CidFromReader should not wrap valid EOF return.

cid.go

@@ -717,6 +717,9 @@ func (r *bufByteReader) ReadByte() (byte, error) {
 // It's recommended to supply a reader that buffers and implements io.ByteReader,
 // as CidFromReader has to do many single-byte reads to decode varints.
 // If the argument only implements io.Reader, single-byte Read calls are used instead.
+//
+// If the Reader is found to yield zero bytes, an io.EOF error is returned directly, in all
+// other error cases, an ErrInvalidCid, wrapping the original error, is returned.
 func CidFromReader(r io.Reader) (int, Cid, error) {
 	// 64 bytes is enough for any CIDv0,
 	// and it's enough for most CIDv1s in practice.
@@ -727,6 +730,11 @@ func CidFromReader(r io.Reader) (int, Cid, error) {
 	// The varint package wants a io.ByteReader, so we must wrap our io.Reader.
 	vers, err := varint.ReadUvarint(br)
 	if err != nil {
+		if err == io.EOF {
+			// First-byte read in ReadUvarint errors with io.EOF, so reader has no data.
+			// Subsequent reads with an EOF will return io.ErrUnexpectedEOF and be wrapped here.
+			return 0, Undef, err
+		}
 		return len(br.dst), Undef, ErrInvalidCid{err}
 	}
 

cid_test.go

@@ -783,6 +783,30 @@ func TestBadCidInput(t *testing.T) {
 	}
 }
 
+func TestFromReaderNoData(t *testing.T) {
+	// Reading no data from io.Reader should return io.EOF, not ErrInvalidCid.
+	n, cid, err := CidFromReader(bytes.NewReader(nil))
+	if err != io.EOF {
+		t.Fatal("Expected io.EOF error")
+	}
+	if cid != Undef {
+		t.Fatal("Expected Undef CID")
+	}
+	if n != 0 {
+		t.Fatal("Expected 0 data")
+	}
+
+	// Read byte indicatiing more data to and check error is ErrInvalidCid.
+	_, _, err = CidFromReader(bytes.NewReader([]byte{0x80}))
+	if !errors.Is(err, ErrInvalidCid{}) {
+		t.Fatal("Expected ErrInvalidCid error")
+	}
+	// Check for expected wrapped error.
+	if !errors.Is(err, io.ErrUnexpectedEOF) {
+		t.Fatal("Expected error", io.ErrUnexpectedEOF)
+	}
+}
+
 func TestBadParse(t *testing.T) {
 	hash, err := mh.Sum([]byte("foobar"), mh.SHA3_256, -1)
 	if err != nil {