[ibrarahmad]

Problem

While lolor is enabled, dropping the extension renames the pg_catalog.lo_* functions back. That must not happen under other connected sessions, since a renamed function keeps its OID and those sessions keep calling the previous implementation. The event trigger enforced this by calling lolor._require_no_other_sessions(), but only when that function existed in the installed schema. After the 1.3.0 library is installed and before ALTER EXTENSION lolor UPDATE has run, the schema is still 1.2.3, the helper does not exist, and DROP EXTENSION lolor with another session connected went through with the rename:

CREATE EXTENSION lolor VERSION '1.2.3';   -- library already 1.3.0
-- another session connected
DROP EXTENSION lolor;                     -- succeeded

The trigger also ignored the return values of SPI_connect() and SPI_execute(), so a failed SPI call silently skipped disable() and the hook then refused the drop with a hint telling the user to run disable() themselves.

Fix

The trigger checks the backend type and counts the other client sessions itself, from C, with the same messages as lolor.disable(), and raises on any SPI failure. The same command now refuses:

ERROR:  cannot drop the lolor extension while other sessions are connected to the database
DETAIL:  1 other session is connected.

lolor.disable() keeps its own checks for direct calls. The SPI query on pg_proc that looked for the helper is gone.

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• Makefile
• README.md
• docs/index.md
• docs/lolor_release_notes.md
• lolor--1.2.3--1.3.0.sql
• src/lolor.c
• t/008_drop_paths.pl
• t/011_version_skew.pl

Walkthrough

The drop trigger returns when native large-object functions are not parked under their _orig names. Otherwise, it checks the backend type and other client sessions before calling lolor.disable(). The session checks refresh PostgreSQL’s statistics snapshot, and the regression test covers a stale snapshot.

Changes: Drop checks

Layer / File(s)	Summary
Drop validation and disable src/lolor.c, lolor--1.2.3--1.3.0.sql, t/008_drop_paths.pl	The trigger checks the backend type and counts other client sessions connected to the current database. The session checks refresh the statistics snapshot. SPI and disable failures raise errors. The test covers a stale snapshot and checks that lolor remains enabled after the refusal path.

Priority: Normal

Merge Risk: Low · previous review a6a54

The drop-path test may fail intermittently during cleanup. Poll for backend exit before the final drop; the remaining risk is limited to test reliability.

Architecture risk: Low · up to ac73f

The change affects 1 system.

Changed systems: src

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — src (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in src/lolor.c: Added local variables for SPI result status, the count of other sessions, and the count query’s null indicator.
• observed — Modified behavior in src/lolor.c: After the storage-drop check, the trigger now exits if native large-object functions are not parked under _orig names; this replaces the old combined SQL-side enabled-state handling for disabled lolor.
• observed — Modified behavior in src/lolor.c: For enabled lolor, the trigger now rejects non-client backends, connects to SPI with explicit failure handling, counts other client backends in the current database excluding itself, and errors if the count is null or positive. If no other sessions are connected, it invokes lolor.disable() and errors unless SPI reports a successful select. This replaces the prior optional _require_no_other_sessions lookup/call and conditional is_enabled()-guarded disable call.

Pre-merge checks: 5

Check name	Status	Explanation
Title check		The title clearly identifies the main change: moving drop session requirement checks into C.
Description check		The description accurately explains the session-safety problem, the C-based fix, SPI error handling, and the related tests.
Docstring Coverage		Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within survi…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 1 functions across 1 files. (2 skipped: 2 unsupported.)

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_{A rabbit checks the session count,
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Actionable comments posted: 1

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Inline comments:
Review comments at @src/lolor.c:
- Around line 609-613: Clear the PostgreSQL statistics snapshot immediately
before the SPI_execute query that counts other client backends in
pg_stat_activity, so the subsequent lolor.disable() check uses refreshed session
activity. Add a test covering a transaction that reads activity, then sees
another client connect before the drop check.

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Actionable comments posted: 1

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Inline comments:
Review comments at @t/008_drop_paths.pl:
- Line 170: Update the test flow after `$other->quit` to repeat the existing
backend-exit poll before the final `DROP EXTENSION lolor`, ensuring the server
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[danolivo]

The direction is right, but I think the patch solves each problem one level too low.

Critical operations should not be permitted if the version of the binary and DB objects disagree.

Claude's finding:

The patch exists because the library may be 1.3.0 while the installed extension is still 1.2.3, so the checks were moved into C. But then the same trigger turns around and calls the possibly-older disable() to do the actual renaming — and the 1.2.2 version of it looks up pg_proc WHERE proname = 'lo_close_orig' without a schema, which is the squatting hole 1.3.0 fixes. So the skew is only half closed, and the next path will need its own fix.

I would make it a rule instead: critical operations — enable(), disable(), migrate_to_native(), migrate_from_native(), check_orphans() and the drop — refuse to run when the binary and the installed extension disagree. If you want to do something critical, make the versions agree first.

pg_stat_activity signals wrong design decisions we have made

The comment on _require_no_other_sessions() already names the right precedent — ALTER DATABASE … RENAME. That path uses CountOtherDBBackends(). So we can close the race properly: take AccessExclusiveLock on (DatabaseRelationId, MyDatabaseId, 0) first, then count. A session that hasn't taken the lock isn't in the ProcArray and waits; one that has is counted. No third case, and the lock is released on rollback as well as commit.

If I understand correctly, this approach has only one minor issue: cached LOLOR function OIDs in the libpq (and other drivers) cache for our own session (we have no more sessions). I think we can mention it in the documentation.

Tests

This PR doesn't cover the scenario: new binaries, old extension, another session connected, DROP EXTENSION. The new test exercises disable() instead, so the C path has no test at all. Please add that case and the ALTER EXTENSION lolor UPDATE path.

Smaller things: the discarded boolean result of disable() and B_STANDALONE_BACKEND being rejected along with the background workers.

[ibrarahmad]

@danolivo thanks, all addressed in 90c9f59.

Version skew. Took it as a rule. The library gets its version from lolor.control through the Makefile and refuses enable(), disable(), both migrations, check_orphans(), fix_orphans() and the drop when it differs from the installed extversion. The drop checks it in C before anything else, so with a 1.3.0 library and 1.2.3 installed it never reaches the old disable(). The 1.3.0 SQL functions go through C for it too. The one thing it cannot cover is 1.2.3's own plpgsql enable()/disable(), which never call into C.

Sessions. Now takes AccessExclusiveLock on (DatabaseRelationId, MyDatabaseId, 0) and then counts, in one C function used by the drop and by require_no_other_sessions(). I kept counting client backends only, from pg_stat_activity with the snapshot cleared, rather than CountOtherDBBackends(). That one counts apply workers and logical walsenders too, so on a Spock node none of these would run without stopping replication first, and they never call the lo* functions by name. The lock still closes the race: a new backend publishes its activity entry while it holds RowExclusiveLock on the database, on 16 through 19, so it is either counted or waits for us. Checked by hand that a new connection waits until the transaction holding the lock ends.

Tests. New t/011: new library with 1.2.3 installed and another session, DROP EXTENSION is refused on the version and renames nothing; ALTER EXTENSION lolor UPDATE works with the session still there; then the drop is refused on the session count; the 1.3.0 SQL entry points refuse under a skewed extversion; and single-user mode.

Smaller ones. The drop now fails if disable() returns false, and B_STANDALONE_BACKEND is accepted. The note about a session's own cached OIDs is in docs/index.md along with the lock behaviour.

[danolivo]

1. The lock settles the correctness question, so this is a “consider” rather than a blocker: CountOtherDBBackends() would remove the SPI call, the pg_stat_activity query and pgstat_clear_snapshot() from this path, and it also reports prepared transactions, which the current count ignores. If you looked at it and decided against it, the reason is worth a line in the comment, since the next reader will ask the same thing.
2. For PG18 and recent, lolor_installed_version should use conventional syscache search instead of direct scan.
3. Not sure we need it right now, but the 'CREATE EXTENSION lolor' command needs the same guard, doesn't it?

[ibrarahmad]

All three are in ac7e43b. On CountOtherDBBackends(): I looked at it and kept the client-backend count on purpose. It also counts walsenders and spock apply workers and terminates autovacuum workers, so on a spock node lolor could not be enabled, disabled or dropped without disabling the subscriptions first, and a prepared transaction cannot call a function, so it does not matter for the rename. That reason is in the comment now. Whether replication processes should count is the question you raised on #67 too, and I'd rather settle it once for both. lolor_installed_version() uses the EXTENSIONNAME syscache on PG 18 and later, where it exists, and keeps the index scan on 16 and 17. And yes, CREATE EXTENSION needs the guard: the 1.0 script renames the pg_catalog functions the same way enable() does. It runs from a ProcessUtility hook before the command, since the object access hook for the extension fires before its catalog row is visible; t/011 covers the refusal. The branch is squashed to two commits against #78.