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>

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]
+}