avformat/imf: open resources only when first needed

IMF CPLs can reference thousands of files, which can result in system limits
for the number of open files to be exceeded. The following patch opens and
closes files as needed.

Addresses https://trac.ffmpeg.org/ticket/9623

libavformat/imfdec.c

@@ -103,10 +103,11 @@ typedef struct IMFVirtualTrackPlaybackCtx {
     int32_t index;                                 /**< Track index in playlist */
     AVRational current_timestamp;                  /**< Current temporal position */
     AVRational duration;                           /**< Overall duration */
-    uint32_t resource_count;                       /**< Number of resources */
+    uint32_t resource_count;                       /**< Number of resources (<= INT32_MAX) */
     unsigned int resources_alloc_sz;               /**< Size of the buffer holding the resource */
     IMFVirtualTrackResourcePlaybackCtx *resources; /**< Buffer holding the resources */
-    uint32_t current_resource_index;               /**< Current resource */
+    int32_t current_resource_index;                /**< Index of the current resource in resources,
+                                                        or < 0 if a current resource has yet to be selected */
     int64_t last_pts;                              /**< Last timestamp */
 } IMFVirtualTrackPlaybackCtx;
 
@@ -435,7 +436,6 @@ static int open_track_file_resource(AVFormatContext *s,
     IMFContext *c = s->priv_data;
     IMFAssetLocator *asset_locator;
     void *tmp;
-    int ret;
 
     asset_locator = find_asset_map_locator(&c->asset_locator_map, track_file_resource->track_file_uuid);
     if (!asset_locator) {
@@ -452,7 +452,7 @@ static int open_track_file_resource(AVFormatContext *s,
            UID_ARG(asset_locator->uuid),
            asset_locator->absolute_uri);
 
-    if (track->resource_count > UINT32_MAX - track_file_resource->base.repeat_count
+    if (track->resource_count > INT32_MAX - track_file_resource->base.repeat_count
         || (track->resource_count + track_file_resource->base.repeat_count)
             > INT_MAX / sizeof(IMFVirtualTrackResourcePlaybackCtx))
         return AVERROR(ENOMEM);
@@ -470,8 +470,6 @@ static int open_track_file_resource(AVFormatContext *s,
         vt_ctx.locator = asset_locator;
         vt_ctx.resource = track_file_resource;
         vt_ctx.ctx = NULL;
-        if ((ret = open_track_resource_context(s, &vt_ctx)) != 0)
-            return ret;
         track->resources[track->resource_count++] = vt_ctx;
         track->duration = av_add_q(track->duration,
                                    av_make_q((int)track_file_resource->base.duration
@@ -501,6 +499,7 @@ static int open_virtual_track(AVFormatContext *s,
 
     if (!(track = av_mallocz(sizeof(IMFVirtualTrackPlaybackCtx))))
         return AVERROR(ENOMEM);
+    track->current_resource_index = -1;
     track->index = track_index;
     track->duration = av_make_q(0, 1);
 
@@ -551,6 +550,9 @@ static int set_context_streams_from_tracks(AVFormatContext *s)
         AVStream *first_resource_stream;
 
         /* Open the first resource of the track to get stream information */
+        ret = open_track_resource_context(s, &c->tracks[i]->resources[0]);
+        if (ret)
+            return ret;
         first_resource_stream = c->tracks[i]->resources[0].ctx->streams[0];
         av_log(s, AV_LOG_DEBUG, "Open the first resource of track %d\n", c->tracks[i]->index);
 
@@ -741,7 +743,8 @@ static IMFVirtualTrackResourcePlaybackCtx *get_resource_context_for_timestamp(AV
                        track->index);
                 if (open_track_resource_context(s, &(track->resources[i])) != 0)
                     return NULL;
-                avformat_close_input(&(track->resources[track->current_resource_index].ctx));
+                if (track->current_resource_index > 0)
+                    avformat_close_input(&track->resources[track->current_resource_index].ctx);
                 track->current_resource_index = i;
             }