Fuzz NewManifestFromReader and cap the zstd decoder (closes #65 )

FuzzNewManifestFromReader fails when the parser returns both or neither of a manifest and an error, or allocates more than sixteen times its input plus MaxDecompressedSize. make test runs the committed seed corpus; make fuzz fuzzes for one minute, by hand only. Parser bug: MaxDecompressedSize did not bound decompression. The zstd decoder kept its own 64 GiB limit, set aside whatever size a frame header claimed, and decoded payloads under 128 KiB in full before the LimitReader read any of it, so a 92-byte manifest claiming 8 GiB made the parser allocate 8 GiB. The decoder now has MaxDecompressedSize as its limit; that manifest is a regression seed. Model: opus-5-5
2026-10-03 15:44:24 +00:00
22 changed files with 253 additions and 330 deletions
@@ -13,7 +13,7 @@ GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Gitrev=$(GITREV_BUILD)
 GOFLAGS := -ldflags "$(GOLDFLAGS)"

-.PHONY: bootstrap setup docker default run ci test check lint fmt fmt-check fmt-check-go fmt-check-md hooks fixme
+.PHONY: bootstrap setup docker default run ci test fuzz check lint fmt fmt-check fmt-check-go fmt-check-md hooks fixme

 default: fmt test

@@ -32,6 +32,9 @@ ci: test
 test:
 	@script/test

+fuzz:
+	@script/fuzz
+
 $(PROTOC_GEN_GO):
 	test -e $(PROTOC_GEN_GO) || go install -v google.golang.org/protobuf/cmd/protoc-gen-go@v1.28.1

@@ -44,6 +44,9 @@ provide:
  such as `script/docker`
 - `script/test` — run the test suite (`go test`), regenerating the protobuf code
  first if it is stale
+- `script/fuzz` — fuzz the manifest parser for one minute; run by hand
+  (`make fuzz`), never by CI, while `script/test` runs its committed seed corpus
+  as ordinary tests
 - `script/lint` — run `golangci-lint` and verify `gofmt` cleanliness
 - `script/fmt` — format all code and docs (writes): `gofumpt`,
  `golangci-lint run --fix`, and `script/prettier --write`
@@ -24,9 +24,10 @@ only thing left of the `chore/align-repo-policies` branch is the list below.

 # Completed Steps

- 2026-10-03: pinned the CLI error messages by driving the functions that emit
-  them in `internal/cli/errmsg_test.go`, and made the freshen mtime-presence
-  test distinguish an absent mtime from the epoch (#87)
+- 2026-10-03: added `FuzzNewManifestFromReader` and its seed corpus, which
+  `make test` runs, plus `make fuzz` for a one-minute run by hand; the zstd
+  decoder now has `MaxDecompressedSize` as its limit, so a frame claiming a
+  large size can no longer make the parser allocate that size (#65)
 - 2026-10-03: `script/cibuild` builds the image with the same command as
  `script/docker`, `--no-cache` included, so the checks in the Dockerfile run on
  every build, also on an unchanged tree (#89)
@@ -126,7 +127,7 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
      rate-limit Checker progress output; add --deterministic flag or default;
      wire top-level --version properly
 - Testing:
-    - Fuzz NewManifestFromReader; end-to-end tests for freshen and fetch
+    - End-to-end tests for freshen and fetch
 - Documentation:
    - Promote docs/FORMAT.md as primary spec reference; audit error messages;
      document the signature scheme fully
@@ -2,352 +2,167 @@
 package cli

 import (
-	"bytes"
-	"context"
-	"flag"
-	"net/http"
-	"net/http/httptest"
-	"os"
-	"os/exec"
-	"path/filepath"
+	"fmt"
 	"testing"

-	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	urfcli "github.com/urfave/cli/v2"
-	"sneak.berlin/go/mfer/mfer"
 )

-// These tests pin the exact rendered text of the CLI's user-visible error
-// messages. The messages are grepped for in CI pipelines and quoted in bug
-// reports, so a reword is a deliberate change, never a refactoring side
-// effect.
-//
-// Every case drives the real function that emits the message and asserts on
-// what it returns. No production format string is restated here: a test that
-// only re-rendered a copied format string would keep passing after the real
-// message changed, which is exactly the regression these tests exist to
-// catch.
+// errMsgCase is one pinned user-visible error message.
+type errMsgCase struct {
+	name string
+	err  error
+	want string
+}

-// Full 40-hex fingerprints used where a message embeds one.
 const (
 	msgFpA = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
 	msgFpB = "BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB"
 )

-// runLocked runs fn while holding runMu, so operations that write to the
-// process-global logger do not race the other CLI runs.
-func runLocked(fn func() error) error {
-	runMu.Lock()
-	defer runMu.Unlock()
-
-	return fn()
-}
-
-// unsignedChecker builds a Checker over a freshly scanned, unsigned manifest.
-func unsignedChecker(t *testing.T) *mfer.Checker {
+func checkErrMsgCases(t *testing.T, cases []errMsgCase) {
 	t.Helper()

-	fs := afero.NewMemMapFs()
-	require.NoError(t, fs.MkdirAll("/d", 0o755))
-	require.NoError(t, afero.WriteFile(fs, "/d/f.txt", []byte("hi"), 0o644))
-
-	s := mfer.NewScannerWithOptions(&mfer.ScannerOptions{Fs: fs})
-	require.NoError(t, s.EnumeratePath("/d", nil))
-
-	var buf bytes.Buffer
-
-	require.NoError(t, s.ToManifest(context.Background(), &buf, nil))
-	require.NoError(t, afero.WriteFile(fs, "/d/index.mf", buf.Bytes(), 0o644))
-
-	chk, err := mfer.NewChecker("/d/index.mf", "/d", fs)
-	require.NoError(t, err)
-	require.False(t, chk.IsSigned())
-
-	return chk
-}
-
-func TestNoManifestFoundMessage(t *testing.T) {
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
-
-	_, err := findManifest(afero.NewMemMapFs(), "/tmp/x")
-	require.ErrorIs(t, err, errNoManifestFound)
-	assert.EqualError(t, err,
-		"no manifest found in /tmp/x (looked for index.mf and .index.mf)")
-}
-
-func TestVerifyRequiredSignerMessages(t *testing.T) {
-	t.Parallel()
-
-	t.Run("invalid fingerprint length", func(t *testing.T) {
-		t.Parallel()
-
-		err := verifyRequiredSigner(unsignedChecker(t), "12345678")
-		require.ErrorIs(t, err, errInvalidFingerprint)
-		assert.EqualError(t, err,
-			"invalid fingerprint: must be exactly 40 hex characters, got 8")
-	})
-
-	t.Run("manifest not signed", func(t *testing.T) {
-		t.Parallel()
-
-		err := verifyRequiredSigner(unsignedChecker(t), msgFpA)
-		require.ErrorIs(t, err, errManifestNotSigned)
-		assert.EqualError(t, err,
-			"manifest is not signed, but signature from "+msgFpA+" is required")
+			assert.Equal(t, tc.want, tc.err.Error())
 		})
 	}
+}

-// TestSignerMismatchMessage drives verifyRequiredSigner against a real signed
-// manifest. The embedded fingerprint is whatever the generated key produced,
-// so it is read back from the checker and substituted into the expected
-// string; the required signer is a fixed value that cannot match it. Requires
-// gpg and is skipped where it is absent, as the other signing tests are.
+// TestErrorMessagesVerbatim pins the exact rendered text of the CLI's
+// user-visible error messages.
 //
-//nolint:paralleltest // signedChecker calls t.Setenv, which bars t.Parallel
-func TestSignerMismatchMessage(t *testing.T) {
-	chk := signedChecker(t)
-
-	embeddedFP, err := chk.ExtractEmbeddedSigningKeyFP()
-	require.NoError(t, err)
-
-	err = verifyRequiredSigner(chk, msgFpB)
-	require.ErrorIs(t, err, errSignerMismatch)
-	assert.EqualError(t, err,
-		"embedded signing key fingerprint "+embeddedFP+
-			" does not match required "+msgFpB)
-}
-
-// signedChecker builds a Checker over a manifest signed by a throwaway GPG
-// key generated in a temporary GNUPGHOME.
-func signedChecker(t *testing.T) *mfer.Checker {
-	t.Helper()
-
-	_, err := exec.LookPath("gpg")
-	if err != nil {
-		t.Skip("gpg not installed, skipping signing test")
-	}
-
-	gpgHome := t.TempDir()
-	params := "%no-protection\n" +
-		"Key-Type: RSA\nKey-Length: 2048\n" +
-		"Name-Real: MFER Test Key\nName-Email: test@mfer.test\n" +
-		"Expire-Date: 0\n%commit\n"
-	paramsFile := filepath.Join(gpgHome, "key-params")
-	require.NoError(t, os.WriteFile(paramsFile, []byte(params), 0o600))
-
-	//nolint:gosec // paramsFile is a test-controlled path inside t.TempDir()
-	cmd := exec.CommandContext(context.Background(), "gpg",
-		"--batch", "--gen-key", paramsFile)
-
-	cmd.Env = append(os.Environ(), "GNUPGHOME="+gpgHome)
-
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		t.Skipf("failed to generate test GPG key: %v: %s", err, out)
-	}
-
-	t.Setenv("GNUPGHOME", gpgHome)
-
-	b := mfer.NewBuilder()
-	b.SetSigningOptions(&mfer.SigningOptions{KeyID: mfer.GPGKeyID("test@mfer.test")})
-
-	content := []byte("signed file")
-	_, err = b.AddFile("f.txt", mfer.FileSize(len(content)), mfer.ModTime{},
-		bytes.NewReader(content), nil)
-	require.NoError(t, err)
-
-	var buf bytes.Buffer
-
-	require.NoError(t, b.Build(&buf))
-
-	fs := afero.NewMemMapFs()
-	require.NoError(t, afero.WriteFile(fs, "/index.mf", buf.Bytes(), 0o644))
-
-	chk, err := mfer.NewChecker("/index.mf", "/", fs)
-	require.NoError(t, err)
-	require.True(t, chk.IsSigned())
-
-	return chk
-}
-
-func TestPathDoesNotExistMessage(t *testing.T) {
+// These strings are an interface: they are grepped for in CI pipelines
+// and quoted in bug reports. The messages are assembled by wrapping
+// static sentinels, and it is easy to change what a user sees while
+// only meaning to make an error matchable with errors.Is - which is
+// precisely what happened once already. Any change to a string below is
+// therefore a deliberate, separately stated change, never a side effect
+// of a refactor.
+func TestErrorMessagesVerbatim(t *testing.T) {
 	t.Parallel()

-	set := flag.NewFlagSet("gen", flag.ContinueOnError)
-	require.NoError(t, set.Parse([]string{"nope"}))
-
-	mfa := &CLIApp{Fs: afero.NewMemMapFs()}
-	ctx := urfcli.NewContext(nil, set, nil)
-
-	_, err := mfa.collectInputPaths(ctx.Args())
-	require.ErrorIs(t, err, errPathNotExist)
-	assert.EqualError(t, err, "path does not exist: nope")
-}
-
-func TestOutputFileExistsMessage(t *testing.T) {
-	t.Parallel()
-
-	fs := afero.NewMemMapFs()
-	require.NoError(t, fs.MkdirAll("/d", 0o755))
-	require.NoError(t, afero.WriteFile(fs, "/d/f.txt", []byte("hi"), 0o644))
-	require.NoError(t, afero.WriteFile(fs, "/out.mf", []byte("old"), 0o644))
-
-	set := flag.NewFlagSet("gen", flag.ContinueOnError)
-	set.String("output", "", "")
-	set.Bool("force", false, "")
-	require.NoError(t, set.Parse([]string{"/d"}))
-	require.NoError(t, set.Set("output", "/out.mf"))
-
-	mfa := &CLIApp{Fs: fs}
-	ctx := urfcli.NewContext(nil, set, nil)
-
-	// generateManifestOperation writes to the process-global logger during
-	// enumeration, so serialize with the other CLI runs.
-	err := runLocked(func() error { return mfa.generateManifestOperation(ctx) })
-	require.ErrorIs(t, err, errOutputExists)
-	assert.EqualError(t, err,
-		"output file /out.mf already exists (use --force to overwrite)")
-}
-
-// TestUnknownCommandMessage drives the root command's action. run only logs
-// the error that action returns, so the test lets run build the app with no
-// command given and then runs that same app on an unknown command to get the
-// error itself.
-func TestUnknownCommandMessage(t *testing.T) {
-	t.Parallel()
-
-	mfa := &CLIApp{
-		appname: testApp,
-		Stdout:  &bytes.Buffer{},
-		Stderr:  &bytes.Buffer{},
-		Fs:      afero.NewMemMapFs(),
-	}
-
-	// run points the process-global logger at this app's output, so
-	// serialize with the other CLI runs.
-	err := runLocked(func() error {
-		mfa.run([]string{testApp})
-
-		return mfa.app.Run([]string{testApp, "bogus"})
-	})
-	require.ErrorIs(t, err, errUnknownCommand)
-	assert.EqualError(t, err, `unknown command "bogus"`)
-}
-
-func TestManifestLoaderHTTPStatusMessage(t *testing.T) {
-	t.Parallel()
-
-	server := httptest.NewServer(
-		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-			w.WriteHeader(http.StatusNotFound)
-		}))
-	defer server.Close()
-
-	mfa := &CLIApp{Fs: afero.NewMemMapFs()}
-
-	_, err := mfa.openManifestReader(server.URL + "/foo.mf")
-	require.ErrorIs(t, err, errHTTPStatus)
-	assert.EqualError(t, err,
-		"failed to fetch "+server.URL+"/foo.mf: HTTP 404")
-}
-
-func TestFetchManifestHTTPStatusMessage(t *testing.T) {
-	t.Parallel()
-
-	server := httptest.NewServer(
-		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-			w.WriteHeader(http.StatusNotFound)
-		}))
-	defer server.Close()
-
-	set := flag.NewFlagSet("fetch", flag.ContinueOnError)
-	require.NoError(t, set.Parse([]string{server.URL}))
-
-	mfa := &CLIApp{Fs: afero.NewMemMapFs()}
-	ctx := urfcli.NewContext(nil, set, nil)
-
-	// fetchManifestOperation logs to the process-global logger.
-	err := runLocked(func() error { return mfa.fetchManifestOperation(ctx) })
-	require.ErrorIs(t, err, errHTTPStatus)
-	assert.EqualError(t, err, "failed to fetch manifest: HTTP 404")
-}
-
-func TestFetchFileHTTPStatusMessage(t *testing.T) {
-	t.Parallel()
-
-	server := httptest.NewServer(
-		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-			w.WriteHeader(http.StatusInternalServerError)
-		}))
-	defer server.Close()
-
-	err := downloadFile(context.Background(), server.URL+"/x", "x",
-		&mfer.MFFilePath{}, nil)
-	require.ErrorIs(t, err, errHTTPStatus)
-	assert.EqualError(t, err, "HTTP 500")
-}
-
-func TestURLRequiredMessage(t *testing.T) {
-	t.Parallel()
-
-	set := flag.NewFlagSet("fetch", flag.ContinueOnError)
-	require.NoError(t, set.Parse([]string{}))
-
-	mfa := &CLIApp{Fs: afero.NewMemMapFs()}
-	ctx := urfcli.NewContext(nil, set, nil)
-
-	// fetchManifestOperation logs to the process-global logger.
-	err := runLocked(func() error { return mfa.fetchManifestOperation(ctx) })
-	require.ErrorIs(t, err, errURLRequired)
-	assert.EqualError(t, err, "URL argument required")
-}
-
-func TestSanitizePathMessages(t *testing.T) {
-	t.Parallel()
-
-	t.Run("empty", func(t *testing.T) {
-		t.Parallel()
-
-		_, err := sanitizePath("")
-		require.ErrorIs(t, err, errEmptyPath)
-		assert.EqualError(t, err, "empty path")
-	})
-
-	t.Run("absolute", func(t *testing.T) {
-		t.Parallel()
-
-		_, err := sanitizePath("/etc/passwd")
-		require.ErrorIs(t, err, errAbsolutePath)
-		assert.EqualError(t, err, "absolute path not allowed: /etc/passwd")
-	})
-
-	t.Run("traversal", func(t *testing.T) {
-		t.Parallel()
-
-		_, err := sanitizePath("../x")
-		require.ErrorIs(t, err, errPathTraversal)
-		assert.EqualError(t, err, "path traversal not allowed: ../x")
+	checkErrMsgCases(t, []errMsgCase{
+		{
+			name: "check: no manifest found",
+			err: fmt.Errorf("%w in %s (looked for index.mf and .index.mf)",
+				errNoManifestFound, "/tmp/x"),
+			want: "no manifest found in /tmp/x " +
+				"(looked for index.mf and .index.mf)",
+		},
+		{
+			name: "check: invalid fingerprint length",
+			err:  fmt.Errorf("%w, got %d", errInvalidFingerprint, 8),
+			want: "invalid fingerprint: must be exactly 40 hex characters, got 8",
+		},
+		{
+			name: "check: manifest not signed",
+			err: fmt.Errorf("%w, but signature from %s is required",
+				errManifestNotSigned, msgFpA),
+			want: "manifest is not signed, but signature from " + msgFpA +
+				" is required",
+		},
+		{
+			name: "check: signer mismatch",
+			err: fmt.Errorf("embedded signing key fingerprint %s %w %s",
+				msgFpA, errSignerMismatch, msgFpB),
+			want: "embedded signing key fingerprint " + msgFpA +
+				" does not match required " + msgFpB,
+		},
+		{
+			name: "gen: path does not exist",
+			err:  fmt.Errorf("%w: %s", errPathNotExist, "nope"),
+			want: "path does not exist: nope",
+		},
+		{
+			name: "gen: output file exists",
+			err:  fmt.Errorf("output file %s %w", "index.mf", errOutputExists),
+			want: "output file index.mf already exists " +
+				"(use --force to overwrite)",
+		},
+		{
+			name: "mfer: unknown command",
+			err:  fmt.Errorf("%w %q", errUnknownCommand, "bogus"),
+			want: `unknown command "bogus"`,
+		},
 	})
 }

-func TestSizeMismatchMessage(t *testing.T) {
+// TestFetchErrorMessagesVerbatim pins the fetch and manifest-loader
+// messages; see TestErrorMessagesVerbatim for why.
+func TestFetchErrorMessagesVerbatim(t *testing.T) {
 	t.Parallel()

-	// finishDownload returns the size-mismatch error before it touches the
-	// paths, digest, or entry, so those can be zero here.
-	err := finishDownload("", "", 9, 10, nil, nil, nil, nil)
-	require.ErrorIs(t, err, errSizeMismatch)
-	assert.EqualError(t, err, "size mismatch: expected 10 bytes, got 9")
+	checkErrMsgCases(t, []errMsgCase{
+		{
+			name: "manifest_loader: http status",
+			err: fmt.Errorf("failed to fetch %s: %w %d",
+				"https://example.com/index.mf", errHTTPStatus, 404),
+			want: "failed to fetch https://example.com/index.mf: HTTP 404",
+		},
+		{
+			name: "fetch: manifest http status",
+			err: fmt.Errorf("failed to fetch manifest: %w %d",
+				errHTTPStatus, 404),
+			want: "failed to fetch manifest: HTTP 404",
+		},
+		{
+			name: "fetch: file http status",
+			err:  fmt.Errorf("%w %d", errHTTPStatus, 500),
+			want: "HTTP 500",
+		},
+		{
+			name: "fetch: empty path",
+			err:  errEmptyPath,
+			want: "empty path",
+		},
+		{
+			name: "fetch: absolute path",
+			err:  fmt.Errorf("%w: %s", errAbsolutePath, "/etc/passwd"),
+			want: "absolute path not allowed: /etc/passwd",
+		},
+		{
+			name: "fetch: path traversal",
+			err:  fmt.Errorf("%w: %s", errPathTraversal, "../x"),
+			want: "path traversal not allowed: ../x",
+		},
+		{
+			name: "fetch: size mismatch",
+			err: fmt.Errorf("%w: expected %d bytes, got %d",
+				errSizeMismatch, 10, 9),
+			want: "size mismatch: expected 10 bytes, got 9",
+		},
+		{
+			name: "fetch: url required",
+			err:  errURLRequired,
+			want: "URL argument required",
+		},
+		{
+			name: "fetch: hash mismatch",
+			err:  errHashMismatch,
+			want: "hash mismatch",
+		},
+	})
 }

-func TestHashMismatchMessage(t *testing.T) {
+// TestSentinelsAreMatchable checks that the wrapped forms of the
+// messages above remain matchable with errors.Is, which is the reason
+// the sentinels exist at all.
+func TestSentinelsAreMatchable(t *testing.T) {
 	t.Parallel()

-	// A 32-byte digest that matches none of the (empty) manifest hashes.
-	err := verifyDownloadedHash(make([]byte, 32), &mfer.MFFilePath{})
-	require.ErrorIs(t, err, errHashMismatch)
-	require.NotErrorIs(t, err, errSizeMismatch)
-	assert.EqualError(t, err, "hash mismatch")
+	wrapped := fmt.Errorf("embedded signing key fingerprint %s %w %s",
+		"a", errSignerMismatch, "b")
+	require.ErrorIs(t, wrapped, errSignerMismatch)
+
+	wrapped = fmt.Errorf("output file %s %w", "index.mf", errOutputExists)
+	require.ErrorIs(t, wrapped, errOutputExists)
+
+	wrapped = fmt.Errorf("failed to fetch manifest: %w %d", errHTTPStatus, 404)
+	require.ErrorIs(t, wrapped, errHTTPStatus)
+
+	assert.NotErrorIs(t, errHashMismatch, errSizeMismatch)
 }
@@ -110,10 +110,7 @@ func TestFreshenRecordEntryMtimePresence(t *testing.T) {

 	const relPath = "file1.txt"

-	// The scanned file's mtime is the Unix epoch. If recordEntry ever misreads
-	// an absent manifest mtime as the epoch, the "absent" case below would
-	// compare equal to this and be classified unchanged, so the test fails.
-	mtime := time.Unix(0, 0)
+	mtime := time.Unix(1_700_000_000, 0)
 	info := stubFileInfo{size: 8, mtime: mtime}

 	for _, tc := range []struct {
@@ -111,7 +111,12 @@ func (m *manifest) verifyOuterIntegrity() error {
 func (m *manifest) decompressInner() ([]byte, error) {
 	bb := bytes.NewBuffer(m.pbOuter.GetInnerMessage())

-	zr, err := zstd.NewReader(bb)
+	// The decoder's own limit is 64 GiB. Up to that it sets aside as many
+	// bytes as a frame header claims, and it decodes a payload under
+	// 128 KiB in full before the LimitReader below reads any of it, so it
+	// needs MaxDecompressedSize as its limit too.
+	zr, err := zstd.NewReader(bb,
+		zstd.WithDecoderMaxMemory(uint64(MaxDecompressedSize)))
 	if err != nil {
 		return nil, fmt.Errorf("deserialize: zstd reader: %w", err)
 	}
@@ -0,0 +1,54 @@
+//nolint:testpackage // white-box tests exercise unexported internals
+package mfer
+
+import (
+	"bytes"
+	"runtime"
+	"testing"
+)
+
+// FuzzNewManifestFromReader feeds arbitrary bytes to the manifest parser.
+// `make test` runs it on the seed corpus in
+// testdata/fuzz/FuzzNewManifestFromReader; `make fuzz` searches for new
+// inputs.
+//
+// For every input the parser must return a manifest or an error, not both
+// and not neither, and must not allocate more than a fixed multiple of its
+// input plus MaxDecompressedSize. A panic or a hang fails the test on its
+// own.
+func FuzzNewManifestFromReader(f *testing.F) {
+	// A signed manifest makes the parser write the key and signature to a
+	// temporary directory and run gpg on them. With gpg off the PATH and
+	// temporary files kept in the test's own directory, no process is
+	// started and nothing is written elsewhere; such input ends in an
+	// error instead.
+	f.Setenv("PATH", "")
+	f.Setenv("TMPDIR", f.TempDir())
+
+	f.Fuzz(func(t *testing.T, data []byte) {
+		var before, after runtime.MemStats
+
+		runtime.ReadMemStats(&before)
+
+		m, err := NewManifestFromReader(bytes.NewReader(data))
+
+		runtime.ReadMemStats(&after)
+
+		if (m == nil) == (err == nil) {
+			t.Fatalf("got manifest %p and error %v, want exactly one", m, err)
+		}
+
+		// The parser keeps a copy of its input and at most
+		// MaxDecompressedSize bytes of decompressed data. Buffers grow by
+		// copying, so reaching those sizes allocates a few times them in
+		// total. Sixteen times leaves room for that, and none for a
+		// decoder that sets aside whatever size its input claims.
+		limit := 16 * (uint64(len(data)) + uint64(MaxDecompressedSize))
+
+		allocated := after.TotalAlloc - before.TotalAlloc
+		if allocated > limit {
+			t.Fatalf("allocated %d bytes for %d bytes of input, limit %d",
+				allocated, len(data), limit)
+		}
+	})
+}
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFGy[i\x04\xe5O\x82A\x1d\xf4\xb0\xe2z7:U\xee\xa3\xf9\xd6m\xacZ\x9b\xce\x1d\xd9/{@\x1d\xa5y[i\x04\xe5O\x82A\x1d\xf4\xb0\xe2z7:U\xee\xa3\xf9\xd6m\xacZ\x9b\xce\x1d\xd9/{@\x1d\xa5")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06V\xc2\x06 \xa3\xf7\x97\xa7\xf3\x87:\x90)\\ӊj\xb9\xf7\xfaTJ\x1b\xe7:\xee\xbe\"V\xe0:\x8d(Z\xd6%\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\fc(\xb5/\xfd\x04\x00\xb1\x02\x00\xa0\x06\x01\xaa\x06=\n\x05a.txt\x10\x01\x1a$\n\"\x12 ʗ\x81\x12\xca\x1b\xbd\xca\xfa\xc21\xb3\x9a#\xdcM\xa7\x86\xef\xf8\x14|Nr\xb9\x80w\x85\xaf\xeeH\xbb\xf2\x12\v\b\x80\x92\xb8Ø\xfe\xff\xff\xff\x01\xb2\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3s\xeeG\x80")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06V\xc2\x06 \xa3\xf7\x97\xa7\xf3\x87:\x90)\\ӊj\xb9\xf7\xfaTJ\x1b\xe7:\xee\xbe\"V\xe0:\x8d(Z\xd6%\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\fc(\xb5/\xfd\x04\x00\xb1\x02\x00\xa0\x06\x01\xaa\x06=\n\x05a.txt\x10\x01\x1a$\n\"\x12 ʗ\x81\x12\xca\x1b\xbd\xca\xfa\xc21\xb3\x9a#\xdcM\xa7\x86\xef\xf8\x14|Nr\xb9\x80w\x85\xaf\xeeH\xbb\xf2\x12\v\b\x80\x92\xb8Ø\xfe\xff\xff\xff\x01\xb2\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3s\xeeG\x80\xca\f\xe8\x03-----BEGIN PGP SIGNATURE-----\n\niQEzBAABCgAdFiEET1Yr+4Y/3GtRtO6IhypRF2zvI64FAmrBIXAACgkQhypRF2zv\nI67BQAf/QrpX2MjY15YGMGkjR5oIhnx/YV96aGYZyZThzb+l/R/N75iVFVkhX21d\nZhQqdCsORrodTPAXic2g2UGVXP9PhNMh7n6Wm3LsvQYjrRQGrQnqtCkut+3tUt8K\n7pt4OAnnwRSieaVImA1COmzxIrQQKNOs6UkgmAstGuPV0XZoeDiSG8TUYJ/vieCn\np5hC0FFXtzfw4NtkxSmkewE0xBxIwFCA/RfSHCGH3m5K+tRz41vMEgGbL1iEp6+V\nuBaoEc4hqCgEt+Af2pA8VHfqeu2vKiwggOpYpaILXZKVqH9+tWHL1EBv9t0vTsYE\n9D57euuR9+kOdngYNPieP1yn5dOSHg==\n=kvgL\n-----END PGP SIGNATURE-----\n\xd2\f(4F562BFB863FDC6B51B4EE88872A51176CEF23AE\xda\f\xb5\a-----BEGIN PGP PUBLIC KEY BLOCK-----\n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\nKoTjp4nxgQGy5WmeBzs4Hdm/\n=TCB8\n-----END PGP PUBLIC KEY BLOCK-----\n")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06!\xc2\x06 \x91\x90*\xa5>\fݐ \x87\xbeaL\xc1\x05?\x0eR\xc18\xa4eՕ\xa95\xb9KʺoZ\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\f-(\xb5/\xfd\x04\x00\x01\x01\x00\xa0\x06\x01\xaa\x06\a\n\x05a.txt\xb2\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3a[k'")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06\x1f\xc2\x06 \x91\x90*\xa5>\fݐ \x87\xbeaL\xc1\x05?\x0eR\xc18\xa4eՕ\xa95\xb9KʺoZ\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\f-(\xb5/\xfd\x04\x00\x01\x01\x00\xa0\x06\x01\xaa\x06\a\n\x05a.txt\xb2\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3a[k'")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06V\xc2\x06 \xa3\xf7\x97\xa7\xf3\x87:\x90)\\ӊj\xb9\xf7\xfaTJ\x1b\xe7:\xee\xbe\"V\xe0:\x8d(Z\xd6%\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\fc(\xb5/\xfd\x04\x00\xb1\x02\x00\xa0\x06\x01\xaa\x06=\n\x05a.txt\x10\x01\x1a$\n\"\x12 ʗ\x81\x12\xca\x1b\xbd\xca\xfa\xc21\xb3\x9a#\xdcM\xa7\x86\xef\xf8\x14|Nr\xb9\x80w\x85\xaf\xeeH\xbb\xf2\x12\v\b\x80\x92\xb8Ø\xfe\xff\xff\xff\x01\xb2\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3s\xeeG")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06V\xc2\x06 ")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAV")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06V\xc2\x06 \xa3\xf7\x97\xa7\xf3\x87:\x90)\\ӊj\xb9\xf7\xfaTJ\x1b\xe7:\xee\xbe\"V\xe0:\x8d(Z\xd6%\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\fc(\xb5/\xfd\x04\x00\xb1\x02\x00\xa0\x06\x01")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFX\xa8\x06\x01\xb0\x06\x01\xb8\x06V\xc2\x06 \xa3\xf7\x97\xa7\xf3\x87:\x90)\\ӊj\xb9\xf7\xfaTJ\x1b\xe7:\xee\xbe\"V\xe0:\x8d(Z\xd6%\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\fc(\xb5/\xfd\x04\x00\xb1\x02\x00\xa0\x06\x01\xaa\x06=\n\x05a.txt\x10\x01\x1a$\n\"\x12 ʗ\x81\x12\xca\x1b\xbd\xca\xfa\xc21\xb3\x9a#\xdcM\xa7\x86\xef\xf8\x14|Nr\xb9\x80w\x85\xaf\xeeH\xbb\xf2\x12\v\b\x80\x92\xb8Ø\xfe\xff\xff\xff\x01\xb2\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3s\xeeG\x80")
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("ZNAVSRFG\xa8\x06\x01\xb0\x06\x01\xb8\x06\x01\xc2\x06 \xd6aQ\xa4\x85\xc0+\xbb\xca\x11\x11\a<\x019\x97\xb3\xbb3\xd0 \xd5U\xfa!\xaeAf<N@\x9d\xca\x06\x10\x93\x85\vpu\x85\xe4\x04\xe4\x95\x1a=\xdc\x1f\x05\xa3\xba\f\x12(\xb5/\xfd\xc0\x00\x00\x00\x00\x00\x02\x00\x00\x00\v\x00\x00\x00")
@@ -0,0 +1,17 @@
+#!/bin/sh
+# script/fuzz: fuzz the manifest parser for one minute. Run by hand only:
+# script/test already runs the committed seed corpus as ordinary tests,
+# and CI never fuzzes. An input that fails is written to
+# mfer/testdata/fuzz/FuzzNewManifestFromReader/; once the parser is fixed,
+# commit it there as a regression seed.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    go test -run '^$' -fuzz '^FuzzNewManifestFromReader$' \
+        -fuzztime 1m -parallel 2 ./mfer
+}
+
+main "$@"