conncache: only allow multiplexing within same multi handle

Connections that are used for HTTP/1.1 Pipelining or HTTP/2 multiplexing
only get additional transfers added to them if the existing connection
is held by the same multi or easy handle. libcurl does not support doing
HTTP/2 streams in different threads using a shared connection.

Closes #2152

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/url.c b/lib/url.c
index b38e88eec..be2105e3b 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -1264,6 +1264,11 @@ ConnectionExists(struct Curl_easy *data,
            already in use so we skip it */
         continue;
 
+      if((check->inuse) && (check->data->multi != needle->data->multi))
+        /* this could be subject for pipeline/multiplex use, but only
+           if they belong to the same multi handle */
+        continue;
+
       if(needle->localdev || needle->localport) {
         /* If we are bound to a specific local end (IP+port), we must not
            re-use a random other one, although if we didn't ask for a