Re: [PATCH] Add NESTED_STATEMENTS option to EXPLAIN

Hi Zsolt,

Attached is v2 of the patch which fixes all three issues:

1. Error during EXPLAIN (e.g., division by zero):
   - Root cause: hooks were not removed on the error path.
   - Fix: wrapped EXPLAIN execution in PG_TRY/PG_FINALLY to guarantee
     hook removal and cleanup regardless of errors.

2. Nested EXPLAIN crash:
   - Root cause: static globals overwritten by the inner EXPLAIN.
   - Fix: added a reentrancy guard — if nested tracking is already
     active, the inner EXPLAIN skips hook installation entirely.

3. Memory leak (TopMemoryContext):
   - Root cause: allocating captured plans in TopMemoryContext with no
     cleanup on error.
   - Fix: replaced TopMemoryContext with a dedicated memory context
     ("Nested EXPLAIN plans") that is created at the start and deleted
     in PG_FINALLY — freeing everything at once on both success and
     error paths.

The v2 patch passes the regression test suite (245 subtests, 0 failures)
and a comprehensive 19-test script covering the original functionality
plus the three bug scenarios, memory leak verification, and a stress
test with 50 nested statements.

Attachments:
1- v2-0001-Add-NESTED_STATEMENTS-option-to-EXPLAIN.patch
2- comprehensive_nested_statements_test_v2.sql
3- test_output_all_v2.txt

Mohamed Ali
AWS RDS