avcodec/encode: Enable encoders to control padding of last frame

Some audio codecs work with atomic units that decode to a fixed
number of audio samples with this number being so small that it is
common to put multiple of these atoms into one packet. In these
cases it makes no sense to pad the last frame to the big frame_size,
so allow encoders to set the number of samples that they want
the last frame to be padded to instead.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/encode.c

@@ -127,12 +127,12 @@ static int encode_make_refcounted(AVCodecContext *avctx, AVPacket *avpkt)
 /**
  * Pad last frame with silence.
  */
-static int pad_last_frame(AVCodecContext *s, AVFrame *frame, const AVFrame *src)
+static int pad_last_frame(AVCodecContext *s, AVFrame *frame, const AVFrame *src, int out_samples)
 {
     int ret;
 
     frame->format         = src->format;
-    frame->nb_samples     = s->frame_size;
+    frame->nb_samples     = out_samples;
     ret = av_channel_layout_copy(&frame->ch_layout, &s->ch_layout);
     if (ret < 0)
         goto fail;
@@ -406,10 +406,15 @@ static int encode_send_frame_internal(AVCodecContext *avctx, const AVFrame *src)
             if (src->nb_samples < avctx->frame_size) {
                 avctx->internal->last_audio_frame = 1;
                 if (!(avctx->codec->capabilities & AV_CODEC_CAP_SMALL_LAST_FRAME)) {
-                ret = pad_last_frame(avctx, dst, src);
-                if (ret < 0)
-                    return ret;
-                goto finish;
+                    int pad_samples = avci->pad_samples ? avci->pad_samples : avctx->frame_size;
+                    int out_samples = (src->nb_samples + pad_samples - 1) / pad_samples * pad_samples;
+
+                    if (out_samples != src->nb_samples) {
+                        ret = pad_last_frame(avctx, dst, src, out_samples);
+                        if (ret < 0)
+                            return ret;
+                        goto finish;
+                    }
                 }
             }
         }

libavcodec/internal.h

@@ -62,6 +62,12 @@ typedef struct AVCodecInternal {
      */
     int last_audio_frame;
 
+    /**
+     * Audio encoders can set this flag during init to indicate that they
+     * want the small last frame to be padded to a multiple of pad_samples.
+     */
+    int pad_samples;
+
     AVBufferRef *pool;
 
     void *thread_ctx;