Use correct connection for cancellation in frontend's parallel slots

While waiting for slots to become available in wait_on_slots() in
parallel_slot.c, the cancellation always relied on the first connection
in the set to do the job.  This could cause problems when this slot's
socket is gone as PQgetCancel() would return NULL in this case.  Rather
than always using the first connection, this changes the logic to use
the first valid connection for the cancellation.

Author: Ranier Vilela
Reviewed-by: Justin Pryzby
Discussion: https://postgr.es/m/CAEudQAokk1h_pUwGXsYS4oVOuf35s1O2o3TXGHpV8=AWikvgHA@mail.gmail.com
Backpatch-through: 14

diff --git a/src/fe_utils/parallel_slot.c b/src/fe_utils/parallel_slot.c
index 69581157c29a2ca12f626ee20a75a54d4b181ed1..dcdad9e30c5cc562ef2d11dc76d9cf09c9c78eed 100644 (file)
--- a/src/fe_utils/parallel_slot.c
+++ b/src/fe_utils/parallel_slot.c
@@ -237,7 +237,7 @@ wait_on_slots(ParallelSlotArray *sa)
    if (cancelconn == NULL)
        return false;
 
-   SetCancelConn(sa->slots->connection);
+   SetCancelConn(cancelconn);
    i = select_loop(maxFd, &slotset);
    ResetCancelConn();