lavfi: use min_perms and rej_perms for out pads.

There are several reasons for doing that:

1. It documents the code for the reader and helps find
   inconsistencies and bugs.

2. For rej_perms, it guarantees the change will be done
   even if the output reference can be created by several
   code paths.

3. It can be used to predict cases where a copy will,
   or will not happen and optimize buffer allocation
   (for example not request a rare direct-rendering buffer
   from a device sink if it will be copied anyway).

Note that a filter is still allowed to manage the permissions
on its own without using these fields.

1 files changed, 6 insertions, 1 deletions

diff --git a/libavfilter/video.c b/libavfilter/video.c
index eb92edd8f2..b7a8b86dad 100644
--- a/libavfilter/video.c
+++ b/libavfilter/video.c
@@ -232,8 +232,9 @@ static void clear_link(AVFilterLink *link)
 int ff_start_frame(AVFilterLink *link, AVFilterBufferRef *picref)
 {
     int (*start_frame)(AVFilterLink *, AVFilterBufferRef *);
+    AVFilterPad *src = link->srcpad;
     AVFilterPad *dst = link->dstpad;
-    int ret, perms = picref->perms;
+    int ret, perms;
     AVFilterCommand *cmd= link->dst->command_queue;
     int64_t pts;
 
@@ -242,6 +243,10 @@ int ff_start_frame(AVFilterLink *link, AVFilterBufferRef *picref)
     if (!(start_frame = dst->start_frame))
         start_frame = default_start_frame;
 
+    av_assert1((picref->perms & src->min_perms) == src->min_perms);
+    picref->perms &= ~ src->rej_perms;
+    perms = picref->perms;
+
     if (picref->linesize[0] < 0)
         perms |= AV_PERM_NEG_LINESIZES;
     /* prepare to copy the picture if it has insufficient permissions */