- Andreas Wurf and Markus Koetter helped me analyze a problem that Andreas got

when uploading files to a single FTP server using multiple easy handle
  handles with the multi interface. Occasionally a handle would stall in
  mysterious ways.

  The problem turned out to be a side-effect of the ConnectionExists()
  function's eagerness to re-use a handle for HTTP pipelining so it would
  select it even if already being in use, due to an inadequate check for its
  chances of being used for pipelnining.

lib/url.c

@@ -2509,6 +2509,11 @@ ConnectionExists(struct SessionHandle *data,
       /* don't do mixed proxy and non-proxy connections */
       continue;
 
+    if(!canPipeline && check->inuse)
+      /* this request can't be pipelined but the checked connection is already
+         in use so we skip it */
+      continue;
+
     if(!needle->bits.httpproxy || needle->protocol&PROT_SSL ||
        (needle->bits.httpproxy && check->bits.httpproxy &&
         needle->bits.tunnel_proxy && check->bits.tunnel_proxy &&