AVCodecContext.extra_hw_frames is added to the size of hardware frame
pools created by libavcodec for APIs which require fixed-size pools.
This allows the user to keep references to a greater number of frames
after decode, which may be necessary for some use-cases.

It is also added to the initial_pool_size value returned by
avcodec_get_hw_frames_parameters() if a fixed-size pool is required.
---
 doc/APIchanges             |  3 +++
 libavcodec/avcodec.h       | 14 ++++++++++++++
 libavcodec/decode.c        |  9 +++++++++
 libavcodec/options_table.h |  1 +
 4 files changed, 27 insertions(+)

diff --git a/doc/APIchanges b/doc/APIchanges
index 0bde3a052..62c7a17e5 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -13,6 +13,9 @@ libavutil:     2017-03-23
 
 API changes, most recent first:
 
+2018-xx-xx - xxxxxxx - lavc 58.x+1.0 - avcodec.h
+  Add AVCodecContext.extra_hw_frames.
+
 2017-xx-xx - xxxxxxx - lavc 58.8.0 - avcodec.h
   Add const to AVCodecContext.hwaccel.
 
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 7eaa0c927..03a3d5bd6 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2739,6 +2739,20 @@ typedef struct AVCodecContext {
      *             AVCodecContext.get_format callback)
      */
     int hwaccel_flags;
+
+    /**
+     * Video decoding only.  Sets the number of extra hardware frames which
+     * the decoder will allocate for use by the caller.  This must be set
+     * before avcodec_open2() is called.
+     *
+     * Some hardware decoders require all frames that they will use for
+     * output to be defined in advance before decoding starts.  For such
+     * decoders, the hardware frame pool must therefore be of a fixed size.
+     * The extra frames set here are on top of any number that the decoder
+     * needs internally in order to operate normally (for example, frames
+     * used as reference pictures).
+     */
+    int extra_hw_frames;
 } AVCodecContext;
 
 /**
diff --git a/libavcodec/decode.c b/libavcodec/decode.c
index 12a95d422..e024a3232 100644
--- a/libavcodec/decode.c
+++ b/libavcodec/decode.c
@@ -791,6 +791,15 @@ int avcodec_get_hw_frames_parameters(AVCodecContext *avctx,
 
     ret = hwa->frame_params(avctx, frames_ref);
     if (ret >= 0) {
+        AVHWFramesContext *frames_ctx = (AVHWFramesContext*)frames_ref->data;
+
+        if (frames_ctx->initial_pool_size) {
+            // If the user has requested that extra output surfaces be
+            // available then add them here.
+            if (avctx->extra_hw_frames > 0)
+                frames_ctx->initial_pool_size += avctx->extra_hw_frames;
+        }
+
         *out_frames_ref = frames_ref;
     } else {
         av_buffer_unref(&frames_ref);
diff --git a/libavcodec/options_table.h b/libavcodec/options_table.h
index 925ef376f..4b0a8344d 100644
--- a/libavcodec/options_table.h
+++ b/libavcodec/options_table.h
@@ -419,6 +419,7 @@ static const AVOption avcodec_options[] = {
 {"side_data_only_packets", NULL, OFFSET(side_data_only_packets), 
AV_OPT_TYPE_INT, { .i64 = 1 }, 0, 1, A|V|E },
 #endif
 {"apply_cropping", NULL, OFFSET(apply_cropping), AV_OPT_TYPE_INT, { .i64 = 1 
}, 0, 1, V | D },
+{"extra_hw_frames", "Number of extra hardware frames to allocate for the 
user", OFFSET(extra_hw_frames), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, 
V|D },
 {NULL},
 };
 
-- 
2.11.0

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