avcodec/utils: Add utility functions for bsf

Several bitstream filters (e.g. dump_extradata, imxdump, mjpeg2jpeg,
mjpegadump, mp3decomp, ...) don't buffer packets; instead, they just
modify the buffer of one packet and don't change any other of the
packet's non-buffer properties. The usual approach of these bitstream
filters is to use separate packets for in- and output as follows:

1. Get the input packet via ff_bsf_get_packet which entails an allocation.
2. Use av_new_packet to allocate a big enough buffer in the output
packet.
3. Perform the actual work of the bitstream filter, i.e. fill the output
buffer.
4. Use av_packet_copy_props to copy the non-buffer fields of the input
packet to the output packet.
5. Free the input packet and return.

This commit adds two utility functions that allow a different approach:
A function to (re)allocate a refcounted buffer with zeroed padding and a
function to replace a packet's buffer and the buffer-related fields with
information from an AVBufferRef. This allows to modify the bitstream
filters as follows:

1. Get the packet via ff_bsf_get_packet_ref.
2. Use ff_buffer_padded_realloc to get a big enough refcounted buffer.
3. Perform the actual work of the bitstream filter.
4. Use ff_packet_replace_buffer to replace the old data in the packet
with the modified one and return.

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

libavcodec/internal.h

@@ -424,6 +424,32 @@ int64_t ff_guess_coded_bitrate(AVCodecContext *avctx);
 int ff_int_from_list_or_default(void *ctx, const char * val_name, int val,
                                 const int * array_valid_values, int default_value);
 
+/**
+ * Reallocate an AVBufferRef of effective size size + grow_by. In addition,
+ * the buffer will have AV_BUFFER_INPUT_PADDING_SIZE bytes of zeroed padding
+ * at the end. The addition is checked for overflow and the size restrictions
+ * are checked, too.
+ *
+ * @param buf     Pointer to pointer to an AVBufferRef. Must not be NULL;
+ *                *buf will be reallocated to the desired size; if *buf is NULL,
+ *                it will be allocated.
+ * @param size    Must be >= 0.
+ * @param grow_by Must be so that size + grow_by >= 0.
+ * @return        Zero on success, negative error code on failure
+ */
+int ff_buffer_padded_realloc(AVBufferRef **buf, int size, int grow_by);
+
+/**
+ * Unreference the packet's buf and replace it with the given buf.
+ * The packet's data and size fields will be updated with the information
+ * from buf, too.
+ *
+ * @param pkt   Pointer to a packet to modify. Must not be NULL.
+ * @param buf   Pointer to an AVBufferRef. Must not be NULL. The
+ *              reference will be owned by pkt afterwards.
+ */
+void ff_packet_replace_buffer(AVPacket *pkt, AVBufferRef *buf);
+
 #if defined(_WIN32) && CONFIG_SHARED && !defined(BUILDING_avcodec)
 #    define av_export_avcodec __declspec(dllimport)
 #else

libavcodec/utils.c

@@ -2232,3 +2232,28 @@ int ff_int_from_list_or_default(void *ctx, const char * val_name, int val,
            "%s %d are not supported. Set to default value : %d\n", val_name, val, default_value);
     return default_value;
 }
+
+int ff_buffer_padded_realloc(AVBufferRef **buf, int size, int grow_by)
+{
+    int err;
+
+    if (size < 0 || (unsigned)size + grow_by > INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE)
+        return AVERROR(EINVAL);
+
+    err = av_buffer_realloc(buf, size + grow_by + AV_INPUT_BUFFER_PADDING_SIZE);
+    if (err < 0)
+        return err;
+    memset((*buf)->data + size + grow_by, 0, AV_INPUT_BUFFER_PADDING_SIZE);
+    (*buf)->size -= AV_INPUT_BUFFER_PADDING_SIZE;
+
+    return 0;
+}
+
+void ff_packet_replace_buffer(AVPacket *pkt, AVBufferRef *buf)
+{
+    av_buffer_unref(&pkt->buf);
+
+    pkt->buf  = buf;
+    pkt->data = buf->data;
+    pkt->size = buf->size;
+}