files: seek anywhere in a re-encode; it restarts from there

A re-encode's playlist is now the whole film on a fixed 4 s grid. A segment
that isn't on disk and isn't coming soon moves the encode there; runs stop
short of segments already made, and gaps are filled after. Every run is cut
the same way (-ss, -output_ts_offset start+10, frag_discont), so segments
from different runs join. Repackaging keeps its growing playlist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

PLAN.md

@@ -772,9 +772,38 @@ something long in one, switch away and back, refresh — it's all still there.
     11.7×, AV1 720p 13×, all on the Arc. `core` and `hamtv` re-encode on the CPU until
     *Allow GPU access* is pressed (libx264 at 9.4× for a 720p→480p clip on `core`); the Mac
     and `gate` need ffmpeg first, which the previews note offers to install.
-  - *Not done, on purpose:* no restart-from-a-seek beyond what is encoded (the encode is
+  - *Not done, on purpose:* no bit-rate ladder or adaptive switching, no subtitles, no
-    faster than watching; a seek far ahead waits), no bit-rate ladder or adaptive
+    keeping a finished re-encode for next time. (Restart-from-a-seek was left out here at
-    switching, no subtitles, no keeping a finished re-encode for next time.
+    first; it came 2026-10-20, the next entry.)
+- **Seek anywhere in a re-encode** — **2026-10-20**. Asked for as "there's no way to seek in
+  the video and have the service just start transcoding from that point onward".
+  - **The playlist is the whole film, up front:** a VOD playlist on a fixed 4 s grid
+    (`segCount`, the last 0.3 s left out so sound running past the picture never lists a
+    segment no frame starts), so the seek bar is the film's from the first frame.
+  - **A segment not on disk** is waited for when the run going now will write it within 3
+    segments; otherwise it is a seek: that run is ended and one starts at the segment asked
+    for. A run stops half a frame short of the next segment already on disk, never over it.
+    When a run ends by itself the next gap is filled, wrapping round, until every segment is
+    there. The newest request moves the run; an older one still waiting never moves it back.
+    A run we end closes its half-done segment short on the way out, so whatever it finished
+    after being asked to stop is deleted.
+  - **Runs are interchangeable** because every one, the first included, is made the same way
+    (`segArgs`): accurate `-ss` to the segment, timestamps put back with
+    `-output_ts_offset start+10` (the +10 keeps every run clear of the muxer's shift for the
+    B-frame delay, which a run from the top would otherwise get and a seek wouldn't), kept
+    absolute in each fragment (`movflags=+frag_discont`), `-start_number`. Tested on MKV
+    and on MPEG-TS with a 2.8 s start time (where `-copyts` was a second out): the init
+    segments come out byte-identical, and a seeked run's segments join a from-the-top run's
+    within a frame. Only the first run's init is served; a later run writes `init-run.mp4`
+    and the log notes if it ever differs.
+  - Repackaged ("copy") streams keep the growing event playlist: the source's keyframes
+    don't sit on a grid, and repackaging is near-instant anyway.
+  - A segment waits at most 45 s, under the hub proxy's 60 s for a node to answer; hls.js
+    is told a segment's first byte may take 50 s. The bar reads "7.3× from 12:00 · 43%
+    ready — seek anywhere".
+  - Tested (`TestSeekRestartsTheEncode`): a 64 s 1080p clip, a request for segment 12 right
+    after the start runs from 48 s, the gap is filled, all 16 segments joined behind the one
+    init decode with no hole.
 - **Files edge sweep** — **2026-10-11** (v0.1.0-214), asked for as "a full bug sweep of
   the Files section, every example and edge case". Every handler and both UI files read
   through with odd inputs in mind, then a battery of ~60 requests against the live hub

internal/api/play.go

@@ -146,24 +146,48 @@ func (s *Server) playStop(w http.ResponseWriter, r *http.Request) {
 }
 
 // playFile: GET /api/files/play/{id}/{file} — the playlist, the init segment, a segment.
+// A seekable stream's playlist is written whole, up front; a segment of it that isn't on disk
+// yet is waited for, or re-encoded from there if the encode isn't heading that way (a seek).
 func (s *Server) playFile(w http.ResponseWriter, r *http.Request) {
 	st, ok := s.plays.Find(r.PathValue("id"))
 	if !ok {
 		jsonErr(w, 404, "no such stream")
 		return
 	}
-	p, ok := st.File(r.PathValue("file"))
+	name := r.PathValue("file")
+	p, ok := st.File(name)
 	if !ok {
 		jsonErr(w, 404, "not a stream file")
 		return
 	}
+	w.Header().Set("Cache-Control", "no-store")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+	if st.Seekable {
+		if name == "index.m3u8" {
+			w.Header().Set("Content-Type", "application/vnd.apple.mpegurl")
+			_, _ = w.Write(st.Playlist())
+			return
+		}
+		if i, isSeg := media.SegmentIndex(name); isSeg {
+			sp, err := st.Segment(r.Context(), i)
+			switch {
+			case errors.Is(err, media.ErrNotInFilm):
+				jsonErr(w, 404, err.Error())
+				return
+			case r.Context().Err() != nil:
+				return // the player moved on
+			case err != nil:
+				jsonErr(w, 503, err.Error())
+				return
+			}
+			p = sp
+		}
+	}
 	switch filepath.Ext(p) {
 	case ".m3u8":
 		w.Header().Set("Content-Type", "application/vnd.apple.mpegurl")
 	default:
 		w.Header().Set("Content-Type", "video/mp4")
 	}
-	w.Header().Set("Cache-Control", "no-store")
-	w.Header().Set("X-Content-Type-Options", "nosniff")
 	http.ServeFile(w, r, p)
 }

internal/media/encode.go

@@ -212,6 +212,69 @@ func hlsArgs(dir, in string, src Info, r Recipe, copyCodec bool) []string {
 
 const hlsSeconds = 4
 
+// tsBase is added to every run's timestamps. A run from the top would otherwise start just
+// below zero (the B-frame delay) and the muxer would shift it to make room, while a run from a
+// seek point would not: with the base, no run is ever shifted, and a segment means the same
+// stretch of the film whichever run made it.
+const tsBase = 10
+
+// segArgs is one run of a seekable re-encode: segments from..to-1 of the fixed 4 s grid.
+// Every run is made the same way, the first included, so their segments are interchangeable:
+// an accurate seek to from×4 s, timestamps put back where that is in the film (+ tsBase),
+// kept as they are in each fragment (frag_discont), numbered from `from`, and an end just
+// short of segment `to`, which is already on disk, so the run never writes over it. Segments
+// land in dir as seg%05d.m4s; init is the init segment's name (only the first run's is used).
+func segArgs(dir, in string, src Info, r Recipe, from, to, n int, init string) []string {
+	fps := 30.0
+	if src.Video != nil && src.Video.FPS > 0 {
+		fps = src.Video.FPS
+	}
+	start := from * hlsSeconds
+	a := []string{"-nostdin", "-hide_banner", "-loglevel", "error", "-y"}
+	a = append(a, r.Pre...)
+	a = append(a, "-ss", strconv.Itoa(start), "-i", in, "-map", "0:v:0", "-map", "0:a:0?", "-sn", "-dn")
+	if r.VF != "" {
+		a = append(a, "-vf", r.VF)
+	}
+	a = append(a, r.Video...)
+	switch {
+	case src.Audio == nil:
+		a = append(a, "-an")
+	case src.Audio.Codec == "aac":
+		a = append(a, "-c:a", "copy")
+	default:
+		a = append(a, "-c:a", "aac", "-b:a", "160k", "-ac", "2")
+	}
+	gop := int(math.Round(fps * hlsSeconds))
+	if gop < 24 {
+		gop = 24
+	}
+	if gop > 300 {
+		gop = 300
+	}
+	a = append(a, "-g", strconv.Itoa(gop), "-force_key_frames", fmt.Sprintf("expr:gte(t,n_forced*%d)", hlsSeconds))
+	if to < n {
+		a = append(a, "-t", strconv.FormatFloat(float64((to-from)*hlsSeconds)-0.5/fps, 'f', 3, 64))
+	}
+	a = append(a, "-output_ts_offset", strconv.Itoa(start+tsBase),
+		"-f", "hls", "-hls_time", strconv.Itoa(hlsSeconds), "-hls_playlist_type", "event", "-hls_segment_type", "fmp4",
+		"-hls_segment_options", "movflags=+frag_discont", "-start_number", strconv.Itoa(from),
+		"-hls_fmp4_init_filename", init, "-hls_segment_filename", dir+"/seg%05d.m4s", "-hls_flags", "independent_segments+temp_file",
+		"-progress", dir+"/progress", "-stats_period", "1", dir+"/run.m3u8")
+	return a
+}
+
+// segCount is how many 4 s segments a seekable re-encode of a file this long lists. The last
+// 0.3 s are left out of the count, so a container whose sound runs on past the picture never
+// lists a segment no frame starts.
+func segCount(duration float64) int {
+	d := duration - 0.3
+	if d < 0 {
+		d = 0
+	}
+	return int(d/hlsSeconds) + 1
+}
+
 func has(list []string, x string) bool {
 	for _, v := range list {
 		if v == x {

internal/media/encode_test.go

@@ -110,6 +110,35 @@ func TestHLSArgs(t *testing.T) {
 	}
 }
 
+func TestSegArgs(t *testing.T) {
+	in := src("h264", 1920, 1080, 25)
+	r := recipes(gridCaps(), "h264", in, 1920, 1080, 10000)[0]
+	first := strings.Join(segArgs("/d", "/v.mkv", in, r, 0, 900, 900, "init.mp4"), " ")
+	for _, want := range []string{"-ss 0 -i /v.mkv", "-output_ts_offset 10", "-start_number 0", "movflags=+frag_discont", "-hls_fmp4_init_filename init.mp4", "-hls_segment_filename /d/seg%05d.m4s", "/d/run.m3u8", "-force_key_frames expr:gte(t,n_forced*4)"} {
+		if !strings.Contains(first, want) {
+			t.Errorf("first run: missing %q in\n%s", want, first)
+		}
+	}
+	if strings.Contains(first, " -t ") {
+		t.Error("a run to the end has no -t")
+	}
+	// a seek: segments 30..44, stopping half a frame short of 45, which is on disk
+	seek := strings.Join(segArgs("/d", "/v.mkv", in, r, 30, 45, 900, "init-run.mp4"), " ")
+	for _, want := range []string{"-ss 120 -i /v.mkv", "-t 59.980", "-output_ts_offset 130", "-start_number 30", "-hls_fmp4_init_filename init-run.mp4"} {
+		if !strings.Contains(seek, want) {
+			t.Errorf("seek run: missing %q in\n%s", want, seek)
+		}
+	}
+	if strings.Index(seek, "-ss 120") > strings.Index(seek, "-i /v.mkv") || strings.Index(seek, "-hwaccel") > strings.Index(seek, "-i /v.mkv") {
+		t.Error("-ss and the decoder's options go before -i")
+	}
+	for d, n := range map[float64]int{9: 3, 8.2: 2, 8.31: 3, 64: 16, 0.1: 1, 3600: 900} {
+		if got := segCount(d); got != n {
+			t.Errorf("segCount(%v) = %d, want %d", d, got, n)
+		}
+	}
+}
+
 func TestParseProgress(t *testing.T) {
 	b := []byte("frame=10\nout_time_us=1000000\nspeed=2.5x\nprogress=continue\nframe=20\nout_time_us=2500000\nspeed=3.1x\nprogress=continue\n")
 	s, sp := parseProgress(b)

internal/media/hls.go

@@ -3,14 +3,26 @@ package media
 // Playing what the browser cannot play as it is: an HLS stream, re-encoded on demand by
 // ffmpeg on the machine's own GPU where it has one it can use, on its CPU otherwise. A
 // stream is one file at one codec and size, in a folder of its own under
-// <data dir>/transcode/: ffmpeg writes an event playlist and fragmented-MP4 segments there
+// <data dir>/transcode/.
-// as fast as it can, and the player reads them as they land, so the range it can seek in
+//
-// grows until the whole file is through — on a GPU, several times faster than watching.
+// A re-encode is seekable anywhere from the start. Its playlist is written up front, for
-// Nothing is kept: a stream nobody has touched for a while is ended and its folder
+// the whole film: a fixed grid of 4 s segments, so the player's timeline is the film's.
-// removed, and the folder is emptied when the daemon starts.
+// ffmpeg runs ahead from the top as fast as it can; a request for a segment it hasn't
+// reached and won't reach soon — a seek ahead, or back into a stretch never made — ends that
+// run and starts another at the segment asked for, which stops on its own where segments
+// already exist. When a run finishes, the next gap is filled, until the whole file is on
+// disk. Every run is made the same way (segArgs), so segments from different runs join.
+//
+// "Repackaged" (copy) can't cut on a fixed grid — its keyframes are the source's — so it is
+// an event playlist that grows as ffmpeg writes it, seekable within what's done. Repackaging
+// runs at disk speed, so that is all of it within seconds.
+//
+// Nothing is kept: a stream nobody has touched for a while is ended and its folder removed,
+// and the folder is emptied when the daemon starts.
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"crypto/rand"
 	"encoding/hex"
@@ -39,15 +51,35 @@ type Stream struct {
 	Source   Info      `json:"source"`
 	Started  time.Time `json:"started"`
 	Root     bool      `json:"root"`
+	Seekable bool      `json:"seekable"` // seeking anywhere restarts the re-encode there
 
 	key      string
 	dir      string
 	mu       sync.Mutex
 	used     time.Time
-	proc     *ffproc
+	proc     *ffproc // the run going now (or the only one, for copy)
 	finished bool
 	failure  string
 	gone     bool // forgotten by the Streamer
+
+	// seekable streams only
+	caps    Caps
+	self    string
+	recipe  Recipe
+	n       int // segments in the playlist
+	end     int // segments past this are not in the file after all (n until a run finds out)
+	run     *segRun
+	runMu   sync.Mutex // one run change at a time; held across ending one run and starting the next
+	asked   time.Time  // when the request that last moved the run arrived: an older one never moves it back
+	initOK  bool       // a later run's init segment was checked against the first's
+	initBad bool
+}
+
+// segRun is one ffmpeg over segments from..to-1.
+type segRun struct {
+	from, to int
+	proc     *ffproc
+	stopped  bool // ended by us: what it wrote after that is not to be trusted
 }
 
 // StreamStatus is what the player asks for while the file is still going through.
@@ -55,9 +87,10 @@ type StreamStatus struct {
 	Running  bool    `json:"running"`
 	Done     bool    `json:"done"`
 	Error    string  `json:"error,omitempty"`
-	Progress float64 `json:"progress"` // seconds of output written
+	Progress float64 `json:"progress"` // seconds of output written (copy), or on disk (seekable)
 	Speed    float64 `json:"speed"`    // × realtime
 	Segments int     `json:"segments"`
+	From     float64 `json:"from"` // seekable: where the run going now started, seconds
 }
 
 // Streamer keeps one machine's streams.
@@ -72,7 +105,15 @@ type Streamer struct {
 const streamIdle = 10 * time.Minute
 const firstSegmentWait = 40 * time.Second
 
+// segmentWait is how long a request for a segment waits for a run to write it.
+const segmentWait = 45 * time.Second // under the hub proxy's 60 s wait for a node to answer
+
+// lookahead: a segment this many past what the run has written is waited for, not
+// restarted at. A few seconds of film, which any engine here writes faster than that.
+const lookahead = 3
+
 var streamFile = regexp.MustCompile(`^(index\.m3u8|init\.mp4|seg\d{5}\.m4s)$`)
+var segName = regexp.MustCompile(`^seg(\d{5})\.m4s$`)
 
 // NewStreamer empties the folder — a daemon restart orphans every stream — and starts the
 // sweeper.
@@ -128,8 +169,15 @@ func (m *Streamer) Open(ctx context.Context, c Caps, root bool, path, codec, qua
 		m.mu.Unlock()
 		return nil, fmt.Errorf("%d videos are already being re-encoded on this machine; one must finish or be closed first", running)
 	}
-	s := &Stream{ID: newID(), Path: path, Codec: codec, Quality: quality, Source: src, Started: time.Now(), Root: root, key: key, used: time.Now()}
+	s := &Stream{ID: newID(), Path: path, Codec: codec, Quality: quality, Source: src, Started: time.Now(), Root: root, key: key, used: time.Now(),
+		caps: c, self: m.self}
 	s.dir = filepath.Join(m.dir, s.ID)
+	// A fixed grid needs the film's length; without one it can only grow as it goes.
+	s.Seekable = codec != "copy" && src.Duration > 0
+	if s.Seekable {
+		s.n = segCount(src.Duration)
+		s.end = s.n
+	}
 	m.streams[s.ID] = s
 	m.mu.Unlock()
 
@@ -149,18 +197,30 @@ func (m *Streamer) Open(ctx context.Context, c Caps, root bool, path, codec, qua
 			return nil, err
 		}
 		_ = clearDir(s.dir)
-		args := hlsArgs(s.dir, path, src, r, codec == "copy")
+		var args []string
+		if s.Seekable {
+			args = segArgs(s.dir, path, src, r, 0, s.n, s.n, "init.mp4")
+		} else {
+			args = hlsArgs(s.dir, path, src, r, codec == "copy")
+		}
 		p, err := startFF(c, m.self, root, args)
 		if err != nil {
 			last = r.Engine + ": " + err.Error()
 			continue
 		}
 		s.mu.Lock()
-		s.proc, s.Engine = p, r.Engine
+		s.proc, s.Engine, s.recipe = p, r.Engine, r
+		if s.Seekable {
+			s.run = &segRun{from: 0, to: s.n, proc: p}
+		}
 		s.mu.Unlock()
 		msg, ok := s.waitFirstSegment(ctx)
 		if ok {
+			if s.Seekable {
+				go s.watchRun(s.run)
+			} else {
 				go s.watch()
+			}
 			log.Printf("media: stream %s: %s → %s %s %dx%d %dk on %s", s.ID, filepath.Base(path), codec, quality, s.Width, s.Height, s.Kbps, r.Engine)
 			return s, nil
 		}
@@ -176,12 +236,12 @@ func (m *Streamer) Open(ctx context.Context, c Caps, root bool, path, codec, qua
 	return nil, &DecodeError{Msg: last}
 }
 
-// waitFirstSegment watches for the playlist's first segment, or ffmpeg giving up.
+// waitFirstSegment watches for the first segment, or ffmpeg giving up.
 func (s *Stream) waitFirstSegment(ctx context.Context) (string, bool) {
 	deadline := time.Now().Add(firstSegmentWait)
 	playlist := filepath.Join(s.dir, "index.m3u8")
 	for {
-		if hasSegments(playlist) {
+		if s.Seekable && exists(s.segPath(0)) || !s.Seekable && hasSegments(playlist) {
 			return "", true
 		}
 		if done, err := s.proc.exited(); done {
@@ -205,7 +265,7 @@ func (s *Stream) waitFirstSegment(ctx context.Context) (string, bool) {
 	}
 }
 
-// watch records how the encode ended.
+// watch records how the encode ended (copy streams: one run, start to end).
 func (s *Stream) watch() {
 	<-s.proc.done
 	s.mu.Lock()
@@ -222,6 +282,250 @@ func (s *Stream) watch() {
 	}
 }
 
+// ---- seekable streams ----
+
+func (s *Stream) segPath(i int) string { return filepath.Join(s.dir, fmt.Sprintf("seg%05d.m4s", i)) }
+
+// written is the last segment of a run on disk, counting on from where it started.
+func (s *Stream) written(r *segRun) int {
+	i := r.from
+	for i < r.to && exists(s.segPath(i)) {
+		i++
+	}
+	return i - 1
+}
+
+// watchRun follows one run to its end. A run that ends by itself — the end of the film, or
+// the segments it was to stop short of — is followed by the next gap. One that failed fails
+// the stream; one we ended is the business of whoever ended it.
+func (s *Stream) watchRun(r *segRun) {
+	<-r.proc.done
+	s.mu.Lock()
+	if s.gone || r.stopped || s.run != r {
+		s.mu.Unlock()
+		return
+	}
+	if r.proc.err != nil {
+		s.failure = r.proc.lastWords()
+		if s.failure == "" {
+			s.failure = r.proc.err.Error()
+		}
+		s.finished = true
+		s.mu.Unlock()
+		log.Printf("media: stream %s: run from segment %d failed: %s", s.ID, r.from, s.failure)
+		return
+	}
+	if w := s.written(r); w+1 < r.to && r.to == s.n {
+		// ran to the end of the file before the end of the grid: the picture is shorter than
+		// the container says, and those last segments will never exist
+		s.end = w + 1
+		log.Printf("media: stream %s: the film ends at segment %d of %d", s.ID, s.end, s.n)
+	}
+	s.checkInit(r)
+	s.mu.Unlock()
+	s.fillNext(r.to)
+}
+
+// checkInit compares a later run's init segment with the first run's, once. They come out
+// identical (same encoder, same settings); if one day they don't, the log says so.
+func (s *Stream) checkInit(r *segRun) {
+	if r.from == 0 || s.initOK {
+		return
+	}
+	a, err1 := os.ReadFile(filepath.Join(s.dir, "init.mp4"))
+	b, err2 := os.ReadFile(filepath.Join(s.dir, "init-run.mp4"))
+	if err1 != nil || err2 != nil {
+		return
+	}
+	s.initOK = true
+	if !bytes.Equal(a, b) && !s.initBad {
+		s.initBad = true
+		log.Printf("media: stream %s: a restarted run's init segment differs from the first (%s); its segments may not join", s.ID, s.Engine)
+	}
+}
+
+// missingFrom is the first segment not on disk at or after i (wrapping round to the start),
+// or -1 when the whole film is there.
+func (s *Stream) missingFrom(i int) int {
+	end := s.end
+	for k := 0; k < end; k++ {
+		j := (i + k) % end
+		if !exists(s.segPath(j)) {
+			return j
+		}
+	}
+	return -1
+}
+
+// fillNext starts a run at the next gap after `after`; when there is none, the stream is done.
+func (s *Stream) fillNext(after int) {
+	s.runMu.Lock()
+	defer s.runMu.Unlock()
+	s.mu.Lock()
+	if s.gone || s.failure != "" || (s.run != nil && !s.run.done()) {
+		s.mu.Unlock()
+		return
+	}
+	end := s.end
+	s.mu.Unlock()
+	if after >= end {
+		after = 0
+	}
+	next := s.missingFrom(after)
+	if next < 0 {
+		s.mu.Lock()
+		s.finished, s.proc = true, nil
+		s.mu.Unlock()
+		log.Printf("media: stream %s: all %d segments on disk", s.ID, end)
+		return
+	}
+	s.startRun(next)
+}
+
+func (r *segRun) done() bool { d, _ := r.proc.exited(); return d }
+
+// startRun ends the run going now and starts one at segment from, stopping short of the next
+// segment already on disk. The run it ends is asked to stop and waited for, and whatever it
+// finished writing after being asked is removed: ffmpeg closes the segment it was in the
+// middle of on its way out, short. Called with runMu held, mu not.
+func (s *Stream) startRun(from int) {
+	s.mu.Lock()
+	old := s.run
+	end := s.end
+	s.mu.Unlock()
+	if old != nil && !old.done() {
+		had := map[int]bool{}
+		for i := old.from; i < old.to; i++ {
+			had[i] = exists(s.segPath(i))
+		}
+		s.mu.Lock()
+		old.stopped = true
+		s.mu.Unlock()
+		old.proc.end()
+		for i := old.from; i < old.to; i++ {
+			if !had[i] && exists(s.segPath(i)) {
+				_ = os.Remove(s.segPath(i))
+			}
+		}
+	}
+	to := from + 1
+	for to < end && !exists(s.segPath(to)) {
+		to++
+	}
+	s.mu.Lock()
+	if s.gone {
+		s.mu.Unlock()
+		return
+	}
+	s.mu.Unlock()
+	init := "init-run.mp4"
+	if !exists(filepath.Join(s.dir, "init.mp4")) {
+		init = "init.mp4"
+	}
+	p, err := startFF(s.caps, s.self, s.Root, segArgs(s.dir, s.Path, s.Source, s.recipe, from, to, s.n, init))
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if err != nil {
+		s.failure, s.finished = "restarting at "+strconv.Itoa(from*hlsSeconds)+" s: "+err.Error(), true
+		return
+	}
+	r := &segRun{from: from, to: to, proc: p}
+	s.run, s.proc, s.finished = r, p, false
+	go s.watchRun(r)
+	log.Printf("media: stream %s: run from %d s to %d s", s.ID, from*hlsSeconds, to*hlsSeconds)
+}
+
+// Playlist is the whole film's, written before most of it exists.
+func (s *Stream) Playlist() []byte {
+	var b strings.Builder
+	b.WriteString("#EXTM3U\n#EXT-X-VERSION:7\n")
+	fmt.Fprintf(&b, "#EXT-X-TARGETDURATION:%d\n", hlsSeconds)
+	b.WriteString("#EXT-X-MEDIA-SEQUENCE:0\n#EXT-X-PLAYLIST-TYPE:VOD\n#EXT-X-INDEPENDENT-SEGMENTS\n#EXT-X-MAP:URI=\"init.mp4\"\n")
+	for i := 0; i < s.n; i++ {
+		d := float64(hlsSeconds)
+		if i == s.n-1 {
+			d = s.Source.Duration - float64(i*hlsSeconds)
+		}
+		fmt.Fprintf(&b, "#EXTINF:%.6f,\nseg%05d.m4s\n", d, i)
+	}
+	b.WriteString("#EXT-X-ENDLIST\n")
+	return []byte(b.String())
+}
+
+// ErrNotInFilm is a segment the playlist lists but the film turned out not to reach.
+var ErrNotInFilm = errors.New("past the end of the film")
+
+// Segment is the path of segment i once it is on disk. When the run going now will write it
+// soon, it waits; otherwise this is a seek, and the run moves there — unless a newer request
+// has moved it since this one arrived.
+func (s *Stream) Segment(ctx context.Context, i int) (string, error) {
+	asked := time.Now()
+	deadline := asked.Add(segmentWait)
+	p := s.segPath(i)
+	for {
+		s.touch()
+		if exists(p) {
+			return p, nil
+		}
+		s.mu.Lock()
+		switch {
+		case s.gone:
+			s.mu.Unlock()
+			return "", errors.New("the stream was closed")
+		case i >= s.end:
+			s.mu.Unlock()
+			return "", ErrNotInFilm
+		case s.failure != "":
+			f := s.failure
+			s.mu.Unlock()
+			return "", errors.New(f)
+		}
+		r := s.run
+		coming := r != nil && !r.done() && r.from <= i && i < r.to && i <= s.written(r)+lookahead
+		move := !coming && !asked.Before(s.asked)
+		if move {
+			s.asked = asked
+		}
+		s.mu.Unlock()
+		if move {
+			s.runMu.Lock()
+			// still the newest, and still not on its way? (a run may have moved for a
+			// newer request while this one waited for the lock)
+			s.mu.Lock()
+			r = s.run
+			coming = r != nil && !r.done() && r.from <= i && i < r.to && i <= s.written(r)+lookahead
+			newest := !asked.Before(s.asked)
+			s.mu.Unlock()
+			if !coming && newest && !exists(p) {
+				log.Printf("media: stream %s: seek to %d s", s.ID, i*hlsSeconds)
+				s.startRun(i)
+			}
+			s.runMu.Unlock()
+			continue
+		}
+		if time.Now().After(deadline) {
+			return "", fmt.Errorf("segment %d didn't arrive in %s", i, segmentWait)
+		}
+		select {
+		case <-ctx.Done():
+			return "", ctx.Err()
+		case <-time.After(150 * time.Millisecond):
+		}
+	}
+}
+
+// SegmentIndex reads the number out of a segment's name.
+func SegmentIndex(name string) (int, bool) {
+	m := segName.FindStringSubmatch(name)
+	if m == nil {
+		return 0, false
+	}
+	i, err := strconv.Atoi(m[1])
+	return i, err == nil
+}
+
+// ---- both kinds ----
+
 func (m *Streamer) Find(id string) (*Stream, bool) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
@@ -238,7 +542,11 @@ func (m *Streamer) Close(id string) {
 		return
 	}
 	s.mu.Lock()
+	s.gone = true // a run ending now is ours, not a gap to fill
 	p := s.proc
+	if s.run != nil {
+		s.run.stopped = true
+	}
 	s.mu.Unlock()
 	if p != nil {
 		p.end()
@@ -282,8 +590,8 @@ func (m *Streamer) Sweep(idle time.Duration) {
 	}
 }
 
-// File is the on-disk path of one of the stream's files — the playlist, the init segment
+// File is the on-disk path of one of the stream's files — the playlist (copy streams; a
-// or a media segment — and nothing else.
+// seekable one's is Playlist), the init segment or a media segment — and nothing else.
 func (s *Stream) File(name string) (string, bool) {
 	if !streamFile.MatchString(name) {
 		return "", false
@@ -300,18 +608,35 @@ func (s *Stream) running() bool {
 	return s.proc != nil && !s.finished
 }
 
-// Status is read from ffmpeg's own progress file and the playlist.
+// Status is read from ffmpeg's own progress file and what is on disk.
 func (s *Stream) Status() StreamStatus {
 	s.mu.Lock()
 	st := StreamStatus{Running: s.proc != nil && !s.finished, Done: s.finished && s.failure == "", Error: s.failure}
+	seekable, end, run := s.Seekable, s.end, s.run
 	s.mu.Unlock()
-	if b, err := os.ReadFile(filepath.Join(s.dir, "progress")); err == nil {
+	b, err := os.ReadFile(filepath.Join(s.dir, "progress"))
+	if err == nil {
 		st.Progress, st.Speed = parseProgress(b)
 	}
+	if !seekable {
 		if b, err := os.ReadFile(filepath.Join(s.dir, "index.m3u8")); err == nil {
 			st.Segments = strings.Count(string(b), "#EXTINF")
 		}
 		return st
+	}
+	for i := 0; i < end; i++ {
+		if exists(s.segPath(i)) {
+			st.Segments++
+		}
+	}
+	st.Progress = float64(st.Segments * hlsSeconds)
+	if st.Progress > s.Source.Duration {
+		st.Progress = s.Source.Duration
+	}
+	if run != nil && st.Running {
+		st.From = float64(run.from * hlsSeconds)
+	}
+	return st
 }
 
 // parseProgress takes the last out_time_us and speed from ffmpeg's -progress output.

internal/media/hls_test.go

@@ -2,9 +2,14 @@ package media
 
 import (
 	"context"
+	"errors"
+	"math"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
 	"testing"
 	"time"
 )
@@ -38,12 +43,16 @@ func TestStreamWithFFmpeg(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if s.Width != 640 || s.Height != 360 || s.Engine == "" {
+	if s.Width != 640 || s.Height != 360 || s.Engine == "" || !s.Seekable {
 		t.Fatalf("stream: %+v", s)
 	}
-	if p, ok := s.File("index.m3u8"); !ok {
+	if _, ok := s.File("index.m3u8"); !ok {
 		t.Fatal("no playlist")
-	} else if !hasSegments(p) {
+	}
+	if pl := string(s.Playlist()); !strings.Contains(pl, "#EXT-X-PLAYLIST-TYPE:VOD") || !strings.HasSuffix(pl, "#EXT-X-ENDLIST\n") || strings.Count(pl, "#EXTINF") != 3 {
+		t.Fatalf("playlist for 9 s:\n%s", pl)
+	}
+	if !exists(s.segPath(0)) {
 		t.Fatal("Open returned before the first segment")
 	}
 	for _, bad := range []string{"../x", "progress", "seg1.m4s", "index.m3u8/", "init.mp4.tmp"} {
@@ -63,7 +72,7 @@ func TestStreamWithFFmpeg(t *testing.T) {
 		}
 		time.Sleep(250 * time.Millisecond)
 	}
-	if !st.Done || st.Error != "" || st.Segments < 2 || st.Progress < 8 {
+	if !st.Done || st.Error != "" || st.Segments != 3 || st.Progress < 8 {
 		t.Fatalf("status at the end: %+v", st)
 	}
 	if init, _ := s.File("init.mp4"); !exists(init) {
@@ -83,3 +92,129 @@ func TestStreamWithFFmpeg(t *testing.T) {
 		t.Fatalf("left behind: %v", left)
 	}
 }
+
+// A seek ahead of the encode: the run moves there, the segment comes from a run that started
+// at it, the gaps are filled afterwards, and the segments of the different runs, joined, are
+// one film with no hole and nothing twice beyond a frame at a join.
+func TestSeekRestartsTheEncode(t *testing.T) {
+	if _, err := exec.LookPath("ffmpeg"); err != nil {
+		t.Skip("no ffmpeg")
+	}
+	c := Probe(context.Background())
+	if c.Error != "" || EngineFor(c, "h264") == "" {
+		t.Skip("nothing here encodes h264")
+	}
+	dir := t.TempDir()
+	clip := filepath.Join(dir, "clip.mkv")
+	// big and busy enough that the first run is still near the start when the seek comes
+	out, err := exec.Command("ffmpeg", "-nostdin", "-v", "error", "-f", "lavfi", "-i", "testsrc2=size=1920x1080:rate=30000/1001:duration=64", "-f", "lavfi", "-i", "sine=frequency=440:duration=64",
+		"-c:v", "libx264", "-preset", "ultrafast", "-g", "250", "-c:a", "libopus", "-shortest", clip).CombinedOutput()
+	if err != nil {
+		t.Skipf("could not make a clip: %s", out)
+	}
+	src, err := Inspect(context.Background(), c, clip, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := NewStreamer(dir, "hamlab", 2)
+	s, err := m.Open(context.Background(), c, false, clip, "h264", "1080p", src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer m.Close(s.ID)
+	if s.n != 16 {
+		t.Fatalf("%d segments for 64 s", s.n)
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+	p, err := s.Segment(ctx, 12)
+	if err != nil {
+		t.Fatal(err)
+	}
+	s.mu.Lock()
+	from := s.run.from
+	s.mu.Unlock()
+	if from != 12 {
+		t.Fatalf("the run after the seek starts at segment %d, not 12 (the first run was already there?)", from)
+	}
+	if got := firstVideoPTS(t, filepath.Join(s.dir, "init.mp4"), p); math.Abs(got-(48+tsBase)) > 0.05 {
+		t.Fatalf("segment 12 starts at %.3f, want %d", got, 48+tsBase)
+	}
+	if _, err := s.Segment(ctx, 16); !errors.Is(err, ErrNotInFilm) {
+		t.Fatalf("past the end: %v", err)
+	}
+	var st StreamStatus
+	for i := 0; i < 480; i++ {
+		if st = s.Status(); st.Done || st.Error != "" {
+			break
+		}
+		time.Sleep(250 * time.Millisecond)
+	}
+	if !st.Done || st.Segments != 16 {
+		t.Fatalf("not filled in: %+v", st)
+	}
+	// the joined film: every segment in order behind the one init segment
+	joined := filepath.Join(dir, "joined.mp4")
+	f, _ := os.Create(joined)
+	for _, n := range append([]string{"init.mp4"}, segNames(16)...) {
+		b, err := os.ReadFile(filepath.Join(s.dir, n))
+		if err != nil {
+			t.Fatal(err)
+		}
+		_, _ = f.Write(b)
+	}
+	f.Close()
+	if out, err := exec.Command("ffmpeg", "-v", "error", "-i", joined, "-f", "null", "-").CombinedOutput(); err != nil || strings.Contains(string(out), "error") {
+		t.Fatalf("the joined film doesn't decode cleanly: %v %s", err, out)
+	}
+	pts := videoPTS(t, joined)
+	if len(pts) < 1900 || pts[0] > tsBase+0.1 || pts[len(pts)-1] < tsBase+63.5 {
+		t.Fatalf("%d frames, %.3f .. %.3f", len(pts), pts[0], pts[len(pts)-1])
+	}
+	for i := 1; i < len(pts); i++ {
+		if d := pts[i] - pts[i-1]; d > 0.07 { // a frame is 0.033; a join may lose up to one
+			t.Fatalf("a hole of %.3f s at %.3f", d, pts[i-1])
+		}
+	}
+}
+
+func segNames(n int) []string {
+	var out []string
+	for i := 0; i < n; i++ {
+		out = append(out, "seg"+strconv.FormatInt(int64(100000+i), 10)[1:]+".m4s")
+	}
+	return out
+}
+
+func videoPTS(t *testing.T, file string) []float64 {
+	out, err := exec.Command("ffprobe", "-v", "error", "-select_streams", "v", "-show_entries", "packet=pts_time", "-of", "csv=p=0", file).Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var pts []float64
+	seen := map[string]bool{}
+	for _, l := range strings.Fields(string(out)) {
+		if seen[l] { // the same frame from both runs at a join
+			continue
+		}
+		seen[l] = true
+		if v, err := strconv.ParseFloat(l, 64); err == nil {
+			pts = append(pts, v)
+		}
+	}
+	sort.Float64s(pts)
+	return pts
+}
+
+func firstVideoPTS(t *testing.T, init, seg string) float64 {
+	dir := t.TempDir()
+	j := filepath.Join(dir, "one.mp4")
+	a, _ := os.ReadFile(init)
+	b, _ := os.ReadFile(seg)
+	_ = os.WriteFile(j, append(a, b...), 0o644)
+	pts := videoPTS(t, j)
+	if len(pts) == 0 {
+		t.Fatal("no frames in the segment")
+	}
+	return pts[0]
+}

web/src/Player.tsx

@@ -3,6 +3,8 @@
 // picker: codec and size, and the machine re-encodes it to HLS on its own GPU (or CPU),
 // which plays natively on Safari and through hls.js everywhere else. The same picker is
 // one tap away even when the file plays, for a 36 Mbps capture over a slow link.
+// A re-encode's timeline is the whole film from the start: seek anywhere, and the machine
+// starts encoding from there (repackaging, which is near-instant, grows as it goes instead).
 import { useCallback, useEffect, useRef, useState } from 'react'
 import { api, apiPath, PlayCaps, PlayStatus, PlayStream } from './api'
 import { Btn, Spinner } from './ui'
@@ -13,6 +15,7 @@ const QUALITY_KEY = 'hamlab.play.quality'
 const remember = (k: string, v: string) => { try { localStorage.setItem(k, v) } catch { /* private mode */ } }
 const recall = (k: string, d: string) => { try { return localStorage.getItem(k) || d } catch { return d } }
 
+const clock = (sec: number) => { const t = Math.floor(sec), h = Math.floor(t / 3600), m = Math.floor((t % 3600) / 60), x = t % 60; return `${h ? `${h}:${String(m).padStart(2, '0')}` : m}:${String(x).padStart(2, '0')}` }
 const fmtMbps = (bps?: number) => (bps ? `${(bps / 1e6).toFixed(bps >= 10e6 ? 0 : 1)} Mbps` : '')
 const describe = (s: PlayStream) => {
   const v = s.source.video
@@ -72,7 +75,10 @@ export default function Player({ path, sudo, name }: { path: string; sudo: boole
         const { default: Hls } = await import('hls.js')
         if (gone) return
         if (!Hls.isSupported()) { setErr('this browser has neither HLS nor MediaSource — nothing can play here'); return }
-        const h = new Hls({ maxBufferLength: 60, backBufferLength: 30 })
+        // a segment the machine hasn't made yet is made when asked for (a seek): its first
+        // byte can take as long as the re-encode takes to start there and write it
+        const slow = { maxTimeToFirstByteMs: 50000, maxLoadTimeMs: 120000, timeoutRetry: { maxNumRetry: 2, retryDelayMs: 0, maxRetryDelayMs: 0 }, errorRetry: { maxNumRetry: 3, retryDelayMs: 1000, maxRetryDelayMs: 8000 } }
+        const h = new Hls({ maxBufferLength: 60, backBufferLength: 30, fragLoadPolicy: { default: slow } })
         hls = h
         h.on(Hls.Events.ERROR, (_e, d) => { if (d.fatal) setErr(`playback failed: ${d.details}`) })
         h.loadSource(url)
@@ -131,7 +137,9 @@ export default function Player({ path, sudo, name }: { path: string; sudo: boole
         <>
           <div className="play-bar">
             <span title={describe(stream.s)}>{describe(stream.s)}</span>
-            {status && status.running && <span>· {status.speed ? `${status.speed.toFixed(1)}×` : ''} {stream.s.source.duration ? `${Math.min(100, Math.round(status.progress / stream.s.source.duration * 100))}% through` : `${Math.round(status.progress)} s done`} — seek within what's done</span>}
+            {status && status.running && (stream.s.seekable
+              ? <span>· {status.speed ? `${status.speed.toFixed(1)}× ` : ''}from {clock(status.from ?? 0)} · {Math.min(100, Math.round(status.progress / stream.s.source.duration * 100))}% ready — seek anywhere</span>
+              : <span>· {status.speed ? `${status.speed.toFixed(1)}×` : ''} {stream.s.source.duration ? `${Math.min(100, Math.round(status.progress / stream.s.source.duration * 100))}% through` : `${Math.round(status.progress)} s done`} — seek within what's done</span>)}
             {status?.done && <span>· all of it is ready</span>}
             {status?.error && <span className="error">· stopped early: {status.error}</span>}
             <span className="spacer" />

web/src/api.ts

@@ -252,8 +252,10 @@ export interface ARCView { detail: ARCDetail; live: ARCLive | null }
 export interface FileEntry { name: string; type: 'dir' | 'file' | 'link' | 'other'; size: number; mode: string; perm: string; mtime: number; user: string; group: string; target?: string; broken?: boolean; preview?: 'image' | 'video' | 'audio' }
 export interface PlayCaps { codecs: { id: string; label: string; engine?: string }[]; qualities: { id: string; label: string; height: number }[]; error?: string }
 export interface PlayTrack { codec: string; profile?: string; pix_fmt?: string; width?: number; height?: number; fps?: number; channels?: number }
-export interface PlayStream { id: string; codec: string; quality: string; engine: string; width: number; height: number; kbps: number; source: { container: string; duration: number; bit_rate?: number; video?: PlayTrack; audio?: PlayTrack } }
+export interface PlayStream { id: string; codec: string; quality: string; engine: string; width: number; height: number; kbps: number; seekable?: boolean; source: { container: string; duration: number; bit_rate?: number; video?: PlayTrack; audio?: PlayTrack } }
-export interface PlayStatus { running: boolean; done: boolean; error?: string; progress: number; speed: number; segments: number }
+// progress: seconds written (repackaged), or seconds of the film on disk (a seekable re-encode,
+// whose run now going started at `from`)
+export interface PlayStatus { running: boolean; done: boolean; error?: string; progress: number; speed: number; segments: number; from?: number }
 export interface ThumbGPU { api: string; device: string; name?: string; codecs?: string[]; reason?: string; permission?: boolean }
 // How previews get made on the machine: ok with an engine, or why not and what would fix it.
 export interface ThumbStatus { ok: boolean; engine: 'gpu' | 'cpu' | ''; summary: string; ffmpeg?: string; gpus: ThumbGPU[]; unusable?: ThumbGPU[]; error?: string; install?: { pm: string; package: string }; gpu_fix?: boolean; user: string; cache: { path: string; entries: number; bytes: number } }