[pgpool-committers: 9843] pgpool: Fix assorted causes of segmentation fault.

[Tatsuo Ishii]

Fix assorted causes of segmentation fault.

It is reported that pgpool and its child process segfault in certain
cases when failover involved.

In pgpool main get_query_result (called from find_primary_node) crashed.

                do_query(slots[backend_id]->con, query, res, PROTO_MAJOR_V3);

It seems slots[0] is NULL here. slots[0] is created by
make_persistent_db_connection_noerror() but it failed with log
message: "find_primary_node: make_persistent_db_connection_noerror
failed on node 0". Note that at the time when
make_persistent_db_connection_noerror() is called, VALID_BACKEND
reported that node 0 is up. This means that failover is ongoing and
the node status used by VALID_BACKEND did not catch up. As a result
get_query_user is called with slots[0] = NULL, which caused the
segfault. Fix is, check slots entry before calling
get_query_result.

Also health check has an issue with connection "slot" memory. It is
managed by HealthCheckMemoryContext. slot is the pointer to the
memory. When elog(ERROR) is raised, pgpool long jumps and resets the
memory context. Thus, slot remains as a pointer to freed memory. To
fix this, always set NULL to slot right after the
HealthCheckMemoryContext call.

Similar issue is found with streaming replication check too and is
also fixed in this commit.

Problem reported and analyzed: Emond Papegaaij
Backpatch-through: v4.4
Discussion:
[pgpool-general: 9070] Re: Segmentation after switchover
https://www.pgpool.net/pipermail/pgpool-general/2024-April/009131.html

Branch
------
V4_4_STABLE

Details
-------
https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=9dee833c632081aaf38c561033e59e1ffc5120a3

Modified Files
--------------
src/main/health_check.c                       | 8 +++++++-
src/main/pgpool_main.c                        | 4 ++--
src/streaming_replication/pool_worker_child.c | 8 +++++++-
3 files changed, 16 insertions(+), 4 deletions(-)