From 3b13aaf844f3b61950f437cb339fb881312ecf7b Mon Sep 17 00:00:00 2001
From: Florian Dold <florian.dold@gmail.com>
Date: Wed, 15 Jan 2020 15:52:12 +0100
Subject: [PATCH] respond with CORS headers for OPTIONS pre-flight request

---
 src/exchange/taler-exchange-httpd.c | 19 ++++++++++++++-----
 src/include/taler_mhd_lib.h         | 10 ++++++++++
 src/mhd/mhd_responses.c             | 23 +++++++++++++++++++++++
 3 files changed, 47 insertions(+), 5 deletions(-)

diff --git a/src/exchange/taler-exchange-httpd.c b/src/exchange/taler-exchange-httpd.c
index e53c04d98..b2c9e4d84 100644
--- a/src/exchange/taler-exchange-httpd.c
+++ b/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) ||
-           (0 == strcmp (method,
-                         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)) )
     {
       /* FIXME: consider caching 'rh' in '**connection_cls' to
          avoid repeated lookup! */
diff --git a/src/include/taler_mhd_lib.h b/src/include/taler_mhd_lib.h
index c836e4469..966ba6a09 100644
--- a/src/include/taler_mhd_lib.h
+++ b/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
diff --git a/src/mhd/mhd_responses.c b/src/mhd/mhd_responses.c
index a8c159300..d26f533e7 100644
--- a/src/mhd/mhd_responses.c
+++ b/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.