audio: drop buffered filter data when seeking

This could lead to (barely) audible artifacts with --af=scaletempo and
modified playback speed.

audio/filter/af.c

@@ -746,3 +746,10 @@ struct af_instance *af_control_any_rev(struct af_stream *s, int cmd, void *arg)
     }
     return NULL;
 }
+
+/* Send control to all filters. Never stop, even if a filter returns AF_OK. */
+void af_control_all(struct af_stream *s, int cmd, void *arg)
+{
+    for (struct af_instance *af = s->first; af; af = af->next)
+        af->control(af, cmd, arg);
+}

audio/filter/af.h

@@ -99,6 +99,7 @@ struct af_stream {
 // Parameters for af_control_*
 enum af_control {
     AF_CONTROL_REINIT = 1,
+    AF_CONTROL_RESET,
     AF_CONTROL_COMMAND_LINE,
     AF_CONTROL_SET_RESAMPLE_RATE,
     AF_CONTROL_SET_FORMAT,
@@ -125,6 +126,7 @@ void af_uninit(struct af_stream *s);
 struct af_instance *af_add(struct af_stream *s, char *name, char **args);
 struct mp_audio *af_play(struct af_stream *s, struct mp_audio *data);
 struct af_instance *af_control_any_rev(struct af_stream *s, int cmd, void *arg);
+void af_control_all(struct af_stream *s, int cmd, void *arg);
 
 double af_calc_filter_multiplier(struct af_stream *s);
 double af_calc_delay(struct af_stream *s);

audio/filter/af_lavrresample.c

@@ -93,11 +93,19 @@ static int get_delay(struct af_resample *s)
 {
     return avresample_get_delay(s->avrctx);
 }
+static void drop_all_output(struct af_resample *s)
+{
+    while (avresample_read(s->avrctx, NULL, 1000) > 0) {}
+}
 #else
 static int get_delay(struct af_resample *s)
 {
     return swr_get_delay(s->avrctx, s->ctx.in_rate);
 }
+static void drop_all_output(struct af_resample *s)
+{
+    while (swr_drop_output(s->avrctx, 1000) > 0) {}
+}
 #endif
 
 static double af_resample_default_cutoff(int filter_size)
@@ -273,6 +281,9 @@ static int control(struct af_instance *af, int cmd, void *arg)
     case AF_CONTROL_SET_RESAMPLE_RATE:
         out->rate = *(int *)arg;
         return AF_OK;
+    case AF_CONTROL_RESET:
+        drop_all_output(s);
+        return AF_OK;
     }
     return AF_UNKNOWN;
 }

audio/filter/af_scaletempo.c

@@ -428,6 +428,11 @@ static int control(struct af_instance *af, int cmd, void *arg)
         }
         return AF_OK;
     }
+    case AF_CONTROL_RESET:
+        s->bytes_queued = 0;
+        s->bytes_to_slide = 0;
+        s->frames_stride_error = 0;
+        memset(s->buf_overlap, 0, s->bytes_overlap);
     }
     return AF_UNKNOWN;
 }

mpvcore/player/playloop.c

@@ -184,6 +184,8 @@ static void seek_reset(struct MPContext *mpctx, bool reset_ao)
 
     if (mpctx->sh_audio) {
         resync_audio_stream(mpctx->sh_audio);
+        if (mpctx->sh_audio->afilter)
+            af_control_all(mpctx->sh_audio->afilter, AF_CONTROL_RESET, NULL);
         if (reset_ao)
             clear_audio_output_buffers(mpctx);
         clear_audio_decode_buffers(mpctx);