vda_h264_dec: backup context before overriding

Some of context fields which must be overrided by this wrapper decoder
may be set and used by user, so we have to save and restore them before
and after invoking the inner decoder to secure user.

libavcodec/vda_h264_dec.c

@@ -56,6 +56,15 @@ typedef struct {
     int h264_initialized;
     struct vda_context vda_ctx;
     enum AVPixelFormat pix_fmt;
+
+    /* for backing-up fields set by user.
+     * we have to gain full control of such fields here */
+    void *hwaccel_context;
+    enum AVPixelFormat (*get_format)(struct AVCodecContext *s, const enum AVPixelFormat * fmt);
+    int (*get_buffer2)(struct AVCodecContext *s, AVFrame *frame, int flags);
+#if FF_API_GET_BUFFER
+    int (*get_buffer)(struct AVCodecContext *c, AVFrame *pic);
+#endif
 } VDADecoderContext;
 
 static enum AVPixelFormat get_format(struct AVCodecContext *avctx,
@@ -90,6 +99,32 @@ static int get_buffer2(AVCodecContext *avctx, AVFrame *pic, int flag)
     return 0;
 }
 
+static inline void set_context(AVCodecContext *avctx)
+{
+    VDADecoderContext *ctx = avctx->priv_data;
+    ctx->hwaccel_context = avctx->hwaccel_context;
+    avctx->hwaccel_context = &ctx->vda_ctx;
+    ctx->get_format = avctx->get_format;
+    avctx->get_format = get_format;
+    ctx->get_buffer2 = avctx->get_buffer2;
+    avctx->get_buffer2 = get_buffer2;
+#if FF_API_GET_BUFFER
+    ctx->get_buffer = avctx->get_buffer;
+    avctx->get_buffer = NULL;
+#endif
+}
+
+static inline void restore_context(AVCodecContext *avctx)
+{
+    VDADecoderContext *ctx = avctx->priv_data;
+    avctx->hwaccel_context = ctx->hwaccel_context;
+    avctx->get_format = ctx->get_format;
+    avctx->get_buffer2 = ctx->get_buffer2;
+#if FF_API_GET_BUFFER
+    avctx->get_buffer = ctx->get_buffer;
+#endif
+}
+
 static int vdadec_decode(AVCodecContext *avctx,
         void *data, int *got_frame, AVPacket *avpkt)
 {
@@ -97,7 +132,9 @@ static int vdadec_decode(AVCodecContext *avctx,
     AVFrame *pic = data;
     int ret;
 
+    set_context(avctx);
     ret = ff_h264_decoder.decode(avctx, data, got_frame, avpkt);
+    restore_context(avctx);
     if (*got_frame) {
         AVBufferRef *buffer = pic->buf[0];
         VDABufferContext *context = av_buffer_get_opaque(buffer);
@@ -130,8 +167,11 @@ static av_cold int vdadec_close(AVCodecContext *avctx)
     /* release buffers and decoder */
     ff_vda_destroy_decoder(&ctx->vda_ctx);
     /* close H.264 decoder */
-    if (ctx->h264_initialized)
+    if (ctx->h264_initialized) {
+        set_context(avctx);
         ff_h264_decoder.close(avctx);
+        restore_context(avctx);
+    }
     return 0;
 }
 
@@ -184,18 +224,11 @@ static av_cold int vdadec_init(AVCodecContext *avctx)
                 "Failed to init VDA decoder: %d.\n", status);
         goto failed;
     }
-    avctx->hwaccel_context = vda_ctx;
-
-    /* changes callback functions */
-    avctx->get_format = get_format;
-    avctx->get_buffer2 = get_buffer2;
-#if FF_API_GET_BUFFER
-    // force the old get_buffer to be empty
-    avctx->get_buffer = NULL;
-#endif
 
     /* init H.264 decoder */
+    set_context(avctx);
     ret = ff_h264_decoder.init(avctx);
+    restore_context(avctx);
     if (ret < 0) {
         av_log(avctx, AV_LOG_ERROR, "Failed to open H.264 decoder.\n");
         goto failed;
@@ -211,7 +244,9 @@ failed:
 
 static void vdadec_flush(AVCodecContext *avctx)
 {
-    return ff_h264_decoder.flush(avctx);
+    set_context(avctx);
+    ff_h264_decoder.flush(avctx);
+    restore_context(avctx);
 }
 
 AVCodec ff_h264_vda_decoder = {