fixed common track scanning bugs with custom workarounds

backend/metadata/flacduration.go

@@ -0,0 +1,148 @@
+package metadata
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"os"
+)
+
+// errInvalidFLACSignature is returned when the file does not contain
+// a valid FLAC stream signature ("fLaC") at the expected position.
+var errInvalidFLACSignature = errors.New(
+	"invalid FLAC signature",
+)
+
+// errInvalidStreamInfo is returned when the first metadata block is
+// not a StreamInfo block or has an unexpected length.
+var errInvalidStreamInfo = errors.New(
+	"invalid StreamInfo metadata block",
+)
+
+// errZeroSampleRate is returned when the StreamInfo block reports a
+// sample rate of zero, which would cause a division by zero.
+var errZeroSampleRate = errors.New(
+	"FLAC StreamInfo sample rate is zero",
+)
+
+// flacSignatureBytes is the four-byte marker that begins every FLAC
+// stream.
+var flacSignatureBytes = [4]byte{'f', 'L', 'a', 'C'}
+
+// streamInfoLength is the fixed size of a FLAC StreamInfo body in
+// bytes.
+const streamInfoLength = 34
+
+// streamInfoBlockType is the metadata block type for StreamInfo.
+const streamInfoBlockType = 0
+
+// getFlacDuration computes the duration of a FLAC file in
+// milliseconds by reading only the StreamInfo metadata block header.
+// It handles an optional prepended ID3v2 tag by seeking past it.
+//
+// This replaces the previous beep/mewkiz-flac decode path which has
+// a bug in its ID3v2 skip logic (bufio over bufseekio causes a
+// position overshoot).
+//
+// The file position is undefined after this call.
+//
+//nolint:mnd // byte offsets and bit shifts from the FLAC spec.
+func getFlacDuration(f *os.File) (int64, error) {
+	audioStart, err := skipID3v2(f)
+	if err != nil {
+		return 0, fmt.Errorf("skipping ID3v2: %w", err)
+	}
+
+	// Read the 4-byte FLAC signature.
+	var sig [4]byte
+
+	if _, err := f.ReadAt(sig[:], audioStart); err != nil {
+		return 0, fmt.Errorf(
+			"reading FLAC signature: %w", err,
+		)
+	}
+
+	if sig != flacSignatureBytes {
+		return 0, fmt.Errorf(
+			"%w: expected %q, got %q",
+			errInvalidFLACSignature, flacSignatureBytes, sig,
+		)
+	}
+
+	// Read the metadata block header (4 bytes) immediately after
+	// the signature.
+	var mbh [4]byte
+
+	if _, err := f.ReadAt(
+		mbh[:], audioStart+4,
+	); err != nil {
+		return 0, fmt.Errorf(
+			"reading metadata block header: %w", err,
+		)
+	}
+
+	blockType := mbh[0] & 0x7F
+
+	blockLen := int64(mbh[1])<<16 |
+		int64(mbh[2])<<8 |
+		int64(mbh[3])
+
+	if blockType != streamInfoBlockType ||
+		blockLen != streamInfoLength {
+		return 0, fmt.Errorf(
+			"%w: type=%d, length=%d",
+			errInvalidStreamInfo, blockType, blockLen,
+		)
+	}
+
+	// Read the 34-byte StreamInfo body.
+	var si [streamInfoLength]byte
+
+	if _, err := f.ReadAt(
+		si[:], audioStart+8,
+	); err != nil {
+		return 0, fmt.Errorf(
+			"reading StreamInfo block: %w", err,
+		)
+	}
+
+	sampleRate, totalSamples := parseFlacStreamInfo(si)
+
+	if sampleRate == 0 {
+		return 0, errZeroSampleRate
+	}
+
+	durationMS := int64(totalSamples) * 1000 /
+		int64(sampleRate)
+
+	return durationMS, nil
+}
+
+// parseFlacStreamInfo extracts the sample rate (20 bits) and total
+// sample count (36 bits) from a 34-byte FLAC StreamInfo body.
+//
+// StreamInfo layout (bytes 10-17 contain the fields we need):
+//
+//	bits  0-19:  sample rate in Hz     (20 bits)
+//	bits 20-22:  number of channels -1 (3 bits, unused here)
+//	bits 23-27:  bits per sample -1    (5 bits, unused here)
+//	bits 28-63:  total samples         (36 bits)
+//
+//nolint:mnd // bit offsets from the FLAC spec.
+func parseFlacStreamInfo(
+	si [streamInfoLength]byte,
+) (sampleRate uint32, totalSamples uint64) {
+	// Bytes 10-13 packed as big-endian uint32 contain sample rate
+	// in the upper 20 bits.
+	packed := binary.BigEndian.Uint32(si[10:14])
+	sampleRate = packed >> 12
+
+	// Total samples: 4 low bits of byte 13, then bytes 14-17.
+	totalSamples = uint64(si[13]&0x0F)<<32 |
+		uint64(si[14])<<24 |
+		uint64(si[15])<<16 |
+		uint64(si[16])<<8 |
+		uint64(si[17])
+
+	return sampleRate, totalSamples
+}