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package main
import (
"bytes"
"encoding/binary"
"encoding/json"
"fmt"
"io"
"os"
"os/exec"
"strconv"
"strings"
)
// The transcript track is identified by the MIME type in its `mett` sample
// entry. This is the string Google Recorder writes; the trailing digit is a
// format version, so it is matched by prefix in case a later version bumps it.
const transcriptMIMEPrefix = "application/transcription"
type probeResult struct {
DurationMs int64
Bytes int64
AudioCodec string
// TranscriptStream is the ffmpeg stream index of the transcription track,
// or -1 when the file has none.
TranscriptStream int
// DataMIMEs maps stream index to the MIME type of each data track, for
// diagnostics when the expected track is missing.
DataMIMEs map[int]string
}
type ffprobeOutput struct {
Streams []struct {
Index int `json:"index"`
CodecType string `json:"codec_type"`
CodecName string `json:"codec_name"`
} `json:"streams"`
Format struct {
Duration string `json:"duration"`
Size string `json:"size"`
} `json:"format"`
}
// probe inspects a recording. Duration and size come from ffprobe; the
// transcript track has to be found by parsing the container directly.
//
// ffprobe is not able to identify it: all three of the recorder's data tracks
// appear as `codec_tag=mett, handler_name=Meta` with no distinguishing field,
// and the MIME type that actually names them lives in the `mett` sample entry,
// which ffprobe does not surface. Picking "the first data track" would work
// today and break silently the day the recorder reorders them, so the boxes
// are read instead.
func probe(ffprobeBin, path string) (*probeResult, error) {
cmd := exec.Command(ffprobeBin,
"-v", "error",
"-print_format", "json",
"-show_format",
"-show_streams",
path,
)
var out, stderr bytes.Buffer
cmd.Stdout = &out
cmd.Stderr = &stderr
if err := cmd.Run(); err != nil {
return nil, fmt.Errorf("ffprobe %s: %w: %s", path, err, strings.TrimSpace(stderr.String()))
}
var fo ffprobeOutput
if err := json.Unmarshal(out.Bytes(), &fo); err != nil {
return nil, fmt.Errorf("parsing ffprobe output: %w", err)
}
res := &probeResult{TranscriptStream: -1, DataMIMEs: map[int]string{}}
secs, err := strconv.ParseFloat(fo.Format.Duration, 64)
if err != nil {
return nil, fmt.Errorf("ffprobe reported no usable duration (%q)", fo.Format.Duration)
}
res.DurationMs = int64(secs * 1000)
res.Bytes, err = strconv.ParseInt(fo.Format.Size, 10, 64)
if err != nil {
return nil, fmt.Errorf("ffprobe reported no usable size (%q)", fo.Format.Size)
}
for _, s := range fo.Streams {
if s.CodecType == "audio" && res.AudioCodec == "" {
res.AudioCodec = s.CodecName
}
}
// Track order in the container is the order ffmpeg assigns stream
// indices, so the nth track is stream n.
mimes, err := trackMIMEs(path)
if err != nil {
return nil, err
}
for i, m := range mimes {
if m == "" {
continue
}
res.DataMIMEs[i] = m
if strings.HasPrefix(m, transcriptMIMEPrefix) && res.TranscriptStream < 0 {
res.TranscriptStream = i
}
}
return res, nil
}
// trackMIMEs returns, per track index, the MIME type declared in a `mett`
// sample entry, or "" for tracks that are not timed metadata.
func trackMIMEs(path string) ([]string, error) {
f, err := os.Open(path)
if err != nil {
return nil, err
}
defer f.Close()
moov, err := findBox(f, "moov")
if err != nil {
return nil, fmt.Errorf("%s: %w", path, err)
}
var mimes []string
for _, trak := range childBoxes(moov, "trak") {
mime := ""
for _, mdia := range childBoxes(trak, "mdia") {
for _, minf := range childBoxes(mdia, "minf") {
for _, stbl := range childBoxes(minf, "stbl") {
for _, stsd := range childBoxes(stbl, "stsd") {
// stsd: 4 bytes version/flags, 4 bytes entry count,
// then sample entries.
if len(stsd) < 8 {
continue
}
for _, mett := range childBoxes(stsd[8:], "mett") {
// MetaDataSampleEntry: 6 reserved bytes,
// 2 data_reference_index, then an optional
// content_encoding string and the mime_format
// string, both NUL terminated.
if len(mett) < 8 {
continue
}
parts := bytes.Split(mett[8:], []byte{0})
for _, p := range parts {
if bytes.Contains(p, []byte("/")) {
mime = string(p)
break
}
}
}
}
}
}
}
mimes = append(mimes, mime)
}
return mimes, nil
}
// findBox scans top level boxes for one of the given type and returns its
// payload. Only the top level is scanned, which is all that is needed to
// reach `moov`.
func findBox(r io.ReadSeeker, want string) ([]byte, error) {
if _, err := r.Seek(0, io.SeekStart); err != nil {
return nil, err
}
var hdr [8]byte
for {
if _, err := io.ReadFull(r, hdr[:]); err != nil {
if err == io.EOF || err == io.ErrUnexpectedEOF {
return nil, fmt.Errorf("no %s box found (not an MP4?)", want)
}
return nil, err
}
size := int64(binary.BigEndian.Uint32(hdr[0:4]))
typ := string(hdr[4:8])
switch size {
case 0:
// Extends to end of file.
if typ != want {
return nil, fmt.Errorf("no %s box found", want)
}
return io.ReadAll(r)
case 1:
// 64 bit size follows the header.
var ext [8]byte
if _, err := io.ReadFull(r, ext[:]); err != nil {
return nil, err
}
size = int64(binary.BigEndian.Uint64(ext[:])) - 16
if size < 0 {
return nil, fmt.Errorf("invalid 64 bit box size in %s", typ)
}
if typ == want {
buf := make([]byte, size)
_, err := io.ReadFull(r, buf)
return buf, err
}
if _, err := r.Seek(size, io.SeekCurrent); err != nil {
return nil, err
}
default:
if size < 8 {
return nil, fmt.Errorf("invalid box size %d for %s", size, typ)
}
if typ == want {
buf := make([]byte, size-8)
_, err := io.ReadFull(r, buf)
return buf, err
}
if _, err := r.Seek(size-8, io.SeekCurrent); err != nil {
return nil, err
}
}
}
}
// childBoxes returns the payloads of every immediate child box of the given
// type. Malformed input yields no children rather than an error: this is a
// best effort read of a file we did not write, and the caller treats "no
// transcript track" as a condition to report, not a crash.
func childBoxes(buf []byte, want string) [][]byte {
var out [][]byte
for off := 0; off+8 <= len(buf); {
size := int(binary.BigEndian.Uint32(buf[off : off+4]))
typ := string(buf[off+4 : off+8])
if size < 8 || off+size > len(buf) {
break
}
if typ == want {
out = append(out, buf[off+8:off+size])
}
off += size
}
return out
}
// extractStream copies one stream out of the container without re-encoding.
func extractStream(ffmpegBin, path string, index int) ([]byte, error) {
cmd := exec.Command(ffmpegBin,
"-v", "error",
"-i", path,
"-map", fmt.Sprintf("0:%d", index),
"-c", "copy",
"-f", "data", "-",
)
var out, stderr bytes.Buffer
cmd.Stdout = &out
cmd.Stderr = &stderr
if err := cmd.Run(); err != nil {
return nil, fmt.Errorf("ffmpeg extracting stream %d: %w: %s", index, err, strings.TrimSpace(stderr.String()))
}
return out.Bytes(), nil
}
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