virSecurityManagerMetadataLock: Skip over duplicate paths

If there are two paths on the list that are the same we need to
lock it only once. Because when we try to lock it the second time
then open() fails. And if it didn't, locking it the second time
would fail for sure. After all, it is sufficient to lock all
paths just once satisfy the caller.
Reported-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Tested-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

src/security/security_manager.c

@@ -1294,16 +1294,35 @@ virSecurityManagerMetadataLock(virSecurityManagerPtr mgr ATTRIBUTE_UNUSED,
      * paths A B and there's another that is trying to lock them
      * in reversed order a deadlock might occur.  But if we sort
      * the paths alphabetically then both processes will try lock
-     * paths in the same order and thus no deadlock can occur. */
+     * paths in the same order and thus no deadlock can occur.
+     * Lastly, it makes searching for duplicate paths below
+     * simpler. */
     qsort(paths, npaths, sizeof(*paths), cmpstringp);
 
     for (i = 0; i < npaths; i++) {
         const char *p = paths[i];
         struct stat sb;
+        size_t j;
         int retries = 10 * 1000;
         int fd;
 
-        if (!p || stat(p, &sb) < 0)
+        if (!p)
+            continue;
+
+        /* If there's a duplicate path on the list, skip it over.
+         * Not only we would fail open()-ing it the second time,
+         * we would deadlock with ourselves trying to lock it the
+         * second time. After all, we've locked it when iterating
+         * over it the first time. */
+        for (j = 0; j < i; j++) {
+            if (STREQ_NULLABLE(p, paths[j]))
+                break;
+        }
+
+        if (i != j)
+            continue;
+
+        if (stat(p, &sb) < 0)
             continue;
 
         if (S_ISDIR(sb.st_mode)) {