ftp: treat server's response 421 as CURLE_OPERATION_TIMEDOUT

Bug: https://bugzilla.redhat.com/650255
Reported by: Simon H.

RELEASE-NOTES

@@ -24,6 +24,7 @@ This release includes the following bugfixes:
  o ftp: prevent server from hanging on closed data connection when stopping
    a transfer before the end of the full transfer (ranges)
  o LDAP: detect non-binary attributes properly
+ o ftp: treat server's response 421 as CURLE_OPERATION_TIMEDOUT
 
 This release includes the following known bugs:
 

lib/ftp.c

@@ -425,6 +425,12 @@ static CURLcode ftp_readresp(curl_socket_t sockfd,
   if(ftpcode)
     *ftpcode = code;
 
+  if(421 == code)
+    /* 421 means "Service not available, closing control connection." and FTP
+     * servers use it to signal that idle session timeout has been exceeded.
+     * If we ignored the response, it could end up hanging in some cases. */
+    return CURLE_OPERATION_TIMEDOUT;
+
   return result;
 }