respond with CORS headers for OPTIONS pre-flight request

src/exchange/taler-exchange-httpd.c

@@ -420,11 +420,20 @@ handle_mhd_request (void *cls,
   for (unsigned int i = 0; NULL != handlers[i].url; i++)
   {
     rh = &handlers[i];
-    if ( (0 == strcmp (url,
-                       rh->url)) &&
-         ( (NULL == rh->method) ||
+    if (0 != strcmp (url, rh->url))
+      continue;
+
+    /* The URL is a match!  What we now do depends on the method. */
+
+    if (0 == strcmp (method, MHD_HTTP_METHOD_OPTIONS))
+    {
+      GNUNET_async_scope_restore (&old_scope);
+      return TALER_MHD_reply_cors_preflight (connection);
+    }
+
+    if ( (NULL == rh->method) ||
          (0 == strcmp (method,
-                         rh->method)) ) )
+                       rh->method)) )
     {
       /* FIXME: consider caching 'rh' in '**connection_cls' to
          avoid repeated lookup! */

src/include/taler_mhd_lib.h

@@ -441,4 +441,14 @@ TALER_MHD_reply_legal (struct MHD_Connection *conn,
                        struct TALER_MHD_Legal *legal);
 
 
+/**
+ * Send back a "204 No Content" response with headers
+ * for the CORS pre-flight request.
+ *
+ * @param connection the MHD connection
+ * @return MHD result code
+ */
+int
+TALER_MHD_reply_cors_preflight (struct MHD_Connection *connection);
+
 #endif

src/mhd/mhd_responses.c

@@ -267,6 +267,29 @@ TALER_MHD_reply_json (struct MHD_Connection *connection,
 }
 
 
+/**
+ * Send back a "204 No Content" response with headers
+ * for the CORS pre-flight request.
+ *
+ * @param connection the MHD connection
+ * @return MHD result code
+ */
+int
+TALER_MHD_reply_cors_preflight (struct MHD_Connection *connection)
+{
+  struct MHD_Response *resp;
+
+  GNUNET_assert (NULL != (resp = MHD_create_response_from_buffer (0, NULL,
+                                                                  MHD_RESPMEM_PERSISTENT)));
+  /* This adds the Access-Control-Allow-Origin header.
+   * All endpoints of the exchange allow CORS. */
+  TALER_MHD_add_global_headers (resp);
+  GNUNET_assert (MHD_YES == MHD_queue_response (connection, MHD_HTTP_NO_CONTENT,
+                                                resp));
+  return MHD_YES;
+}
+
+
 /**
  * Function to call to handle the request by building a JSON
  * reply from a format string and varargs.