avcodec/cbs: Factor out common code for writing units

All cbs-functions to write units share a common pattern:
1. They check whether they have a write buffer (that is used to store
the unit's data until the needed size becomes known after writing the
unit when a dedicated buffer will be allocated).
2. They use this buffer for a PutBitContext.
3. The (codec-specific) writing takes place through the PutBitContext.
4. The return value is checked. AVERROR(ENOSPC) here always indicates
that the buffer was too small and leads to a reallocation of said
buffer.
5. The final buffer will be allocated and the data copied.

This commit factors this common code out in a single function in cbs.c.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 7c92eaace2b338e0b3acc18e1543b365610578fd)

1 files changed, 7 insertions, 0 deletions

diff --git a/libavcodec/cbs.h b/libavcodec/cbs.h
index fe57e7b2a5..1974cf3e66 100644
--- a/libavcodec/cbs.h
+++ b/libavcodec/cbs.h
@@ -210,6 +210,13 @@ typedef struct CodedBitstreamContext {
      * From AV_LOG_*; defaults to AV_LOG_TRACE.
      */
     int trace_level;
+
+    /**
+     * Write buffer. Used as intermediate buffer when writing units.
+     * For internal use of cbs only.
+     */
+    uint8_t *write_buffer;
+    size_t   write_buffer_size;
 } CodedBitstreamContext;