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fix(stream): retry thumbnail extraction with output-seek on seek-index failure

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Fast input seek (-ss before -i) fails on files whose seek index is imprecise
or mildly corrupt: the demuxer lands mid-EBML element ("invalid as first byte
of an EBML number") and decodes no frame, so the web scrubber showed a broken
image (2026-06-03, anime MKVs: 15/15 prewarm thumbnails failed). When the fast
path yields no frame, retry once with output seek (-ss after -i, decode from
the start) + -err_detect ignore_err. Applied in both the on-demand handler
(buildThumbnailArgsAccurate) and the prewarm extractor (ExtractThumbnailJPEG).
Cost is paid only when the fast path fails, so healthy files keep the cheap path.

Regression test: TestBuildThumbnailArgsAccurate.
This commit is contained in:
Deivid Soto 2026-06-03 18:55:49 +02:00
parent 1814d59e09
commit 547b0d4e37
3 changed files with 120 additions and 11 deletions
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43
internal/engine/stream_server.go
View file

@ -1050,12 +1050,23 @@ func (ss *StreamServer) thumbnailHandler(w http.ResponseWriter, r *http.Request)
cmd.Stderr = &stderr
out, err := cmd.Output()
if err != nil || len(out) == 0 {
// A seek past EOF yields no frame — a benign empty output, not an error
// worth alarming on. Log at most a short line for diagnosis.
log.Printf("[thumbnail] no frame (pos=%.1f w=%d path=%q): err=%v %s",
// Fast input-seek (-ss before -i) can fail on files whose seek index is
// imprecise or mildly corrupt: the demuxer lands mid-EBML element
// ("invalid as first byte of an EBML number") and no frame decodes.
// Retry once with the slow but robust output-seek path before giving up
// (2026-06-03: anime MKVs returned a broken image in the web scrubber).
log.Printf("[thumbnail] input-seek failed (pos=%.1f w=%d path=%q): err=%v %s — retrying output-seek",
pos, width, rawPath, err, strings.TrimSpace(stderr.String()))
http.Error(w, "thumbnail failed", http.StatusInternalServerError)
return
var stderr2 strings.Builder
cmd2 := exec.CommandContext(ctx, ss.ffmpegPath, buildThumbnailArgsAccurate(rawPath, pos, width)...)
cmd2.Stderr = &stderr2
out, err = cmd2.Output()
if err != nil || len(out) == 0 {
log.Printf("[thumbnail] no frame after output-seek fallback (pos=%.1f w=%d path=%q): err=%v %s",
pos, width, rawPath, err, strings.TrimSpace(stderr2.String()))
http.Error(w, "thumbnail failed", http.StatusInternalServerError)
return
}
}
// Write-through so the next request (and trickplay re-hover) is a cache hit.
if ss.cacheThumbnails {
@ -1194,6 +1205,28 @@ func buildThumbnailArgs(path string, posSec float64, width int) []string {
}
}
// buildThumbnailArgsAccurate is the robust fallback for files whose seek index
// is imprecise or mildly corrupt, where the fast input seek (-ss before -i)
// lands mid-EBML element and decodes no frame. `-ss` AFTER `-i` is an output
// (decode) seek — slower (decodes from the start) but reliable — and
// `-err_detect ignore_err` tolerates minor stream corruption encountered along
// the way. Only used after buildThumbnailArgs fails, so its extra cost is paid
// solely for files the fast path can't handle.
func buildThumbnailArgsAccurate(path string, posSec float64, width int) []string {
return []string{
"-nostdin",
"-loglevel", "error",
"-err_detect", "ignore_err",
"-i", path,
"-ss", strconv.FormatFloat(posSec, 'f', 3, 64),
"-frames:v", "1",
"-vf", fmt.Sprintf("scale=%d:-2", width),
"-an", "-sn",
"-f", "mjpeg",
"pipe:1",
}
}
// parseThumbPos parses a non-negative seconds offset; defaults to 0 on garbage.
func parseThumbPos(s string) float64 {
if s == "" {