demux: always use AVPacket

This is a simplification, because it lets us use the AVPacket
functions, instead of handling the details manually.

It also allows the libavcodec rawvideo decoder to use reference
counting, so it doesn't have to memcpy() the full image data. The change
in av_common.c enables this.

This change is somewhat risky, because we rely on the following AVPacket
implementation details and assumptions:
- av_packet_ref() doesn't access the input padding, and just copies the
  data. By the API, AVPacket is always padded, and we violate this. The
  lavc implementation would have to go out of its way to make this a
  real problem, though.
- We hope that the way we make the AVPacket refcountable in av_common.c
  is actually supported API-usage. It's hard to tell whether it is.

Of course we still use our own "old" demux_packet struct, just so that
libav* API usage is somewhat isolated.

1 files changed, 32 insertions, 43 deletions

diff --git a/demux/packet.c b/demux/packet.c
index 77ea78f7cc..2008113600 100644
--- a/demux/packet.c
+++ b/demux/packet.c
@@ -29,55 +29,58 @@
 static void packet_destroy(void *ptr)
 {
     struct demux_packet *dp = ptr;
-    talloc_free(dp->avpacket);
-    av_free(dp->allocation);
+    av_packet_unref(dp->avpacket);
 }
 
-static struct demux_packet *create_packet(size_t len)
+// This actually preserves only data and side data, not PTS/DTS/pos/etc.
+// It also allows avpkt->data==NULL with avpkt->size!=0 - the libavcodec API
+// does not allow it, but we do it to simplify new_demux_packet().
+struct demux_packet *new_demux_packet_from_avpacket(struct AVPacket *avpkt)
 {
-    if (len > 1000000000) {
+    if (avpkt->size > 1000000000) {
         fprintf(stderr, "Attempt to allocate demux packet over 1 GB!\n");
         abort();
     }
     struct demux_packet *dp = talloc(NULL, struct demux_packet);
     talloc_set_destructor(dp, packet_destroy);
     *dp = (struct demux_packet) {
-        .len = len,
         .pts = MP_NOPTS_VALUE,
         .dts = MP_NOPTS_VALUE,
         .duration = -1,
         .pos = -1,
         .stream = -1,
+        .avpacket = talloc_zero(dp, AVPacket),
     };
-    return dp;
-}
-
-struct demux_packet *new_demux_packet(size_t len)
-{
-    struct demux_packet *dp = create_packet(len);
-    dp->buffer = av_malloc(len + FF_INPUT_BUFFER_PADDING_SIZE);
-    if (!dp->buffer) {
-        fprintf(stderr, "Memory allocation failure!\n");
+    av_init_packet(dp->avpacket);
+    int r = -1;
+    if (avpkt->data) {
+        // We hope that this function won't need/access AVPacket input padding,
+        // because otherwise new_demux_packet_from() wouldn't work.
+        r = av_packet_ref(dp->avpacket, avpkt);
+    } else {
+        r = av_new_packet(dp->avpacket, avpkt->size);
+    }
+    if (r < 0) {
+        fprintf(stderr, "Out of memory when referencing packet.\n");
         abort();
     }
-    memset(dp->buffer + len, 0, FF_INPUT_BUFFER_PADDING_SIZE);
-    dp->allocation = dp->buffer;
+    dp->buffer = dp->avpacket->data;
+    dp->len = dp->avpacket->size;
     return dp;
 }
 
-// data must already have suitable padding, and does not copy the data
-struct demux_packet *new_demux_packet_fromdata(void *data, size_t len)
+// Input data doesn't need to be padded.
+struct demux_packet *new_demux_packet_from(void *data, size_t len)
 {
-    struct demux_packet *dp = create_packet(len);
-    dp->buffer = data;
-    return dp;
+    AVPacket pkt = { .data = data, .size = len };
+    return new_demux_packet_from_avpacket(&pkt);
 }
 
-struct demux_packet *new_demux_packet_from(void *data, size_t len)
+struct demux_packet *new_demux_packet(size_t len)
 {
-    struct demux_packet *dp = new_demux_packet(len);
-    memcpy(dp->buffer, data, len);
-    return dp;
+    assert(len <= INT_MAX);
+    AVPacket pkt = { .data = NULL, .size = len };
+    return new_demux_packet_from_avpacket(&pkt);
 }
 
 void demux_packet_shorten(struct demux_packet *dp, size_t len)
@@ -92,28 +95,14 @@ void free_demux_packet(struct demux_packet *dp)
     talloc_free(dp);
 }
 
-static void destroy_avpacket(void *pkt)
-{
-    av_free_packet(pkt);
-}
-
 struct demux_packet *demux_copy_packet(struct demux_packet *dp)
 {
     struct demux_packet *new = NULL;
     if (dp->avpacket) {
-        assert(dp->buffer == dp->avpacket->data);
-        assert(dp->len == dp->avpacket->size);
-        AVPacket *newavp = talloc_zero(NULL, AVPacket);
-        talloc_set_destructor(newavp, destroy_avpacket);
-        av_init_packet(newavp);
-        if (av_packet_ref(newavp, dp->avpacket) < 0)
-            abort();
-        new = new_demux_packet_fromdata(newavp->data, newavp->size);
-        new->avpacket = newavp;
-    }
-    if (!new) {
-        new = new_demux_packet(dp->len);
-        memcpy(new->buffer, dp->buffer, new->len);
+        new = new_demux_packet_from_avpacket(dp->avpacket);
+    } else {
+        // Some packets might be not created by new_demux_packet*().
+        new = new_demux_packet_from(dp->buffer, dp->len);
     }
     new->pts = dp->pts;
     new->dts = dp->dts;