prefer avio_check() over url_exist()

The problem with url_exist() is that it tries to open a resource in
RDONLY mode. If the file is a FIFO and there is already a reading
client, the open() call will hang.

By using avio_check() with access mode of 0, the second reading
process will check if the file exists without attempting to open it,
thus avoiding the lock.

Fix issue #1663.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

ffmpeg.c

@@ -3733,7 +3733,7 @@ static void opt_output_file(const char *filename)
             (strchr(filename, ':') == NULL ||
              filename[1] == ':' ||
              av_strstart(filename, "file:", NULL))) {
-            if (url_exist(filename)) {
+            if (avio_check(filename, 0) == 0) {
                 if (!using_stdin) {
                     fprintf(stderr,"File '%s' already exists. Overwrite ? [y/N] ", filename);
                     fflush(stderr);

ffserver.c

@@ -3679,7 +3679,7 @@ static void build_feed_streams(void)
     for(feed = first_feed; feed != NULL; feed = feed->next_feed) {
         int fd;
 
-        if (url_exist(feed->feed_filename)) {
+        if (avio_check(feed->feed_filename, AVIO_FLAG_READ) > 0) {
             /* See if it matches */
             AVFormatContext *s;
             int matches = 0;
@@ -3752,7 +3752,7 @@ static void build_feed_streams(void)
                 unlink(feed->feed_filename);
             }
         }
-        if (!url_exist(feed->feed_filename)) {
+        if (avio_check(feed->feed_filename, AVIO_FLAG_WRITE) <= 0) {
             AVFormatContext s1 = {0}, *s = &s1;
 
             if (feed->readonly) {

libavformat/img2.c

@@ -131,11 +131,11 @@ static int find_image_range(int *pfirst_index, int *plast_index,
         if (av_get_frame_filename(buf, sizeof(buf), path, first_index) < 0){
             *pfirst_index =
             *plast_index = 1;
-            if(url_exist(buf))
+            if (avio_check(buf, AVIO_FLAG_READ) > 0)
                 return 0;
             return -1;
         }
-        if (url_exist(buf))
+        if (avio_check(buf, AVIO_FLAG_READ) > 0)
             break;
     }
     if (first_index == 5)
@@ -153,7 +153,7 @@ static int find_image_range(int *pfirst_index, int *plast_index,
             if (av_get_frame_filename(buf, sizeof(buf), path,
                                       last_index + range1) < 0)
                 goto fail;
-            if (!url_exist(buf))
+            if (avio_check(buf, AVIO_FLAG_READ) <= 0)
                 break;
             range = range1;
             /* just in case... */