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fix(transcoding): place -ss before -i for fast input seeking (#5492)

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Move the ffmpeg -ss (seek/offset) parameter before -i in all transcoding
commands so ffmpeg uses input seeking instead of output seeking. Per the
ffmpeg docs, placing -ss before -i seeks at the demuxer level by keyframe
(very fast), and since FFmpeg 2.1 it is also frame-accurate when
transcoding. The previous placement after -i caused ffmpeg to decode and
discard all audio up to the seek point, which was unnecessarily slow —
especially problematic for lengthy files (4+ hours).

Both code paths are updated: buildDynamicArgs (for default formats) and
createFFmpegCommand (for custom templates without %t). A database
migration updates existing default commands in the transcoding table.
This commit is contained in:
Deluan Quintão
2026-05-13 17:17:20 -03:00
committed by GitHub
parent 2b3b879c57
commit 24e526e09a
5 changed files with 86 additions and 19 deletions
Download Patch File Download Diff File Expand all files Collapse all files
consts/consts.go
+4 -4
View File
@@ -153,25 +153,25 @@ var (
Name: "mp3 audio",
TargetFormat: "mp3",
DefaultBitRate: 192,
Command: "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -f mp3 -",
Command: "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -f mp3 -",
},
{
Name: "opus audio",
TargetFormat: "opus",
DefaultBitRate: 128,
Command: "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -",
Command: "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -",
},
{
Name: "aac audio",
TargetFormat: "aac",
DefaultBitRate: 256,
Command: "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -",
Command: "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -",
},
{
Name: "flac audio",
TargetFormat: "flac",
DefaultBitRate: 0,
Command: "ffmpeg -i %s -ss %t -map 0:a:0 -v 0 -c:a flac -f flac -",
Command: "ffmpeg -ss %t -i %s -map 0:a:0 -v 0 -c:a flac -f flac -",
},
}
)
core/ffmpeg/ffmpeg.go
+15 -4
View File
@@ -10,6 +10,7 @@ import (
"os"
"os/exec"
"path/filepath"
"slices"
"strconv"
"strings"
"sync"
@@ -394,12 +395,13 @@ func isDefaultCommand(format, command string) bool {
// including all transcoding parameters (bitrate, sample rate, channels).
func buildDynamicArgs(opts TranscodeOptions) []string {
cmdPath, _ := ffmpegCmd()
args := []string{cmdPath, "-i", opts.FilePath}
args := []string{cmdPath}
if opts.Offset > 0 {
args = append(args, "-ss", strconv.Itoa(opts.Offset))
}
args = append(args, "-i", opts.FilePath)
args = append(args, "-map", "0:a:0")
if codec, ok := formatCodecMap[opts.Format]; ok {
@@ -491,11 +493,20 @@ func createFFmpegCommand(cmd, path string, maxBitRate, offset int) []string {
var args []string
for _, s := range fixCmd(cmd) {
if strings.Contains(s, "%s") {
if offset > 0 && !strings.Contains(cmd, "%t") {
// Pre-input seeking: ffmpeg seeks at the demuxer level (fast)
// instead of decoding all frames up to the offset (slow).
insertAt := len(args)
for i := len(args) - 1; i >= 0; i-- {
if args[i] == "-i" {
insertAt = i
break
}
}
args = slices.Insert(args, insertAt, "-ss", strconv.Itoa(offset))
}
s = strings.ReplaceAll(s, "%s", path)
args = append(args, s)
if offset > 0 && !strings.Contains(cmd, "%t") {
args = append(args, "-ss", strconv.Itoa(offset))
}
} else {
s = strings.ReplaceAll(s, "%t", strconv.Itoa(offset))
s = strings.ReplaceAll(s, "%b", strconv.Itoa(maxBitRate))
core/ffmpeg/ffmpeg_test.go
+10 -9
View File
@@ -47,15 +47,15 @@ var _ = Describe("ffmpeg", func() {
})
Context("when command has time offset param", func() {
It("creates a valid command line with offset", func() {
args := createFFmpegCommand("ffmpeg -i %s -b:a %bk -ss %t mp3 -", "/music library/file.mp3", 123, 456)
Expect(args).To(Equal([]string{"ffmpeg", "-i", "/music library/file.mp3", "-b:a", "123k", "-ss", "456", "mp3", "-"}))
args := createFFmpegCommand("ffmpeg -ss %t -i %s -b:a %bk mp3 -", "/music library/file.mp3", 123, 456)
Expect(args).To(Equal([]string{"ffmpeg", "-ss", "456", "-i", "/music library/file.mp3", "-b:a", "123k", "mp3", "-"}))
})
})
Context("when command does not have time offset param", func() {
It("adds time offset after the input file name", func() {
It("adds time offset before the input file name", func() {
args := createFFmpegCommand("ffmpeg -i %s -b:a %bk mp3 -", "/music library/file.mp3", 123, 456)
Expect(args).To(Equal([]string{"ffmpeg", "-i", "/music library/file.mp3", "-ss", "456", "-b:a", "123k", "mp3", "-"}))
Expect(args).To(Equal([]string{"ffmpeg", "-ss", "456", "-i", "/music library/file.mp3", "-b:a", "123k", "mp3", "-"}))
})
})
})
@@ -82,16 +82,16 @@ var _ = Describe("ffmpeg", func() {
Describe("isDefaultCommand", func() {
It("returns true for known default mp3 command", func() {
Expect(isDefaultCommand("mp3", "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -f mp3 -")).To(BeTrue())
Expect(isDefaultCommand("mp3", "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -f mp3 -")).To(BeTrue())
})
It("returns true for known default opus command", func() {
Expect(isDefaultCommand("opus", "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -")).To(BeTrue())
Expect(isDefaultCommand("opus", "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -")).To(BeTrue())
})
It("returns true for known default aac command", func() {
Expect(isDefaultCommand("aac", "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -")).To(BeTrue())
Expect(isDefaultCommand("aac", "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -")).To(BeTrue())
})
It("returns true for known default flac command", func() {
Expect(isDefaultCommand("flac", "ffmpeg -i %s -ss %t -map 0:a:0 -v 0 -c:a flac -f flac -")).To(BeTrue())
Expect(isDefaultCommand("flac", "ffmpeg -ss %t -i %s -map 0:a:0 -v 0 -c:a flac -f flac -")).To(BeTrue())
})
It("returns false for a custom command", func() {
Expect(isDefaultCommand("mp3", "ffmpeg -i %s -b:a %bk -custom-flag -f mp3 -")).To(BeFalse())
@@ -165,8 +165,9 @@ var _ = Describe("ffmpeg", func() {
Offset: 30,
})
Expect(args).To(Equal([]string{
"ffmpeg", "-i", "/music/file.mp3",
"ffmpeg",
"-ss", "30",
"-i", "/music/file.mp3",
"-map", "0:a:0",
"-c:a", "libmp3lame",
"-b:a", "192k",
db/migrations/20260513173954_move_ss_before_input.go
+55
View File
@@ -0,0 +1,55 @@
package migrations
import (
"context"
"database/sql"
"github.com/pressly/goose/v3"
)
func init() {
goose.AddMigrationContext(upMoveSsBeforeInput, downMoveSsBeforeInput)
}
// ssSeekPairs maps old commands (output seeking) to new commands (input seeking).
// Index 0 = old (after -i), index 1 = new (before -i).
var ssSeekPairs = [][2]string{
{
"ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -f mp3 -",
"ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -f mp3 -",
},
{
"ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -",
"ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -",
},
{
"ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -",
"ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -",
},
{
"ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f ipod -movflags frag_keyframe+empty_moov -",
"ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a aac -f ipod -movflags frag_keyframe+empty_moov -",
},
{
"ffmpeg -i %s -ss %t -map 0:a:0 -v 0 -c:a flac -f flac -",
"ffmpeg -ss %t -i %s -map 0:a:0 -v 0 -c:a flac -f flac -",
},
}
func upMoveSsBeforeInput(_ context.Context, tx *sql.Tx) error {
for _, p := range ssSeekPairs {
if _, err := tx.Exec(`UPDATE transcoding SET command = ? WHERE command = ?`, p[1], p[0]); err != nil {
return err
}
}
return nil
}
func downMoveSsBeforeInput(_ context.Context, tx *sql.Tx) error {
for _, p := range ssSeekPairs {
if _, err := tx.Exec(`UPDATE transcoding SET command = ? WHERE command = ?`, p[0], p[1]); err != nil {
return err
}
}
return nil
}
tests/mock_transcoding_repo.go
+2 -2
View File
@@ -19,9 +19,9 @@ func (m *MockTranscodingRepo) FindByFormat(format string) (*model.Transcoding, e
case "opus":
return &model.Transcoding{ID: "opus1", TargetFormat: "opus", DefaultBitRate: 96}, nil
case "flac":
return &model.Transcoding{ID: "flac1", TargetFormat: "flac", DefaultBitRate: 0, Command: "ffmpeg -i %s -ss %t -map 0:a:0 -v 0 -c:a flac -f flac -"}, nil
return &model.Transcoding{ID: "flac1", TargetFormat: "flac", DefaultBitRate: 0, Command: "ffmpeg -ss %t -i %s -map 0:a:0 -v 0 -c:a flac -f flac -"}, nil
case "aac":
return &model.Transcoding{ID: "aac1", TargetFormat: "aac", DefaultBitRate: 256, Command: "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f ipod -movflags frag_keyframe+empty_moov -"}, nil
return &model.Transcoding{ID: "aac1", TargetFormat: "aac", DefaultBitRate: 256, Command: "ffmpeg -ss %t -i %s -map 0:a:0 -b:a %bk -v 0 -c:a aac -f ipod -movflags frag_keyframe+empty_moov -"}, nil
default:
return nil, model.ErrNotFound
}