ffmpeg: abort runaway encodes when FPS/filter explodes frame count

ffmpeg/decoder.h

@@ -13,6 +13,9 @@ struct input_ctx {
   int vi, ai; // video and audio stream indices
   int dv, da; // flags whether to drop video or audio
 
+  // Decoded results for current transcode call (for early abort checks)
+  output_results *decoded_res;
+
   // Hardware decoding support
   AVBufferRef *hw_device_ctx;
   enum AVHWDeviceType hw_type;

ffmpeg/encoder.c

@@ -4,6 +4,7 @@
 #include <libavcodec/avcodec.h>
 #include <libavfilter/buffersrc.h>
 #include <libavfilter/buffersink.h>
+#include <string.h>
 
 static int add_video_stream(struct output_ctx *octx, struct input_ctx *ictx)
 {
@@ -625,6 +626,23 @@ int process_out(struct input_ctx *ictx, struct output_ctx *octx, AVCodecContext
         }
       }
 
+    // Check for runaway encodes where the FPS filter produces too many frames
+    // Unclear what causes these
+    if (is_video && frame && ictx->decoded_res && ictx->decoded_res->frames > 0) {
+      if (ictx->ic && ictx->ic->iformat &&
+          !strcmp(ictx->ic->iformat->name, "image2")) {
+        // Image sequence input can legitimately expand frame counts.
+        goto after_runaway_check;
+      }
+      int64_t decoded_frames = ictx->decoded_res->frames;
+      if ((int64_t)octx->res->frames + 1 > 25 * decoded_frames) {
+        av_frame_unref(frame);
+        ret = lpms_ERR_ENC_RUNAWAY;
+        goto proc_cleanup;
+      }
+    }
+after_runaway_check:
+
       ret = encode(encoder, frame, octx, ost);
 skip:
     av_frame_unref(frame);

ffmpeg/ffmpeg_errors.go

@@ -20,6 +20,7 @@ var lpmsErrors = []struct {
 	{Code: C.lpms_ERR_INPUT_CODEC, Desc: "Unsupported input codec"},
 	{Code: C.lpms_ERR_INPUT_NOKF, Desc: "No keyframes in input"},
 	{Code: C.lpms_ERR_UNRECOVERABLE, Desc: "Unrecoverable state, restart process"},
+	{Code: C.lpms_ERR_ENC_RUNAWAY, Desc: "Encoded frames runaway"},
 }
 
 func error_map() map[int]error {

ffmpeg/ffmpeg_test.go

@@ -1536,6 +1536,33 @@ func TestTranscoder_PassthroughFPS(t *testing.T) {
 	run(cmd)
 }
 
+func TestTranscoder_EncodedFrameRunaway(t *testing.T) {
+	run, dir := setupTest(t)
+	defer os.RemoveAll(dir)
+
+	// generate a sample with 10 seconds between frames
+	// transcode to 5 fps == 50 output frames per input frame
+	// should trigger runaway frame detection (output > 25x input)
+
+	cmd := `
+        cp "$1/../transcoder/test.ts" test.ts
+        ffmpeg -i test.ts -vf "setpts=N*10/TB" -frames:v 5 -c:v libx264 -bf 0 -fps_mode vfr -an lowfps.mp4
+        ffprobe -v warning -select_streams v -show_entries format=duration lowfps.mp4 | grep duration=40.0
+    `
+	run(cmd)
+
+	profile := P144p30fps16x9
+	profile.Framerate = 5
+	in := &TranscodeOptionsIn{Fname: dir + "/lowfps.mp4"}
+	out := []TranscodeOptions{{
+		Oname:   dir + "/out-60fps.ts",
+		Profile: profile,
+	}}
+
+	_, err := Transcode3(in, out)
+	assert.EqualError(t, err, "Encoded frames runaway")
+}
+
 func TestTranscoder_PassthroughFPS_AdjustTimestamps(t *testing.T) {
 	// check timestamp adjustments for fps passthrough
 

ffmpeg/transcoder.c

@@ -20,6 +20,7 @@ const int lpms_ERR_PACKET_ONLY = FFERRTAG('P','K','O','N');
 const int lpms_ERR_FILTER_FLUSHED = FFERRTAG('F','L','F','L');
 const int lpms_ERR_OUTPUTS = FFERRTAG('O','U','T','P');
 const int lpms_ERR_UNRECOVERABLE = FFERRTAG('U', 'N', 'R', 'V');
+const int lpms_ERR_ENC_RUNAWAY = FFERRTAG('E', 'N', 'R', 'W');
 
 //
 //  Notes on transcoder internals:
@@ -326,6 +327,8 @@ int transcode(struct transcode_thread *h,
   int nb_outputs = h->nb_outputs;
   int outputs_ready = 0, hit_eof = 0;
 
+  ictx->decoded_res = decoded_results;
+
   ipkt = av_packet_alloc();
   if (!ipkt) LPMS_ERR(transcode_cleanup, "Unable to allocated packet");
   dframe = av_frame_alloc();
@@ -552,6 +555,7 @@ int transcode(struct transcode_thread *h,
         avformat_close_input(&ictx->ic);
         ictx->ic = NULL;
     }
+    ictx->decoded_res = NULL;
     return 0;
   }
 
@@ -562,6 +566,7 @@ int transcode(struct transcode_thread *h,
   }
 
 transcode_cleanup:
+  ictx->decoded_res = NULL;
   if (dframe) av_frame_free(&dframe);
   if (ipkt) av_packet_free(&ipkt);  // needed for early exits
   if (frame_queue) queue_free(&frame_queue);

ffmpeg/transcoder.h

@@ -17,6 +17,7 @@ extern const int lpms_ERR_PACKET_ONLY;
 extern const int lpms_ERR_FILTER_FLUSHED;
 extern const int lpms_ERR_OUTPUTS;
 extern const int lpms_ERR_UNRECOVERABLE;
+extern const int lpms_ERR_ENC_RUNAWAY;
 
 struct transcode_thread;