internal/depot/remote.go

@@ -0,0 +1,248 @@
+package depot
+
+import (
+	"context"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// NewBackend returns the backend for name ("local"|"cdn"|"bunny").
+// localRoot is used only for "local". Fails closed: "bunny" with empty
+// StorageHost or empty ReadKey returns an error (never prompts);
+// unknown name returns an error.
+func NewBackend(name, localRoot string, cfg Config) (Backend, error) {
+	switch name {
+	case "local":
+		if localRoot == "" {
+			return nil, errors.New("local backend requires a non-empty root")
+		}
+		return &LocalBackend{Root: localRoot}, nil
+	case "cdn":
+		return &httpBackend{
+			name:   "cdn",
+			client: newHTTPClient(cfg),
+			cfg:    cfg,
+		}, nil
+	case "bunny":
+		if cfg.StorageHost == "" {
+			return nil, errors.New("bunny backend requires BUNNY_STORAGE_HOST")
+		}
+		if cfg.ReadKey == "" {
+			return nil, errors.New("bunny backend requires BUNNY_STORAGE_READ_PASSWORD or BUNNY_STORAGE_PASSWORD")
+		}
+		return &httpBackend{
+			name:   "bunny",
+			client: newHTTPClient(cfg),
+			cfg:    cfg,
+		}, nil
+	default:
+		return nil, fmt.Errorf("unknown backend %q", name)
+	}
+}
+
+// newHTTPClient builds an IPv4-only client per spec (HEAD probes are banned;
+// IPv6 dial hazards are out of scope for this depot).
+func newHTTPClient(cfg Config) *http.Client {
+	transport := &http.Transport{
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			d := net.Dialer{Timeout: cfg.ConnectTimeout}
+			return d.DialContext(ctx, "tcp4", addr)
+		},
+		MaxIdleConnsPerHost: cfg.ProbeJobs,
+	}
+	return &http.Client{Transport: transport, Timeout: cfg.ProbeMaxTime}
+}
+
+// httpBackend implements Backend for both cdn (read-only) and bunny
+// (read/write) over HTTP, sharing probe/get/put logic.
+type httpBackend struct {
+	name   string
+	client *http.Client
+	cfg    Config
+}
+
+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) cdnURL(sha string) string {
+	return fmt.Sprintf("%s/%s", b.cfg.CDNBase, BlobKey(sha))
+}
+
+// rangeProbe issues GET <url> with Range: bytes=0-0 and classifies the
+// response. Never HEAD (banned by spec).
+func (b *httpBackend) rangeProbe(ctx context.Context, url string, headers map[string]string) (ProbeState, bool, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return Unconfirmed, true, err
+	}
+	req.Header.Set("Range", "bytes=0-0")
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+
+	resp, err := b.client.Do(req)
+	if err != nil {
+		return Unconfirmed, true, err
+	}
+	defer resp.Body.Close()
+	_, _ = io.Copy(io.Discard, resp.Body)
+
+	switch {
+	case resp.StatusCode >= 200 && resp.StatusCode < 300:
+		return Present, false, nil
+	case resp.StatusCode == 404 || resp.StatusCode == 410:
+		return Absent, false, nil
+	default:
+		return Unconfirmed, true, nil
+	}
+}
+
+// Probe returns the existence state of sha at this backend.
+func (b *httpBackend) Probe(ctx context.Context, sha string) (ProbeState, bool, error) {
+	if b.name == "cdn" {
+		return b.rangeProbe(ctx, b.cdnURL(sha), nil)
+	}
+	return b.rangeProbe(ctx, b.storageURL(sha), map[string]string{"AccessKey": b.cfg.ReadKey})
+}
+
+// Put uploads src for sha. cdn is read-only.
+func (b *httpBackend) Put(ctx context.Context, sha, src string) error {
+	if b.name == "cdn" {
+		return errors.New("cdn backend is read-only")
+	}
+	if b.cfg.WriteKey == "" {
+		return errors.New("bunny backend requires BUNNY_STORAGE_PASSWORD to write")
+	}
+
+	state, _, err := b.Probe(ctx, sha)
+	if err != nil {
+		return err
+	}
+	if state == Present {
+		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
+}
+
+// Get fetches sha into dest via temp file + rename, re-hashing and deleting
+// on mismatch. Mirrors bash _depot_bunny_get: try CDN first, fall back to
+// storage with ReadKey.
+func (b *httpBackend) Get(ctx context.Context, sha, dest string) error {
+	destDir := filepath.Dir(dest)
+	if err := os.MkdirAll(destDir, 0755); err != nil {
+		return err
+	}
+
+	tmpFile, err := os.CreateTemp(destDir, ".tmp-")
+	if err != nil {
+		return err
+	}
+	tmpName := tmpFile.Name()
+	defer os.Remove(tmpName)
+
+	resp, err := b.fetch(ctx, sha)
+	if err != nil {
+		tmpFile.Close()
+		return err
+	}
+	defer resp.Body.Close()
+
+	hasher := sha256.New()
+	tee := io.TeeReader(resp.Body, hasher)
+	_, err = io.Copy(tmpFile, tee)
+	tmpFile.Close()
+	if err != nil {
+		return err
+	}
+
+	gotHash := fmt.Sprintf("%x", hasher.Sum(nil))
+	if gotHash != sha {
+		os.Remove(tmpName)
+		return fmt.Errorf("hash mismatch: expected %s, got %s", sha, gotHash)
+	}
+
+	return os.Rename(tmpName, dest)
+}
+
+// fetch tries the CDN URL first (no auth), falling back to the storage URL
+// with ReadKey on any non-2xx response or transport error.
+func (b *httpBackend) fetch(ctx context.Context, sha string) (*http.Response, error) {
+	if b.name != "cdn" {
+		if resp, err := b.get(ctx, b.cdnURL(sha), nil); err == nil && resp.StatusCode >= 200 && resp.StatusCode < 300 {
+			return resp, nil
+		} else if err == nil {
+			resp.Body.Close()
+		}
+		resp, err := b.get(ctx, b.storageURL(sha), map[string]string{"AccessKey": b.cfg.ReadKey})
+		if err != nil {
+			return nil, err
+		}
+		if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+			resp.Body.Close()
+			return nil, fmt.Errorf("bunny get %s: unexpected status %d", sha, resp.StatusCode)
+		}
+		return resp, nil
+	}
+
+	resp, err := b.get(ctx, b.cdnURL(sha), nil)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		resp.Body.Close()
+		return nil, fmt.Errorf("cdn get %s: unexpected status %d", sha, resp.StatusCode)
+	}
+	return resp, nil
+}
+
+func (b *httpBackend) get(ctx context.Context, url string, headers map[string]string) (*http.Response, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+	return b.client.Do(req)
+}

internal/depot/remote_test.go

@@ -0,0 +1,473 @@
+package depot
+
+import (
+	"context"
+	"crypto/sha256"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+func shaOf(s string) string {
+	return fmt.Sprintf("%x", sha256.Sum256([]byte(s)))
+}
+
+type recordedReq struct {
+	Method  string
+	Path    string
+	Range   string
+	Headers http.Header
+}
+
+// fakeBunny is a minimal recording fake for Bunny storage/CDN endpoints.
+type fakeBunny struct {
+	mu        sync.Mutex
+	requests  []recordedReq
+	headCount int
+	blobs     map[string][]byte
+	statusFor map[string]int // sha -> status code override for probe/get
+}
+
+func newFakeBunny() *fakeBunny {
+	return &fakeBunny{blobs: map[string][]byte{}, statusFor: map[string]int{}}
+}
+
+func (f *fakeBunny) handler() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		f.mu.Lock()
+		f.requests = append(f.requests, recordedReq{
+			Method:  r.Method,
+			Path:    r.URL.Path,
+			Range:   r.Header.Get("Range"),
+			Headers: r.Header.Clone(),
+		})
+		if r.Method == http.MethodHead {
+			f.headCount++
+			f.mu.Unlock()
+			w.WriteHeader(500)
+			return
+		}
+		f.mu.Unlock()
+
+		sha := strings.TrimPrefix(r.URL.Path, "/")
+		// last path element is the sha
+		parts := strings.Split(sha, "/")
+		sha = parts[len(parts)-1]
+
+		if r.Method == http.MethodPut {
+			buf, _ := io.ReadAll(r.Body)
+			f.mu.Lock()
+			f.blobs[sha] = buf
+			f.mu.Unlock()
+			w.WriteHeader(201)
+			return
+		}
+
+		// GET (range probe / actual get)
+		f.mu.Lock()
+		status, hasStatus := f.statusFor[sha]
+		data, ok := f.blobs[sha]
+		f.mu.Unlock()
+
+		if hasStatus {
+			w.WriteHeader(status)
+			return
+		}
+		if !ok {
+			w.WriteHeader(404)
+			return
+		}
+		w.WriteHeader(206)
+		_, _ = w.Write(data)
+	}
+}
+
+func (f *fakeBunny) requestsSnapshot() []recordedReq {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	out := make([]recordedReq, len(f.requests))
+	copy(out, f.requests)
+	return out
+}
+
+func testCfg(storageHost, readKey, writeKey string) Config {
+	return Config{
+		CDNBase:        "http://127.0.0.1:1", // unreachable by default; overridden per test
+		StorageHost:    storageHost,
+		StorageZone:    "sow-assets-depot",
+		ReadKey:        readKey,
+		WriteKey:       writeKey,
+		ProbeJobs:      4,
+		Jobs:           4,
+		ConnectTimeout: 2 * time.Second,
+		ProbeMaxTime:   2 * time.Second,
+		ConfirmRetries: 3,
+		ConfirmMax:     200,
+	}
+}
+
+// hostPort returns a StorageHost value for tests: a full "http://host:port"
+// base URL, which storageURL() passes through unchanged (bypassing the
+// production https:// default so httptest.NewServer works without TLS).
+func hostPort(srv *httptest.Server) string {
+	return srv.URL
+}
+
+func TestProbeUsesRangeGetNotHead(t *testing.T) {
+	sha := shaOf("present-blob")
+	f := newFakeBunny()
+	f.blobs[sha] = []byte("hello")
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+
+	cfg := testCfg(hostPort(srv), "readkey", "writekey")
+	b, err := NewBackend("bunny", "", cfg)
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+
+	state, transient, err := b.Probe(context.Background(), sha)
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	if state != Present || transient {
+		t.Fatalf("expected Present/non-transient, got %v/%v", state, transient)
+	}
+
+	reqs := f.requestsSnapshot()
+	if len(reqs) != 1 {
+		t.Fatalf("expected exactly 1 request, got %d", len(reqs))
+	}
+	if reqs[0].Method != "GET" {
+		t.Fatalf("expected GET, got %s", reqs[0].Method)
+	}
+	if reqs[0].Range != "bytes=0-0" {
+		t.Fatalf("expected Range bytes=0-0, got %q", reqs[0].Range)
+	}
+	if f.headCount != 0 {
+		t.Fatalf("expected zero HEAD requests, got %d", f.headCount)
+	}
+}
+
+func TestProbeStates(t *testing.T) {
+	cases := []struct {
+		status        int
+		wantState     ProbeState
+		wantTransient bool
+	}{
+		{200, Present, false},
+		{206, Present, false},
+		{404, Absent, false},
+		{428, Unconfirmed, true},
+		{503, Unconfirmed, true},
+	}
+
+	for _, c := range cases {
+		t.Run(fmt.Sprintf("status_%d", c.status), func(t *testing.T) {
+			sha := shaOf(fmt.Sprintf("blob-%d", c.status))
+			f := newFakeBunny()
+			f.statusFor[sha] = c.status
+			srv := httptest.NewServer(f.handler())
+			defer srv.Close()
+
+			cfg := testCfg(hostPort(srv), "readkey", "writekey")
+			b, err := NewBackend("bunny", "", cfg)
+			if err != nil {
+				t.Fatalf("NewBackend: %v", err)
+			}
+
+			state, transient, err := b.Probe(context.Background(), sha)
+			if err != nil && c.status < 500 {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if state != c.wantState {
+				t.Fatalf("status %d: expected state %v, got %v", c.status, c.wantState, state)
+			}
+			if transient != c.wantTransient {
+				t.Fatalf("status %d: expected transient=%v, got %v", c.status, c.wantTransient, transient)
+			}
+			if f.headCount != 0 {
+				t.Fatalf("expected zero HEAD requests, got %d", f.headCount)
+			}
+		})
+	}
+}
+
+func TestBunnyPutSequence(t *testing.T) {
+	t.Run("absent sha", func(t *testing.T) {
+		sha := shaOf("new-blob")
+		f := newFakeBunny()
+		srv := httptest.NewServer(f.handler())
+		defer srv.Close()
+
+		cfg := testCfg(hostPort(srv), "readkey", "writekey")
+		b, err := NewBackend("bunny", "", cfg)
+		if err != nil {
+			t.Fatalf("NewBackend: %v", err)
+		}
+
+		dir := t.TempDir()
+		srcPath := filepath.Join(dir, "src")
+		if err := os.WriteFile(srcPath, []byte("new-blob-content"), 0644); err != nil {
+			t.Fatal(err)
+		}
+
+		if err := b.Put(context.Background(), sha, srcPath); err != nil {
+			t.Fatalf("Put: %v", err)
+		}
+
+		reqs := f.requestsSnapshot()
+		if len(reqs) != 2 {
+			t.Fatalf("expected 2 requests (probe, put), got %d: %+v", len(reqs), reqs)
+		}
+		if reqs[0].Method != "GET" || reqs[0].Range != "bytes=0-0" {
+			t.Fatalf("expected first request to be range probe GET, got %+v", reqs[0])
+		}
+		if reqs[1].Method != "PUT" {
+			t.Fatalf("expected second request to be PUT, got %+v", reqs[1])
+		}
+		if got := reqs[1].Headers.Get("Checksum"); got != strings.ToUpper(sha) {
+			t.Fatalf("expected Checksum %s, got %s", strings.ToUpper(sha), got)
+		}
+		if got := reqs[1].Headers.Get("AccessKey"); got != "writekey" {
+			t.Fatalf("expected AccessKey writekey, got %s", got)
+		}
+		if f.headCount != 0 {
+			t.Fatalf("expected zero HEAD requests, got %d", f.headCount)
+		}
+	})
+
+	t.Run("present sha", func(t *testing.T) {
+		sha := shaOf("existing-blob")
+		f := newFakeBunny()
+		f.blobs[sha] = []byte("existing-blob-content")
+		srv := httptest.NewServer(f.handler())
+		defer srv.Close()
+
+		cfg := testCfg(hostPort(srv), "readkey", "writekey")
+		b, err := NewBackend("bunny", "", cfg)
+		if err != nil {
+			t.Fatalf("NewBackend: %v", err)
+		}
+
+		dir := t.TempDir()
+		srcPath := filepath.Join(dir, "src")
+		if err := os.WriteFile(srcPath, []byte("existing-blob-content"), 0644); err != nil {
+			t.Fatal(err)
+		}
+
+		if err := b.Put(context.Background(), sha, srcPath); err != nil {
+			t.Fatalf("Put: %v", err)
+		}
+
+		reqs := f.requestsSnapshot()
+		if len(reqs) != 1 {
+			t.Fatalf("expected probe-only (1 request) since present, got %d: %+v", len(reqs), reqs)
+		}
+		if reqs[0].Method != "GET" {
+			t.Fatalf("expected GET probe, got %+v", reqs[0])
+		}
+	})
+}
+
+func TestReadWriteKeySplit(t *testing.T) {
+	sha := shaOf("split-blob")
+	f := newFakeBunny()
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+
+	cfg := testCfg(hostPort(srv), "readkeyXXX", "writekeyYYY")
+	b, err := NewBackend("bunny", "", cfg)
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+
+	dir := t.TempDir()
+	srcPath := filepath.Join(dir, "src")
+	if err := os.WriteFile(srcPath, []byte("split-blob-content"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := b.Put(context.Background(), sha, srcPath); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	reqs := f.requestsSnapshot()
+	if len(reqs) != 2 {
+		t.Fatalf("expected 2 requests, got %d", len(reqs))
+	}
+	if got := reqs[0].Headers.Get("AccessKey"); got != "readkeyXXX" {
+		t.Fatalf("probe expected AccessKey readkeyXXX, got %s", got)
+	}
+	if got := reqs[1].Headers.Get("AccessKey"); got != "writekeyYYY" {
+		t.Fatalf("put expected AccessKey writekeyYYY, got %s", got)
+	}
+	if reqs[0].Headers.Get("AccessKey") == reqs[1].Headers.Get("AccessKey") {
+		t.Fatalf("expected distinct read/write keys")
+	}
+}
+
+func TestBunnyNoReadKeyFailsClosed(t *testing.T) {
+	// Grep-based tripwire: no non-test depot source may reference stdin/tty.
+	dir := "."
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("ReadDir: %v", err)
+	}
+	for _, e := range entries {
+		name := e.Name()
+		if e.IsDir() || !strings.HasSuffix(name, ".go") || strings.HasSuffix(name, "_test.go") {
+			continue
+		}
+		data, err := os.ReadFile(filepath.Join(dir, name))
+		if err != nil {
+			t.Fatalf("ReadFile %s: %v", name, err)
+		}
+		s := string(data)
+		if strings.Contains(s, "os.Stdin") || strings.Contains(s, "bufio.NewReader(os.Stdin)") || strings.Contains(s, "/dev/tty") {
+			t.Fatalf("%s references stdin/tty; credential prompts are banned", name)
+		}
+	}
+
+	// Confirm undrained pipe: swap in an os.Pipe as stdin-equivalent to prove
+	// no read occurs.
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("Pipe: %v", err)
+	}
+	oldStdin := os.Stdin
+	os.Stdin = r
+	defer func() { os.Stdin = oldStdin; r.Close() }()
+
+	cfg := testCfg("storage.example.com", "", "writekey")
+	_, err = NewBackend("bunny", "", cfg)
+	if err == nil {
+		t.Fatal("expected error for empty ReadKey")
+	}
+	if !strings.Contains(err.Error(), "ReadKey") && !strings.Contains(err.Error(), "BUNNY_STORAGE_READ_PASSWORD") && !strings.Contains(err.Error(), "BUNNY_STORAGE_PASSWORD") {
+		t.Fatalf("expected error to mention missing env var, got: %v", err)
+	}
+
+	w.Close()
+	// If code read from stdin it would have blocked already (pipe with no
+	// writer-side data yet); assert the pipe is still open/undrained by
+	// writing now and reading it back ourselves.
+	if _, err := w.Write([]byte("x")); err == nil {
+		t.Fatal("expected write to closed pipe writer to fail")
+	}
+}
+
+func TestBunnyNoStorageHostFailsClosed(t *testing.T) {
+	cfg := testCfg("", "readkey", "writekey")
+	_, err := NewBackend("bunny", "", cfg)
+	if err == nil {
+		t.Fatal("expected error for empty StorageHost")
+	}
+	if !strings.Contains(err.Error(), "StorageHost") && !strings.Contains(err.Error(), "BUNNY_STORAGE_HOST") {
+		t.Fatalf("expected error to mention missing StorageHost/BUNNY_STORAGE_HOST, got: %v", err)
+	}
+}
+
+func TestGetHashMismatchDeletes(t *testing.T) {
+	sha := shaOf("get-mismatch-blob")
+	f := newFakeBunny()
+	f.blobs[sha] = []byte("wrong content")
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+
+	cfg := testCfg(hostPort(srv), "readkey", "writekey")
+	// force CDN unreachable so Get falls back to storage
+	cfg.CDNBase = "http://127.0.0.1:1"
+	b, err := NewBackend("bunny", "", cfg)
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "dest")
+
+	err = b.Get(context.Background(), sha, dest)
+	if err == nil {
+		t.Fatal("expected hash mismatch error")
+	}
+	if _, statErr := os.Stat(dest); !os.IsNotExist(statErr) {
+		t.Fatalf("expected dest to be removed on mismatch, stat err: %v", statErr)
+	}
+}
+
+func TestCDNPutReadOnly(t *testing.T) {
+	cfg := testCfg("storage.example.com", "readkey", "writekey")
+	b, err := NewBackend("cdn", "", cfg)
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+
+	dir := t.TempDir()
+	srcPath := filepath.Join(dir, "src")
+	if err := os.WriteFile(srcPath, []byte("data"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	err = b.Put(context.Background(), shaOf("anything"), srcPath)
+	if err == nil {
+		t.Fatal("expected error for cdn Put (read-only)")
+	}
+	if !strings.Contains(err.Error(), "read-only") {
+		t.Fatalf("expected read-only error, got: %v", err)
+	}
+}
+
+// TestNoHeadInSources is the anti-HEAD regression tripwire from field note 3:
+// grep-assert no http.MethodHead/.Head( usage in non-test depot sources.
+func TestNoHeadInSources(t *testing.T) {
+	dir := "."
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("ReadDir: %v", err)
+	}
+	for _, e := range entries {
+		name := e.Name()
+		if e.IsDir() || !strings.HasSuffix(name, ".go") || strings.HasSuffix(name, "_test.go") {
+			continue
+		}
+		data, err := os.ReadFile(filepath.Join(dir, name))
+		if err != nil {
+			t.Fatalf("ReadFile %s: %v", name, err)
+		}
+		s := string(data)
+		if strings.Contains(s, "http.MethodHead") || strings.Contains(s, ".Head(") {
+			t.Fatalf("%s references HEAD probe (banned)", name)
+		}
+	}
+}
+
+// TestLocalBackendViaFactory sanity-checks NewBackend("local", ...).
+func TestLocalBackendViaFactory(t *testing.T) {
+	root := t.TempDir()
+	b, err := NewBackend("local", root, Config{})
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+	lb, ok := b.(*LocalBackend)
+	if !ok {
+		t.Fatalf("expected *LocalBackend, got %T", b)
+	}
+	if lb.Root != root {
+		t.Fatalf("expected Root=%s, got %s", root, lb.Root)
+	}
+
+	if _, err := NewBackend("local", "", Config{}); err == nil {
+		t.Fatal("expected error for empty local root")
+	}
+
+	if _, err := NewBackend("nonsense", "", Config{}); err == nil {
+		t.Fatal("expected error for unknown backend name")
+	}
+}