seekless filesize retrieving support in 7 lines of code, also doesnt break compatibility

Originally committed as revision 7395 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avio.c

@@ -148,9 +148,12 @@ offset_t url_filesize(URLContext *h)
 {
     offset_t pos, size;
 
+    size= url_seek(h, 0, AVSEEK_SIZE);
+    if(size<0){
     pos = url_seek(h, 0, SEEK_CUR);
     size = url_seek(h, -1, SEEK_END)+1;
     url_seek(h, pos, SEEK_SET);
+    }
     return size;
 }
 

libavformat/avio.h

@@ -69,6 +69,8 @@ void url_set_interrupt_cb(URLInterruptCB *interrupt_cb);
 /* not implemented */
 int url_poll(URLPollEntry *poll_table, int n, int timeout);
 
+#define AVSEEK_SIZE 0x10000
+
 typedef struct URLProtocol {
     const char *name;
     int (*url_open)(URLContext *h, const char *filename, int flags);

libavformat/aviobuf.c

@@ -170,8 +170,11 @@ offset_t url_fsize(ByteIOContext *s)
 
     if (!s->seek)
         return -EPIPE;
+    size = s->seek(s->opaque, 0, AVSEEK_SIZE);
+    if(size<0){
     size = s->seek(s->opaque, -1, SEEK_END) + 1;
     s->seek(s->opaque, s->pos, SEEK_SET);
+    }
     return size;
 }