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 684327885d3565b9d5a2820564e351d2d8eba7b9..4bf053697a3afce315d432090350a639cfa00a75 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();