fix(nwsync): stream emit so peak memory tracks the largest resource (#76) (#78)

Closes #76. Part of #54.

`nwsync emit` held roughly 5× the artifact size in RAM, so ovh-main (7 GB, no swap) OOM-killed it on any hak over ~1.4 GB. That blocked the backfill in sow-assets-manifest and would have killed the next release rebuilding a large hak.

## What it does

- `erf.ReadIndex` / `erf.ReadPayload` — parse the header and resource table only, read one payload on demand. `erf.Read` keeps its shape but returns payloads as subslices of the buffer instead of fresh copies, which removes one full copy for the `pipeline` callers too. Callers must not mutate `Resource.Data`; the doc comment says so and no caller does.
- `nwsync.Emit` opens the artifact, hashes it by streaming for the key check (through a section reader, so the file offset stays put), then hashes, compresses and stores one resource at a time. The archive is never resident. Shadowed duplicate resrefs are now never read at all.
- Bounds checks in `ReadIndex` moved to `int64`, so key/resource-list offsets can no longer overflow.

## Not in the issue, but memory-motivated

The zstd blob encoder ran at the default concurrency, which is one encoder per CPU, each holding a window-sized history — about 200 MB of live heap doing nothing on a 24-core runner. `EncodeAll` is single-threaded per call, so concurrency 1 costs nothing. `TestSingleThreadedEncoderMatchesDefault` pins the claim that blobs come out byte-identical.

## Measured

Peak heap during emit, sampled 1 ms:

| hak | before | after |
|-----|--------|-------|
| 8 MB | 155 MB | 21 MB |
| 64 MB | 289 MB | 22 MB |

Flat, as the acceptance asks. `TestEmitPeakMemoryDoesNotScaleWithArtifactSize` fails if the 64 MB fixture costs more than the 8 MB one plus 24 MB of slack.

## Gaps

- The regression check measures Go heap, not RSS, and its largest fixture is 64 MB — a multi-GB run was not done here. A 2.15 GB hak now needs about the same ~22 MB the 64 MB one does, so the 5 GB budget is not close, but that is inference from the flat curve, not a measurement.
- mmap was suggested in the issue and skipped. Payload buffers are still anonymous, but they are one resource each (≤15 MiB), so making them file-backed buys nothing now.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #78
Reviewed-by: xtul <mpiasecki720@protonmail.com>
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>

docs/command-surface.md

@@ -70,6 +70,35 @@ 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
 
 Existing scripts may continue using these names indefinitely. They are accepted

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 {
+	index, err := ReadIndex(bytes.NewReader(data), int64(len(data)))
-		return Archive{}, fmt.Errorf("erf file too small: %d bytes", len(data))
+	if err != nil {
+		return Archive{}, err
 	}
 
-	var hdr header
+	resources := make([]Resource, 0, len(index.Entries))
-	if err := binary.Read(bytes.NewReader(data[:headerSize]), binary.LittleEndian, &hdr); err != nil {
+	for _, entry := range index.Entries {
-		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 = append(resources, Resource{
-			Name: resref,
+			Name: entry.Name,
-			Type: key.ResourceType,
+			Type: entry.Type,
-			Data: payload,
+			Data: data[entry.Offset : entry.Offset+entry.Size],
-			Size: int64(entry.Size),
+			Size: entry.Size,
 		})
 	}
 
 	return Archive{
-		FileType:  string(hdr.FileType[:]),
+		FileType:  index.FileType,
-		Version:   string(hdr.Version[:]),
+		Version:   index.Version,
 		Resources: resources,
 	}, 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)
+	blobEncoder, _ = zstd.NewWriter(nil, zstd.WithEncoderConcurrency(1))
-	blobDecoder, _ = zstd.NewReader(nil)
+	blobDecoder, _ = zstd.NewReader(nil, zstd.WithDecoderConcurrency(1))
 )
 
 // compressBlob wraps data in NWCompressedBuffer framing.

internal/nwsync/emit.go

@@ -1,10 +1,10 @@
 package nwsync
 
 import (
-	"bytes"
 	"context"
 	"crypto/sha1"
 	"fmt"
+	"io"
 	"os"
 	"path"
 	"path/filepath"
@@ -79,11 +79,18 @@ type EmitOptions struct {
 // 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.ReadFile(options.ArtifactPath)
+	artifact, err := os.Open(options.ArtifactPath)
 	if err != nil {
 		return EmitResult{}, fmt.Errorf("read artifact: %w", err)
 	}
-	if err := checkArtifactKey(options.ArtifactKey, artifact); err != nil {
+	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
@@ -98,7 +105,7 @@ func Emit(options EmitOptions) (EmitResult, error) {
 		return EmitResult{}, err
 	}
 
-	resources, err := readArtifact(options.ArtifactPath, artifact, name)
+	index, err := readArtifactIndex(options.ArtifactPath, artifact, info.Size(), name)
 	if err != nil {
 		return EmitResult{}, err
 	}
@@ -108,7 +115,7 @@ func Emit(options EmitOptions) (EmitResult, error) {
 		return EmitResult{}, err
 	}
 
-	entries, blobs, onDiskBytes, err := emitResources(resources, target)
+	entries, blobs, onDiskBytes, err := emitResources(artifact, index, target)
 	if err != nil {
 		return EmitResult{}, err
 	}
@@ -137,60 +144,64 @@ func openSink(outDir string, injected sink) (sink, error) {
 	return newZoneSink(context.Background(), os.Getenv)
 }
 
-// readArtifact returns the resources of an ERF/HAK/MOD, or the single resource
+// readArtifactIndex locates the resources of an ERF/HAK/MOD, or the single
-// a loose file represents. Upstream's resman does the same dispatch on the
+// resource a loose file represents, without reading any payload. Upstream's
-// file's first three bytes. name is the artifact's published name, which for a
+// resman does the same dispatch on the file's first three bytes. name is the
-// loose file is also its resref.
+// artifact's published name, which for a loose file is also its resref.
-func readArtifact(path string, data []byte, name string) ([]erf.Resource, error) {
+func readArtifactIndex(path string, artifact io.ReaderAt, size int64, name string) ([]erf.IndexEntry, error) {
-	if len(data) >= 3 {
+	magic := make([]byte, 3)
-		switch string(data[: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)
 	}
-	}
 	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{{
+	return []erf.IndexEntry{{Name: name, Type: restype, Offset: 0, Size: size}}, nil
-		Name: name,
-		Type: restype,
-		Data: data,
-	}}, nil
 }
 
-func emitResources(resources []erf.Resource, target sink) ([]Entry, int, int64, error) {
+// emitResources hashes, compresses and stores one resource at a time, reading
+// each payload from the artifact only when its turn comes. Peak memory
+// therefore tracks the largest single resource, not the archive: a 2 GB hak
+// must emit inside a runner's few spare GB.
+func emitResources(artifact io.ReaderAt, index []erf.IndexEntry, target sink) ([]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))
+	order := make([]Identity, 0, len(index))
-	latest := make(map[Identity]erf.Resource, len(resources))
+	latest := make(map[Identity]erf.IndexEntry, len(index))
 	var tooBig []string
-	for _, resource := range resources {
+	for _, entry := range index {
-		if _, ok := erf.ExtensionForResourceType(resource.Type); !ok {
+		if _, ok := erf.ExtensionForResourceType(entry.Type); !ok {
-			return nil, 0, 0, fmt.Errorf("resref %s is not resolvable (unknown restype %d)", resource.Name, resource.Type)
+			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 {
+		if entry.Size > fileSizeLimit {
-			tooBig = append(tooBig, fmt.Sprintf("%s: %d bytes > %d", resource.Name, len(resource.Data), 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)
@@ -201,15 +212,18 @@ func emitResources(resources []erf.Resource, target sink) ([]Entry, int, int64,
 	var blobs int
 	var onDiskBytes int64
 	for _, identity := range order {
-		resource := latest[identity]
+		payload, err := erf.ReadPayload(artifact, latest[identity])
-		sum := sha1.Sum(resource.Data)
+		if err != nil {
+			return nil, 0, 0, err
+		}
+		sum := sha1.Sum(payload)
 		entries = append(entries, Entry{
 			SHA1:    sum,
-			Size:    uint32(len(resource.Data)),
+			Size:    uint32(len(payload)),
 			ResRef:  identity.ResRef,
 			ResType: identity.ResType,
 		})
-		written, err := target.putBlob(fmt.Sprintf("%x", sum), func() []byte { return compressBlob(resource.Data) })
+		written, err := target.putBlob(fmt.Sprintf("%x", sum), func() []byte { return compressBlob(payload) })
 		if err != nil {
 			return nil, 0, 0, err
 		}

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/sink.go

@@ -6,6 +6,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"fmt"
+	"io"
 	"os"
 	"path"
 	"path/filepath"
@@ -193,14 +194,18 @@ func resolveIndexKey(artifactKey, outDir string) (string, error) {
 // 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.
-func checkArtifactKey(artifactKey string, artifact []byte) error {
+// 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)
 	}
-	sum := sha256.Sum256(artifact)
+	hash := sha256.New()
-	if got := hex.EncodeToString(sum[:]); got != digest {
+	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