[jbylund]

Part 3 of the #35 stack, on top of #119 and #120.

DISCARD ALL is what pgbouncer sends between pooled clients, so every pooled deployment sends it — and a session with its own state to drop had no way to hear about it. The statement was answered entirely by the connection.

async def discard(self, kind: str) -> None:
    """Told about DISCARD ALL | PLANS | SEQUENCES | TEMP."""

TEMPORARY arrives as TEMP, the two being one thing in Postgres, so a session has one spelling to match.

Two places I deviated from #35's sketch

All four kinds reach the hook, not just ALL. The connection returns early for PLANS/SEQUENCES/TEMP — "a mimic has none of those to throw away" — which is true of pg_mimic and not of a session, which may have all three. Keeping the early return would have left three of the four kinds unreachable, and they are the ones where the hook is the only thing that could act.

Told before the connection clears, not after. #35's sketch says "the connection has already cleared the state it owns". That is right for set_parameter, which records and undoes on refusal — but undoing a DISCARD ALL means snapshotting the whole connection: settings, prepared statements, portals, savepoints, listeners. Telling first buys the same atomicity for nothing, so a session that refuses leaves everything as it was:

SET work_mem = '32MB';
DISCARD ALL;    -- ERROR: this pool does not recycle connections
SHOW work_mem;  -- 32MB, untouched

The two hooks now differ in order, deliberately, and each docstring says why.

What a session sees for DISCARD ALL

The statement first, then what it did — one set_parameter(name, None, None) per setting still held, from #120:

('discard', 'ALL')
('set', 'work_mem', None)
('set', 'row_security', None)

Unchanged

25001 still outranks the hook: DISCARD ALL inside a transaction is refused before it means anything, so a session is never told about one that cannot run. The narrow forms are still allowed there, as in Postgres.

Tests

11 in a new tests/test_discard_hook.py — every kind, the TEMPORARY/TEMP equivalence, the narrow forms reaching a session the connection ignores them for, the atomic refusal, both transaction rules, and a session that does not override it being unaffected. Nine fail without the change.

Suite: 886 passed, 17 xfailed.

Remaining

4. dotted custom GUCs in the SET grammar, and _PASSED_THROUGH_SETTINGS reworked — where the role / session_authorization finding from An on_execute effect may be awaited #119 lands.