add ffprobe tag reader for webm, mp4, mkv

This is not well-tested tag reader. So FFProbe.Read() only success in
following condition:
- FFProbeScore == 100

cmd/gonic/gonic.go

@@ -45,6 +45,7 @@ import (
 	"go.senan.xyz/gonic/scrobble"
 	"go.senan.xyz/gonic/server/ctrladmin"
 	"go.senan.xyz/gonic/server/ctrlsubsonic"
+	"go.senan.xyz/gonic/tags/ffprobe"
 	"go.senan.xyz/gonic/tags/tagcommon"
 	"go.senan.xyz/gonic/tags/taglib"
 	"go.senan.xyz/gonic/transcode"
@@ -185,7 +186,7 @@ func main() {
 
 	tagReader := tagcommon.ChainReader{
 		taglib.TagLib{},
-		// ffprobe reader?
+		ffprobe.FFProbe{},
 		// nfo reader?
 	}
 

tags/ffprobe/errors.go

@@ -0,0 +1,9 @@
+package ffprobe
+
+import "errors"
+
+var (
+	ErrNoMediaStreams        = errors.New("no media streams")
+	ErrNoMediaDuration       = errors.New("no media duration")
+	ErrFFProbeScroeNotEnough = errors.New("ffprobe score not enough")
+)

tags/ffprobe/ffprobe.go

@@ -0,0 +1,97 @@
+package ffprobe
+
+import (
+	"bytes"
+	"encoding/json"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"go.senan.xyz/gonic/tags/tagcommon"
+)
+
+type FFProbe struct{}
+
+func (FFProbe) CanRead(absPath string) bool {
+	switch ext := strings.ToLower(filepath.Ext(absPath)); ext {
+	case ".webm", ".mp4", ".mkv":
+		return true
+	}
+	return false
+}
+
+func (FFProbe) Read(absPath string) (tagcommon.Info, error) {
+	cmd := exec.Command(
+		"ffprobe",
+		"-v", "quiet",
+		"-print_format", "json",
+		"-show_format",
+		"-show_streams",
+	)
+	stdout := bytes.NewBuffer(make([]byte, 0, 1024))
+	stderr := bytes.NewBuffer(make([]byte, 0, 1024))
+	cmd.Stdout = stdout
+	cmd.Stderr = stderr
+	cmd.Args = append(cmd.Args, absPath)
+	if err := cmd.Run(); err != nil {
+		return nil, err
+	}
+
+	var mi MediaInfo
+	if err := json.NewDecoder(stdout).Decode(&mi); err != nil {
+		return nil, err
+	}
+
+	if len(mi.Streams) == 0 {
+		return nil, ErrNoMediaStreams
+	}
+
+	if mi.Format.ProbeScore < 100 {
+		return nil, ErrFFProbeScroeNotEnough
+	}
+
+	if mi.Format.Duration == "" {
+		return nil, ErrNoMediaDuration
+	}
+
+	ret := &info{absPath, mi}
+
+	return ret, nil
+}
+
+type info struct {
+	abspath   string
+	mediaInfo MediaInfo
+}
+
+func (i *info) Title() string          { return "" }
+func (i *info) BrainzID() string       { return "" }
+func (i *info) Artist() string         { return tagcommon.FallbackArtist }
+func (i *info) Artists() []string      { return []string{tagcommon.FallbackArtist} }
+func (i *info) Album() string          { return "" }
+func (i *info) AlbumArtist() string    { return "" }
+func (i *info) AlbumArtists() []string { return []string{} }
+func (i *info) AlbumBrainzID() string  { return "" }
+func (i *info) Genre() string          { return tagcommon.FallbackGenre }
+func (i *info) Genres() []string       { return []string{tagcommon.FallbackGenre} }
+func (i *info) TrackNumber() int       { return 0 }
+func (i *info) DiscNumber() int        { return 0 }
+func (i *info) Year() int              { return 0 }
+
+func (i *info) ReplayGainTrackGain() float32 { return 0 }
+func (i *info) ReplayGainTrackPeak() float32 { return 0 }
+func (i *info) ReplayGainAlbumGain() float32 { return 0 }
+func (i *info) ReplayGainAlbumPeak() float32 { return 0 }
+
+func (i *info) Length() int {
+	ret, _ := strconv.ParseFloat(i.mediaInfo.Format.Duration, 64)
+	return int(ret)
+}
+
+func (i *info) Bitrate() int {
+	ret, _ := strconv.Atoi(i.mediaInfo.Format.BitRate)
+	return ret
+}
+
+func (i *info) AbsPath() string { return i.abspath }

tags/ffprobe/ffprobe_output_struct.go

@@ -0,0 +1,27 @@
+package ffprobe
+
+type Stream struct {
+	Index         int    `json:"index"`
+	CodecName     string `json:"codec_name"`
+	CodecLongName string `json:"codec_long_name"`
+	CodecType     string `json:"codec_type"`
+	SampleRate    string `json:"sample_rate"`
+	Channels      int    `json:"channels"`
+	ChannelLayout string `json:"channel_layout"`
+}
+
+type Format struct {
+	Filename       string `json:"filename"`
+	NbStreams      int    `json:"nb_streams"`
+	FormatName     string `json:"format_name"`
+	FormatLongName string `json:"format_long_name"`
+	Duration       string `json:"duration"`
+	Size           string `json:"size"`
+	BitRate        string `json:"bit_rate"`
+	ProbeScore     int    `json:"probe_score"`
+}
+
+type MediaInfo struct {
+	Streams []Stream `json:"streams"`
+	Format  Format   `json:"format"`
+}