conncache: use the closure handle when disconnecting surplus connections

Use the closure handle for disconnecting connection cache entries so
that anything that happens during the disconnect is not stored and
associated with the 'data' handle which already just finished a transfer
and it is important that details from the unrelated disconnect does not
taint meta-data in the data handle.

Like storing the response code.

This also adjust test 1506. Unfortunately it also removes a key part of
the test that verifies that a connection is closed since when this
output vanishes (because the closure handle is used), we don't know
exactly that the connection actually gets closed in this test...

Reported-by: ohyeaah on github
Fixes #12367
Closes #12405

lib/conncache.c

@@ -389,8 +389,16 @@ bool Curl_conncache_return_conn(struct Curl_easy *data,
 
     conn_candidate = Curl_conncache_extract_oldest(data);
     if(conn_candidate) {
-      /* the winner gets the honour of being disconnected */
-      Curl_disconnect(data, conn_candidate, /* dead_connection */ FALSE);
+      /* Use the closure handle for this disconnect so that anything that
+         happens during the disconnect is not stored and associated with the
+         'data' handle which already just finished a transfer and it is
+         important that details from this (unrelated) disconnect does not
+         taint meta-data in the data handle. */
+      struct conncache *connc = data->state.conn_cache;
+      connc->closure_handle->state.buffer = data->state.buffer;
+      connc->closure_handle->set.buffer_size = data->set.buffer_size;
+      Curl_disconnect(connc->closure_handle, conn_candidate,
+                      /* dead_connection */ FALSE);
     }
   }
 

tests/data/test1506

@@ -86,7 +86,6 @@ Accept: */*
 * Connection #0 to host server1.example.com left intact
 * Connection #1 to host server2.example.com left intact
 * Connection #2 to host server3.example.com left intact
-* Closing connection
 * Connection #3 to host server4.example.com left intact
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