Refuse a path listed twice in a manifest (closes #170)

gen given arguments whose files share a path, such as gen a b with a.txt
in both, or gen . ., now fails while listing the files, before hashing
any, naming the path and both files. Builder.AddFile and
Builder.AddFileWithHash refuse a path already added. Loading refuses a
manifest that lists a path twice, compared byte for byte; fetch keeps
its own letter-case check. The Path Rules in docs/FORMAT.md say each
path appears at most once. The decode-size test listed one path 1000
times; each entry now has its own path of the same length.

Model: opus-5-5

mfer/builder.go

@@ -38,6 +38,7 @@ var (
 	errNegativeSize     = errors.New("size cannot be negative")
 	errHashNotMultihash = errors.New("hash is not a valid multihash")
 	errHashTooShort     = errors.New("hash digest is too short")
+	errDuplicatePath    = errors.New("duplicate path")
 )
 
 // ValidatePath checks that a file path conforms to manifest path invariants:
@@ -115,6 +116,7 @@ type FileHashProgress struct {
 type Builder struct {
 	mu                sync.Mutex
 	files             []*MFFilePath
+	paths             map[string]bool // the path of each entry in files
 	createdAt         time.Time
 	includeTimestamps bool
 	signingOptions    *SigningOptions
@@ -125,6 +127,7 @@ type Builder struct {
 func NewBuilder() *Builder {
 	return &Builder{
 		files:     make([]*MFFilePath, 0),
+		paths:     make(map[string]bool),
 		createdAt: time.Now(),
 	}
 }
@@ -138,6 +141,7 @@ func (b *Builder) SetSeed(seed string) {
 }
 
 // AddFile reads file content from reader, computes hashes, and adds to manifest.
+// A path already added is refused once the file is read.
 // Only mode's permission bits (mode.Perm()) are recorded; 0 records none.
 // Progress updates are sent to the progress channel (if non-nil) without blocking.
 // Returns the number of bytes read.
@@ -204,11 +208,7 @@ func (b *Builder) AddFile(
 		Mode:  uint32(mode.Perm()),
 	}
 
-	b.mu.Lock()
-	b.files = append(b.files, entry)
-	b.mu.Unlock()
-
-	return totalRead, nil
+	return totalRead, b.addEntry(entry)
 }
 
 // sendFileHashProgress sends a progress update without blocking.
@@ -234,8 +234,9 @@ func (b *Builder) FileCount() int {
 // AddFileWithHash adds a file entry with a pre-computed hash.
 // This is useful when the hash is already known (e.g., from an existing manifest).
 // Only mode's permission bits (mode.Perm()) are recorded; 0 records none.
-// Returns an error if path is invalid, size is negative, or hash is not a
-// multihash with a digest of at least 32 bytes, as long as SHA-256's.
+// Returns an error if path is invalid or already added, size is negative,
+// or hash is not a multihash with a digest of at least 32 bytes, as long
+// as SHA-256's.
 func (b *Builder) AddFileWithHash(
 	path RelFilePath,
 	size FileSize,
@@ -277,11 +278,7 @@ func (b *Builder) AddFileWithHash(
 		Mode:  uint32(mode.Perm()),
 	}
 
-	b.mu.Lock()
-	b.files = append(b.files, entry)
-	b.mu.Unlock()
-
-	return nil
+	return b.addEntry(entry)
 }
 
 // SetIncludeTimestamps controls whether the manifest includes a createdAt timestamp.
@@ -349,3 +346,19 @@ func (b *Builder) Build(ctx context.Context, w io.Writer) error {
 
 	return nil
 }
+
+// addEntry adds entry to the manifest unless an entry with its path is
+// already there.
+func (b *Builder) addEntry(entry *MFFilePath) error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	if b.paths[entry.GetPath()] {
+		return fmt.Errorf("%w %q", errDuplicatePath, entry.GetPath())
+	}
+
+	b.paths[entry.GetPath()] = true
+	b.files = append(b.files, entry)
+
+	return nil
+}