eliminate stat/fopen race in example

src/examples/https_fileserver_example.c

@@ -125,6 +125,7 @@ http_ahc (void *cls,
   struct MHD_Response *response;
   int ret;
   FILE *file;
+  int fd;
   struct stat buf;
 
   if (0 != strcmp (method, MHD_HTTP_METHOD_GET))
@@ -137,12 +138,22 @@ http_ahc (void *cls,
     }
   *ptr = NULL;                  /* reset when done */
 
-  if ( (0 == stat (&url[1], &buf)) &&
-       (S_ISREG (buf.st_mode)) )
-    file = fopen (&url[1], "rb");
-  else
-    file = NULL;
-  if (file == NULL)
+  file = fopen (&url[1], "rb");
+  if (NULL != file)
+    {
+      fd = fileno (file);
+      if (-1 == fd)
+        return MHD_NO; /* internal error */
+      if ( (0 != fstat (fd, &buf)) ||
+           (! S_ISREG (buf.st_mode)) )
+        {
+          /* not a regular file, refuse to serve */
+          fclose (file);
+          file = NULL;
+        }
+    }
+
+  if (NULL == file)
     {
       response = MHD_create_response_from_buffer (strlen (EMPTY_PAGE),
 						  (void *) EMPTY_PAGE,
@@ -155,7 +166,7 @@ http_ahc (void *cls,
       response = MHD_create_response_from_callback (buf.st_size, 32 * 1024,     /* 32k PAGE_NOT_FOUND size */
                                                     &file_reader, file,
                                                     &file_free_callback);
-      if (response == NULL)
+      if (NULL == response)
 	{
 	  fclose (file);
 	  return MHD_NO;
@@ -166,6 +177,7 @@ http_ahc (void *cls,
   return ret;
 }
 
+
 int
 main (int argc, char *const *argv)
 {