avio: add avio_check()

The new function is more flexible than url_exist(), as it allows to
specify which access flags to check, and does not require an explicit
open of the checked resource.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/avio.c

@@ -374,6 +374,25 @@ int url_exist(const char *filename)
     return 1;
 }
 
+int avio_check(const char *url, int flags)
+{
+    URLContext *h;
+    int ret = ffurl_alloc(&h, url, flags);
+    if (ret)
+        return ret;
+
+    if (h->prot->url_check) {
+        ret = h->prot->url_check(h, flags);
+    } else {
+        ret = ffurl_connect(h);
+        if (ret >= 0)
+            ret = flags;
+    }
+
+    ffurl_close(h);
+    return ret;
+}
+
 int64_t ffurl_size(URLContext *h)
 {
     int64_t pos, size;

libavformat/avio.h

@@ -134,6 +134,7 @@ typedef struct URLProtocol {
     int priv_data_size;
     const AVClass *priv_data_class;
     int flags;
+    int (*url_check)(URLContext *h, int mask);
 } URLProtocol;
 
 typedef struct URLPollEntry {
@@ -346,6 +347,23 @@ attribute_deprecated int url_close_buf(AVIOContext *s);
  */
 int url_exist(const char *url);
 
+/**
+ * Return AVIO_* access flags corresponding to the access permissions
+ * of the resource in url, or a negative value corresponding to an
+ * AVERROR code in case of failure. The returned access flags are
+ * masked by the value in flags.
+ *
+ * @note This function is intrinsically unsafe, in the sense that the
+ * checked resource may change its existence or permission status from
+ * one call to another. Thus you should not trust the returned value,
+ * unless you are sure that no other processes are accessing the
+ * checked resource.
+ *
+ * @note This function is slightly broken until next major bump
+ *       because of AVIO_RDONLY == 0. Don't use it until then.
+ */
+int avio_check(const char *url, int flags);
+
 /**
  * The callback is called in blocking functions to test regulary if
  * asynchronous interruption is needed. AVERROR_EXIT is returned

libavformat/url.h

@@ -59,6 +59,7 @@ typedef struct URLProtocol {
     int priv_data_size;
     const AVClass *priv_data_class;
     int flags;
+    int (*url_check)(URLContext *h, int mask);
 } URLProtocol;
 #endif