- 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.

CHANGES

@@ -6,6 +6,17 @@
 
                                   Changelog
 
+Daniel Stenberg (19 Nov 2008)
+- 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.
+
 Daniel Fandrich (17 Nov 2008)
 - Added more compiler warning options for gcc 4.3
 

RELEASE-NOTES

@@ -17,6 +17,8 @@ This release includes the following bugfixes:
  o fixed several calls to memory functions that didn't check return codes
  o memory leak for SSL connects with libcurl/NSS when CURLOPT_ISSUERCERT was
    used
+ o re-use of connections with the multi interface when multiple handles used
+   the same server
 
 This release includes the following known bugs:
 
@@ -29,6 +31,7 @@ Other curl-related news:
 This release would not have looked like this without help, code, reports and
 advice from friends like these:
 
- Yang Tse, Daniel Fandrich, Jim Meyering, Christian Krause
+ Yang Tse, Daniel Fandrich, Jim Meyering, Christian Krause, Andreas Wurf,
+ Markus Koetter
 
         Thanks! (and sorry if I forgot to mention someone)

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 &&