Merge pull request #1943 from danog/fix-mp4-stsc-quadratic

MP4: parse the sample-to-chunk table in linear time

Author: danog

src/API.php

@@ -54,7 +54,7 @@ final class API extends AbstractAPI
      *
      * @var string
      */
-    public const RELEASE = '8.15.3';
+    public const RELEASE = '8.15.4';
     /**
      * We're not logged in.
      *

src/Mp4.php

@@ -650,12 +650,15 @@ private function parseSampleTables(int $trackId, array $tables, array &$progress
         $sample = 0;
         $time = 0;
         $chunkCount = \count($chunkOffsets);
+        // The entries of the sample-to-chunk table are sorted by first chunk: walk them along with the chunks, rescanning
+        // the table for each chunk is quadratic (minutes of blocked event loop for a file with a long table).
+        $perChunk = 0;
+        $stscIndex = 0;
+        $stscCount = \count($stsc);
         foreach ($chunkOffsets as $chunkIndex => $chunkOffset) {
-            $perChunk = 0;
-            foreach ($stsc as [$firstChunk, $samplesPerChunk]) {
-                if ($firstChunk <= $chunkIndex + 1) {
-                    $perChunk = $samplesPerChunk;
-                }
+            while ($stscIndex < $stscCount && $stsc[$stscIndex][0] <= $chunkIndex + 1) {
+                $perChunk = $stsc[$stscIndex][1];
+                $stscIndex++;
             }
             $offset = $chunkOffset;
             for ($i = 0; $i < $perChunk && $sample < $sampleCount; $i++, $sample++) {

tests/danog/MadelineProto/Mp4Test.php

@@ -108,4 +108,49 @@ public function testEveryFrameIsDemuxedWithItsPresentationTimestamp(array $flags
             @unlink($file);
         }
     }
+
+    /**
+     * Interleaved audio and video make a sample-to-chunk table about as long as the chunk list: it used to be rescanned
+     * for each chunk, blocking the event loop for minutes before the first frame of a film.
+     */
+    public function testLongSampleToChunkTableIsParsedInLinearTime(): void
+    {
+        exec('ffmpeg -hide_banner -encoders 2>/dev/null', $encoders, $code);
+        if ($code !== 0 || !str_contains(implode("\n", $encoders), 'libx264')) {
+            self::markTestSkipped('ffmpeg with libx264 is needed');
+        }
+        $file = tempnam(sys_get_temp_dir(), 'mp4').'.mp4';
+        try {
+            exec(implode(' ', array_map(escapeshellarg(...), [
+                'ffmpeg', '-hide_banner', '-loglevel', 'error', '-y',
+                '-f', 'lavfi', '-i', 'testsrc2=size=64x48:rate=25', '-f', 'lavfi', '-i', 'sine=sample_rate=48000',
+                '-t', '1800', '-c:v', 'libx264', '-preset', 'ultrafast', '-g', '250', '-pix_fmt', 'yuv420p',
+                '-c:a', 'aac', '-b:a', '16k', '-movflags', '+faststart', $file,
+            ])), $output, $code);
+            self::assertSame(0, $code, implode("\n", $output));
+            $data = (string) file_get_contents($file);
+            $pos = strpos($data, 'stsc');
+            $entries = 0;
+            for (; $pos !== false; $pos = strpos($data, 'stsc', $pos + 1)) {
+                $entries = max($entries, unpack('N', $data, $pos + 8)[1]);
+            }
+            self::assertGreaterThan(10000, $entries, 'The sample-to-chunk table is too short to tell');
+
+            $start = hrtime(true);
+            $mp4 = new Mp4(new ReadableBuffer($data));
+            $video = 0;
+            foreach ($mp4->frames as $frame) {
+                if ($video === 0) {
+                    // Quadratic, this took about 20 seconds.
+                    self::assertLessThan(2_000_000_000, hrtime(true) - $start, 'The sample tables took too long to parse');
+                }
+                if ($frame['codec'] === 'V_MPEG4/ISO/AVC') {
+                    $video++;
+                }
+            }
+            self::assertSame(1800 * 25, $video);
+        } finally {
+            @unlink($file);
+        }
+    }
 }