffix(artifactKey, extension) + ".nsym", nil
+}
+
+// resolveIndexKey is where emit writes an artifact's index and where assemble
+// reads it from. On the zone that is beside the artifact; locally the indexes
+// sit flat beside the data tree, so upstream's output and ours diff directly.
+func resolveIndexKey(artifactKey, outDir string) (string, error) {
+	key, err := indexKey(artifactKey)
+	if err != nil {
+		return "", err
+	}
+	if outDir != "" {
+		return path.Base(key), nil
+	}
+	return key, nil
+}
+
+// checkArtifactKey fails closed when the key's embedded digest is not the
+// digest of the bytes being emitted. Publishing an index under the wrong key
+// silently pairs a manifest with the wrong artifact.
+// artifact is hashed by streaming, so a multi-gigabyte hak is never resident.
+func checkArtifactKey(artifactKey string, artifact io.Reader) error {
+	base := path.Base(artifactKey)
+	digest := strings.TrimSuffix(base, path.Ext(base))
+	if len(digest) != 64 {
+		return fmt.Errorf("artifact key %q does not name a sha256", artifactKey)
+	}
+	hash := sha256.New()
+	if _, err := io.Copy(hash, artifact); err != nil {
+		return fmt.Errorf("hash artifact: %w", err)
+	}
+	if got := hex.EncodeToString(hash.Sum(nil)); got != digest {
+		return fmt.Errorf("artifact key %q names digest %s but the file hashes to %s", artifactKey, digest, got)
+	}
+	return nil
+}

internal/nwsync/zone_test.go

@@ -0,0 +1,249 @@
+package nwsync
+
+import (
+	"crypto/sha1"
+	"crypto/sha256"
+	"encoding/hex"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// fakeZone is a Bunny-shaped object store: PUT stores, GET reads, and the
+// Checksum header is verified the way Bunny verifies it.
+type fakeZone struct {
+	mu      sync.Mutex
+	objects map[string][]byte
+	puts    []string
+	failOn  func(key string) bool // when true, the PUT fails
+}
+
+func newFakeZone(t *testing.T) (*fakeZone, func(string) string) {
+	t.Helper()
+	zone := &fakeZone{objects: map[string][]byte{}}
+	server := httptest.NewServer(zone)
+	t.Cleanup(server.Close)
+	getenv := func(name string) string {
+		switch name {
+		case "NWSYNC_STORAGE_ZONE":
+			return "sow-nwsync"
+		case "NWSYNC_STORAGE_PASSWORD":
+			return "write-key"
+		case "BUNNY_STORAGE_HOST":
+			return server.URL
+		}
+		return ""
+	}
+	return zone, getenv
+}
+
+func (z *fakeZone) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	key := strings.TrimPrefix(r.URL.Path, "/sow-nwsync/")
+	switch r.Method {
+	case http.MethodPut:
+		if z.failOn != nil && z.failOn(key) {
+			http.Error(w, "boom", http.StatusInternalServerError)
+			return
+		}
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		sum := sha256.Sum256(body)
+		if want := strings.ToUpper(hex.EncodeToString(sum[:])); r.Header.Get("Checksum") != want {
+			http.Error(w, "checksum mismatch", http.StatusBadRequest)
+			return
+		}
+		z.mu.Lock()
+		z.objects[key] = body
+		z.puts = append(z.puts, key)
+		z.mu.Unlock()
+		w.WriteHeader(http.StatusCreated)
+	case http.MethodGet:
+		z.mu.Lock()
+		body, ok := z.objects[key]
+		z.mu.Unlock()
+		if !ok {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+		_, _ = w.Write(body)
+	default:
+		http.Error(w, "unsupported", http.StatusMethodNotAllowed)
+	}
+}
+
+func (z *zoneSinkFixture) emit(t *testing.T, path string) EmitResult {
+	t.Helper()
+	result, err := Emit(EmitOptions{
+		ArtifactKey:  artifactKey(t, path),
+		ArtifactPath: path,
+		Sink:         z.sink,
+	})
+	if err != nil {
+		t.Fatalf("emit %s: %v", path, err)
+	}
+	return result
+}
+
+type zoneSinkFixture struct {
+	zone *fakeZone
+	sink sink
+}
+
+func newZoneFixture(t *testing.T) *zoneSinkFixture {
+	t.Helper()
+	zone, getenv := newFakeZone(t)
+	target, err := newZoneSink(t.Context(), getenv)
+	if err != nil {
+		t.Fatalf("zone sink: %v", err)
+	}
+	return &zoneSinkFixture{zone: zone, sink: target}
+}
+
+func sha1Of(body []byte) [20]byte { return sha1.Sum(body) }
+
+func TestEmitUploadsBlobsThenIndex(t *testing.T) {
+	fixture := newZoneFixture(t)
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	body := []byte("texture bytes")
+	writeHak(t, hak, map[string][]byte{"bloodstain1.tga": body, "copy1.txi": body})
+
+	key := artifactKey(t, hak)
+	result := fixture.emit(t, hak)
+
+	index, err := indexKey(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := fixture.zone.objects[index]; !ok {
+		t.Fatalf("no index at %s; zone holds %v", index, fixture.zone.puts)
+	}
+	if result.BlobsWritten != 1 {
+		t.Errorf("uploaded %d blobs, want 1 (identical content shares a blob)", result.BlobsWritten)
+	}
+
+	// The index is the publication marker, so it must land after every blob it
+	// names — including its own sidecar.
+	last := fixture.zone.puts[len(fixture.zone.puts)-1]
+	if last != index {
+		t.Errorf("index landed at position %d of %d; it must be last", len(fixture.zone.puts), len(fixture.zone.puts))
+	}
+	for _, key := range fixture.zone.puts[:len(fixture.zone.puts)-1] {
+		if strings.HasPrefix(key, "data/sha1/") || key == index+".json" {
+			continue
+		}
+		t.Errorf("unexpected object uploaded before the index: %s", key)
+	}
+}
+
+func TestEmitSkipsBlobsAlreadyInTheZone(t *testing.T) {
+	fixture := newZoneFixture(t)
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	writeHak(t, hak, map[string][]byte{"bloodstain1.tga": []byte("blood")})
+
+	first := fixture.emit(t, hak)
+	if first.BlobsWritten != 1 {
+		t.Fatalf("first emit uploaded %d blobs, want 1", first.BlobsWritten)
+	}
+	second := fixture.emit(t, hak)
+	if second.BlobsWritten != 0 {
+		t.Errorf("re-emit uploaded %d blobs, want 0 (a blob name is its content)", second.BlobsWritten)
+	}
+}
+
+func TestEmitLeavesNoIndexWhenAnUploadFails(t *testing.T) {
+	fixture := newZoneFixture(t)
+	fixture.zone.failOn = func(key string) bool { return strings.HasPrefix(key, "data/sha1/") }
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	writeHak(t, hak, map[string][]byte{"bloodstain1.tga": []byte("blood")})
+
+	_, err := Emit(EmitOptions{
+		ArtifactKey:  artifactKey(t, hak),
+		ArtifactPath: hak,
+		Sink:         fixture.sink,
+	})
+	if err == nil {
+		t.Fatal("emit reported success after an upload failed")
+	}
+	for key := range fixture.zone.objects {
+		if strings.HasSuffix(key, ".nsym") {
+			t.Errorf("a half-emitted artifact published an index: %s", key)
+		}
+	}
+}
+
+func TestEmitRejectsAKeyThatDoesNotMatchTheFile(t *testing.T) {
+	fixture := newZoneFixture(t)
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	writeHak(t, hak, map[string][]byte{"bloodstain1.tga": []byte("blood")})
+
+	wrong := "artifacts/haks/sha256/00/11/" + strings.Repeat("0", 64) + ".hak"
+	_, err := Emit(EmitOptions{ArtifactKey: wrong, ArtifactPath: hak, Sink: fixture.sink})
+	if err == nil || !strings.Contains(err.Error(), "hashes to") {
+		t.Fatalf("emit published under a key that names another artifact: %v", err)
+	}
+}
+
+func TestAssembleReadsIndexesFromTheZone(t *testing.T) {
+	fixture := newZoneFixture(t)
+	dir := t.TempDir()
+	topBody := []byte("2da from sow_top")
+	assetBody := []byte("2da from the asset hak")
+	top := filepath.Join(dir, "sow_top.hak")
+	core := filepath.Join(dir, "sow_core_01.hak")
+	writeHak(t, top, map[string][]byte{"appearance.2da": topBody})
+	writeHak(t, core, map[string][]byte{"appearance.2da": assetBody, "bloodstain1.tga": []byte("blood")})
+	tlkPath := filepath.Join(dir, "sow_tlk.tlk")
+	if err := os.WriteFile(tlkPath, []byte("TLK V3.0 payload"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	fixture.emit(t, top)
+	fixture.emit(t, core)
+	if _, err := Emit(EmitOptions{
+		ArtifactKey:  artifactKey(t, tlkPath),
+		ArtifactPath: tlkPath,
+		As:           "sow_tlk.tlk",
+		Sink:         fixture.sink,
+	}); err != nil {
+		t.Fatalf("emit tlk: %v", err)
+	}
+
+	result, err := Assemble(AssembleOptions{
+		ArtifactKeys: []string{artifactKey(t, top), artifactKey(t, core)},
+		TLKKey:       artifactKey(t, tlkPath),
+		GroupID:      2,
+		Sink:         fixture.sink,
+	})
+	if err != nil {
+		t.Fatalf("assemble: %v", err)
+	}
+	if result.Entries != 3 {
+		t.Fatalf("merged %d entries, want 3 (appearance.2da is shadowed, the TLK adds one)", result.Entries)
+	}
+
+	manifest, ok := fixture.zone.objects["manifests/"+result.SHA1]
+	if !ok {
+		t.Fatalf("no merged manifest in the zone; it holds %v", fixture.zone.puts)
+	}
+	entries, err := readManifest(manifest)
+	if err != nil {
+		t.Fatalf("parse merged manifest: %v", err)
+	}
+	for _, entry := range entries {
+		if entry.ResRef == "appearance" && entry.SHA1 != sha1Of(topBody) {
+			t.Errorf("appearance.2da resolved to the shadowed hak, not the first one given")
+		}
+	}
+}