From f6dab80a9875e7b4d9ba905a739ce380dc891776 Mon Sep 17 00:00:00 2001
From: Zoe Roux <zoe.roux@zoriya.dev>
Date: Mon, 1 Jul 2024 16:38:42 +0000
Subject: [PATCH] Make fragments of ~6 seconds instead of a fragment per
 keyframe

---
 transcoder/src/keyframes.go | 19 +++++++++++++------
 transcoder/src/stream.go    |  9 ++++-----
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/transcoder/src/keyframes.go b/transcoder/src/keyframes.go
index 96cbf6d6..ab902c55 100644
--- a/transcoder/src/keyframes.go
+++ b/transcoder/src/keyframes.go
@@ -4,12 +4,16 @@ import (
 	"bufio"
 	"fmt"
 	"log"
+	"math"
 	"os/exec"
 	"strconv"
 	"strings"
 	"sync"
 )
 
+// In seconds, the spec recomands 6 but since we don't control keyframes we go over more often than not.
+const OptimalFragmentDuration = float64(5)
+
 type Keyframe struct {
 	Sha         string
 	Keyframes   []float64
@@ -97,7 +101,7 @@ func GetKeyframes(sha string, path string) *Keyframe {
 }
 
 func getKeyframes(path string, kf *Keyframe, sha string) error {
-	defer printExecTime("ffprobe analysis for %s", path)()
+	defer printExecTime("keyframe extraction for %s", path)()
 	// run ffprobe to return all IFrames, IFrames are points where we can split the video in segments.
 	// We ask ffprobe to return the time of each frame and it's flags
 	// We could ask it to return only i-frames (keyframes) with the -skip_frame nokey but using it is extremly slow
@@ -123,6 +127,7 @@ func getKeyframes(path string, kf *Keyframe, sha string) error {
 
 	ret := make([]float64, 0, 1000)
 	max := 100
+	last_frame := math.Inf(-1)
 	done := 0
 	// sometimes, videos can start at a timing greater than 0:00. We need to take that into account
 	// and only list keyframes that come after the start of the video (without that, our segments count
@@ -154,12 +159,14 @@ func getKeyframes(path string, kf *Keyframe, sha string) error {
 			return err
 		}
 
-		// Before, we wanted to only save keyframes with at least 3s betweens
-		// to prevent segments of 0.2s but sometimes, the -f segment muxer discards
-		// the segment time and decide to cut at a random keyframe. Having every keyframe
-		// handled as a segment prevents that.
+		// The -f hls encoder decides to not cut at every keyframes (even if we ask it to) when they are too close by.
+		// Instead we can cut every X seconds at the next keyframe, we'll use that as a marker.
+		if fpts < (last_frame+OptimalFragmentDuration) {
+			continue
+		}
 
 		ret = append(ret, fpts)
+		last_frame = fpts
 
 		if len(ret) == max {
 			kf.add(ret)
@@ -190,7 +197,7 @@ func getKeyframes(path string, kf *Keyframe, sha string) error {
 }
 
 func getDummyKeyframes(path string, sha string) ([]float64, error) {
-	dummyKeyframeDuration := float64(2)
+	dummyKeyframeDuration := OptimalFragmentDuration
 	info, err := GetInfo(path, sha)
 	if err != nil {
 		return nil, err
diff --git a/transcoder/src/stream.go b/transcoder/src/stream.go
index 43761c1c..7cdcd013 100644
--- a/transcoder/src/stream.go
+++ b/transcoder/src/stream.go
@@ -267,8 +267,7 @@ func (ts *Stream) run(start int32) error {
 	args = append(args, ts.handle.getTranscodeArgs(toSegmentStr(segments))...)
 	args = append(args,
 		"-f", "hls",
-		// Cut at every keyframes.
-		"-hls_time", "0",
+		"-hls_time", fmt.Sprint(OptimalFragmentDuration),
 		"-start_number", fmt.Sprint(start_segment),
 		"-hls_segment_type", "fmp4",
 		"-hls_fmp4_init_filename", fmt.Sprintf("%s/init.mp4", outpath),
@@ -302,7 +301,7 @@ func (ts *Stream) run(start int32) error {
 		scanner := bufio.NewScanner(stdout)
 		format := ts.handle.getSegmentName()
 		should_stop := false
-		is_init_ready:= false
+		is_init_ready := false
 
 		for scanner.Scan() {
 			line := scanner.Text()
@@ -361,7 +360,7 @@ func (ts *Stream) run(start int32) error {
 
 	go func() {
 		err := cmd.Wait()
-		if exiterr, ok := err.(*exec.ExitError); ok && exiterr.ExitCode() == 255 {
+		if exiterr, ok := err.(*exec.ExitError); ok && (exiterr.ExitCode() == 255 || exiterr.ExitCode() == -1) {
 			log.Printf("ffmpeg %d was killed by us", encoder_id)
 		} else if err != nil {
 			log.Printf("ffmpeg %d occurred an error: %v: %s", encoder_id, err, stderr.String())
@@ -385,11 +384,11 @@ func (ts *Stream) GetIndex() (string, error) {
 #EXT-X-VERSION:7
 #EXT-X-PLAYLIST-TYPE:EVENT
 #EXT-X-START:TIME-OFFSET=0
-#EXT-X-TARGETDURATION:4
 #EXT-X-MEDIA-SEQUENCE:0
 #EXT-X-INDEPENDENT-SEGMENTS
 #EXT-X-MAP:URI="init.mp4"
 `
+	index += fmt.Sprintf("#EXT-X-TARGETDURATION:%d\n", int(OptimalFragmentDuration))
 	length, is_done := ts.file.Keyframes.Length()
 
 	for segment := int32(0); segment < length-1; segment++ {