lavf: add internal AVIOContext.maxsize

This allows simple and generic limiting of allocations used for packets.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avio.h

@@ -120,6 +120,12 @@ typedef struct {
      * A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable.
      */
     int seekable;
+
+    /**
+     * max filesize, used to limit allocations
+     * This field is internal to libavformat and access from outside is not allowed.
+     */
+     int64_t maxsize;
 } AVIOContext;
 
 /* unbuffered I/O */

libavformat/utils.c

@@ -267,7 +267,15 @@ AVInputFormat *av_find_input_format(const char *short_name)
 
 int av_get_packet(AVIOContext *s, AVPacket *pkt, int size)
 {
-    int ret= av_new_packet(pkt, size);
+    int ret;
+
+    if(s->maxsize>0){
+        int64_t remaining= s->maxsize - avio_tell(s);
+        if(remaining>=0)
+            size= FFMIN(size, remaining);
+    }
+
+    ret= av_new_packet(pkt, size);
 
     if(ret<0)
         return ret;