conncache: use conn->data to know if a transfer owns it

- make sure an already "owned" connection isn't returned unless
  multiplexed.

- clear ->data when returning the connection to the cache again

Regression since 7.62.0 (probably in commit 1b76c38904f0)

Bug: https://curl.haxx.se/mail/lib-2019-03/0064.html

Closes #3686

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/conncache.c b/lib/conncache.c
index c975c456e..39302ba7b 100644
--- a/lib/conncache.c
+++ b/lib/conncache.c
@@ -433,6 +433,7 @@ bool Curl_conncache_return_conn(struct connectdata *conn)
     data->multi->maxconnects;
   struct connectdata *conn_candidate = NULL;
 
+  conn->data = NULL; /* no owner anymore */
   if(maxconnects > 0 &&
      Curl_conncache_size(data) > maxconnects) {
     infof(data, "Connection cache is full, closing the oldest one.\n");
@@ -476,7 +477,7 @@ Curl_conncache_extract_bundle(struct Curl_easy *data,
   while(curr) {
     conn = curr->ptr;
 
-    if(!CONN_INUSE(conn)) {
+    if(!CONN_INUSE(conn) && !conn->data) {
       /* Set higher score for the age passed since the connection was used */
       score = Curl_timediff(now, conn->now);
 
@@ -534,7 +535,7 @@ Curl_conncache_extract_oldest(struct Curl_easy *data)
     while(curr) {
       conn = curr->ptr;
 
-      if(!CONN_INUSE(conn)) {
+      if(!CONN_INUSE(conn) && !conn->data) {
         /* Set higher score for the age passed since the connection was used */
         score = Curl_timediff(now, conn->now);