[httpd] Implement httpd_send_error, a modified evhttp_send_error, which

can include CORS headers (credit @bjoernricks and libevent)

src/httpd.h

@@ -13,13 +13,45 @@ enum httpd_send_flags
 void
 httpd_stream_file(struct evhttp_request *req, int id);
 
-// Returns a pointer to an evbuffer with gzipped. Must be freed by caller.
+/*
+ * Gzips an evbuffer
+ *
+ * @in  in       Data to be compressed
+ * @return       Compressed data - must be freed by caller
+ */
 struct evbuffer *
 httpd_gzip_deflate(struct evbuffer *in);
 
+/*
+ * This wrapper around evhttp_send_reply should be used whenever a request may
+ * come from a browser. It will automatically gzip if feasible, but the caller
+ * may direct it not to. It will set CORS headers as appropriate. Should be
+ * thread safe.
+ *
+ * @in  req      The evhttp request struct
+ * @in  code     HTTP code, e.g. 200
+ * @in  reason   A brief explanation of the error - if NULL the standard meaning
+                 of the error code will be used
+ * @in  evbuf    Data for the response body
+ * @in  flags    See flags above
+ */
 void
 httpd_send_reply(struct evhttp_request *req, int code, const char *reason, struct evbuffer *evbuf, enum httpd_send_flags flags);
 
+/*
+ * This is a substitute for evhttp_send_error that should be used whenever an
+ * error may be returned to a browser. It will set CORS headers as appropriate,
+ * which is not possible with evhttp_send_error, because it clears the headers.
+ * Should be thread safe.
+ *
+ * @in  req      The evhttp request struct
+ * @in  error    HTTP code, e.g. 200
+ * @in  reason   A brief explanation of the error - if NULL the standard meaning
+                 of the error code will be used
+ */
+void
+httpd_send_error(struct evhttp_request *req, int error, const char *reason);
+
 char *
 httpd_fixup_uri(struct evhttp_request *req);