Reject manifests whose file entries decode far larger than their bytes (closes #123)

Before decoding the manifest, the parser adds up what decoding sets
aside for each file entry, hash, timestamp and MIME type, however short
its encoding, and refuses the manifest once that passes 8 times the
decompressed size; manifests mfer writes come to at most about 7.15
times. Empty entries decoded to about 50 times their size: under 1 KB of
manifest allocated about 500 MB. Fields the decoder does not know are
dropped; kept, they took up to 5 times more. A test refuses entries
counted just over 8 times and loads them just under. The fuzz ceiling
rises from 16 to 20 times the input and decompressed data; seeds of
empty entries and of empty hashes fail it without the fix.

Model: opus-5-5

mfer/deserialize_path_test.go

@@ -6,7 +6,10 @@ import (
 	"context"
 	"crypto/sha256"
 	"fmt"
+	"strconv"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/google/uuid"
 	"github.com/klauspost/compress/zstd"
@@ -114,6 +117,113 @@ func TestDeserializeRejectsInvalidEntryPaths(t *testing.T) {
 	}
 }
 
+// Entries of a path, an empty hash, an empty MIME type and empty modification
+// and change times are counted at 416 bytes each (160 + 112 + 16 + 64 + 64)
+// and take 16 bytes plus the path to encode. A 35-character path makes that
+// 51 bytes, about 8.2 times: refused, and leaving any one of the five
+// uncounted, even the MIME type, brings it under 8. A 37-character path makes
+// it 53 bytes, about 7.8 times: loaded.
+func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		pathLen int
+		refused bool
+	}{
+		{35, true},
+		{37, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(strconv.Itoa(tt.pathLen), func(t *testing.T) {
+			t.Parallel()
+
+			entry := protowire.AppendTag(nil, 1, protowire.BytesType) // MFFilePath.path
+			entry = protowire.AppendString(entry, strings.Repeat("a", tt.pathLen))
+			entry = protowire.AppendTag(entry, 3, protowire.BytesType) // MFFilePath.hashes
+			entry = protowire.AppendBytes(entry, nil)
+			entry = protowire.AppendTag(entry, 301, protowire.BytesType) // MFFilePath.mimeType
+			entry = protowire.AppendBytes(entry, nil)
+			entry = protowire.AppendTag(entry, 302, protowire.BytesType) // MFFilePath.mtime
+			entry = protowire.AppendBytes(entry, nil)
+			entry = protowire.AppendTag(entry, 303, protowire.BytesType) // MFFilePath.ctime
+			entry = protowire.AppendBytes(entry, nil)
+
+			id := uuid.New()
+			inner := protowire.AppendTag(nil, 102, protowire.BytesType) // MFFile.uuid
+			inner = protowire.AppendBytes(inner, id[:])
+
+			for range 1000 {
+				inner = protowire.AppendTag(inner, 101, protowire.BytesType) // MFFile.files
+				inner = protowire.AppendBytes(inner, entry)
+			}
+
+			_, err := NewManifestFromReader(bytes.NewReader(wrapInner(t, id, inner)))
+			if tt.refused {
+				require.ErrorIs(t, err, errDecodedTooLarge)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}
+
+// Fields the decoder does not know are dropped, in the outer message, the inner
+// message and a file entry, so that they take no memory once loaded.
+func TestDeserializeDropsUnknownFields(t *testing.T) {
+	t.Parallel()
+
+	unknown := protowire.AppendTag(nil, 99, protowire.BytesType) // in no message
+	unknown = protowire.AppendBytes(unknown, []byte("not known"))
+
+	entry := protowire.AppendTag(nil, 1, protowire.BytesType) // MFFilePath.path
+	entry = protowire.AppendString(entry, "a")
+	entry = append(entry, unknown...)
+
+	id := uuid.New()
+	inner := protowire.AppendTag(nil, 101, protowire.BytesType) // MFFile.files
+	inner = protowire.AppendBytes(inner, entry)
+	inner = protowire.AppendTag(inner, 102, protowire.BytesType) // MFFile.uuid
+	inner = protowire.AppendBytes(inner, id[:])
+	inner = append(inner, unknown...)
+
+	data := wrapInner(t, id, inner)
+	data = append(data, unknown...) // the outer message ends the file
+
+	m, err := NewManifestFromReader(bytes.NewReader(data))
+	require.NoError(t, err)
+	require.Len(t, m.Files(), 1)
+	assert.Empty(t, m.pbOuter.ProtoReflect().GetUnknown())
+	assert.Empty(t, m.pbInner.ProtoReflect().GetUnknown())
+	assert.Empty(t, m.Files()[0].ProtoReflect().GetUnknown())
+}
+
+// Many empty files with names of at most three characters and modification
+// times at the epoch make about the densest manifest mfer writes: it takes
+// about 7 times its size to decode, and still loads. A signature would not
+// change the inner message, so none is added.
+func TestDeserializeLoadsDensestManifest(t *testing.T) {
+	t.Parallel()
+
+	hash := make([]byte, 34) // multihash: 2-byte prefix + 32-byte SHA-256
+
+	b := NewBuilder()
+	b.SetIncludeTimestamps(true)
+
+	const files = 10000
+	for i := range files {
+		name := RelFilePath(strconv.FormatInt(int64(i), 36))
+		require.NoError(t, b.AddFileWithHash(name, 0, ModTime(time.Unix(0, 0)), hash))
+	}
+
+	var buf bytes.Buffer
+	require.NoError(t, b.Build(context.Background(), &buf))
+
+	m, err := NewManifestFromReader(&buf)
+	require.NoError(t, err)
+	assert.Len(t, m.Files(), files)
+}
+
 func TestDeserializeValidManifestRoundTrips(t *testing.T) {
 	t.Parallel()