Re: Re: [PATCH] Fix segmentation fault caused by reentrancy in RI_Fkey_cascade_del (ri_triggers.c)

Hi,

A correction to my review upthread. I said I would report the
027_stream_regress assertion separately; that was already done, four
days before the review went out:

Intermittent Assert("plan->magic == _SPI_PLAN_MAGIC") in
027_stream_regress
https://www.google.com/url?q=https://postgr.es/m/CACRpqq9WihNyHJXDQp_t%2Bi-i3R2fG6v%2BNC-XX4NKn4K6h8NrwA@mail.gmail.com&source=gmail&ust=1790347576108000&sa=E

Apologies for the tense -- I wrote that paragraph before posting and
forgot to fix it. There is nothing new in it, I just did not want
anyone holding off on a report that already exists.

Two things from that thread are worth repeating here, because they
bear on this patch more directly than my review made out.

First, the assert is older than the RI work. I ran it at c36a0df195d
(2026-08-20), which predates the RI fast-path series, and it still
fails there. So it is not fallout from 6fc2a486417, e2c812f1475,
2c45694a240 or c62b330912e, and it is independent of v2-0002. My
review only said the failure was pre-existing on current master; the
bisect point makes that a good deal firmer.

Second, on the counts. The 5/15 and 9/15 in my review were a separate
batch from the 7 of 20 on master in the earlier thread, both at
9e17d25e79d. Together that is 12 of 35 unpatched runs, 34%, which is
in line with the 60% I saw patched given the sample sizes. I still
would not read anything into the difference.

No action needed here. #6825 stands where I left it, Waiting on
Author for the refcount points.

Regards,
Trakshan Mishra

On Thu, Sep 24, 2026 05:31 PM, Trakshan Mishra <trakshanmishra477(at)gmail(dot)com>
wrote:

> Hi Lucas,
>
> I picked this up from the PG20-2 commitfest (#6825) as a first-time
> reviewer. Summary up front: the bug is real and reproducible, the patch
> does fix it, but I think the refcount bookkeeping needs another round.
>
> Test environment:
> master @ 9e17d25e79d
> Linux x86_64, gcc 15.2.0
> meson, --buildtype=debug -Dcassert=true
>
>
> == Submission review ==
>
> v3-0001 (the isolation test) applies cleanly.
>
> v2-0002 (the fix) does *not* apply to current master. It conflicts in
> the "Local data" block around ri_triggers.c:251 -- the RI fast-path work
> that landed since you posted (c62b330912e, 2c45694a240, e2c812f1475 and
> neighbours) restructured that area. "git apply -3" resolves it without
> a real conflict, so this is just a rebase, but a v4 on top of current
> master would help the next reviewer and cfbot.
>
> "git apply" reports 12 whitespace errors across the two patches: 4 in
> the .spec file and 8 in ri_triggers.c. Several are tabs immediately
> after an opening brace, e.g.
>
> ri_PreparedPlanExecutionStarted(SPIPlanPtr plan)
> {<tab>
>
> pgindent should clear these.
>
>
> == Feature test ==
>
> I can confirm the crash on unpatched master. Applying only v3-0001 and
> running the new isolation test:
>
> client backend (PID 23625) was terminated by signal 11: Segmentation
> fault
> DETAIL: Failed process was running: DELETE FROM
> crash_reentrancia_tabla_autoreferencial WHERE id = 1;
> LOG: terminating any other active server processes
> LOG: all server processes terminated; reinitializing
>
> With v2-0002 applied the same test passes and the server stays up. So
> the patch does address the reported crash. Thanks for the clear
> reproducer -- the advisory-lock handshake to line up the invalidation
> was a nice touch.
>
>
> == Coding review ==
>
> 1. The refcount is leaked whenever the RI query throws.
>
> In ri_PerformCheck() the increment and decrement bracket
> SPI_execute_snapshot() with no PG_TRY/PG_FINALLY:
>
> ri_PreparedPlanExecutionStarted(qplan);
> spi_result = SPI_execute_snapshot(qplan, ...);
> ri_PreparedPlanExecutionFinished(qplan);
>
> Any ereport(ERROR) from inside the RI query longjmps past the Finished()
> call, so the count is never given back. This is not an exotic path -- a
> BEFORE DELETE trigger on the referencing table that raises will do it,
> and so will statement_timeout, query cancel or a deadlock during the
> cascade.
>
> I instrumented the hash table locally to check, using a parent/child
> pair with ON DELETE CASCADE where the child has a BEFORE DELETE trigger
> that raises, and ten cascade deletes each caught by an EXCEPTION block:
>
> RI_REFCOUNT_DEBUG entries=1 plan=0x60f5822cfd30 refcount=1
> RI_REFCOUNT_DEBUG entries=1 plan=0x60f5822cfd30 refcount=2
> RI_REFCOUNT_DEBUG entries=1 plan=0x60f5822cfd30 refcount=3
> ...
> RI_REFCOUNT_DEBUG entries=1 plan=0x60f5822cfd30 refcount=10
>
> The count rises monotonically and never comes back down. Once that has
> happened ri_PreparedPlanCanRelease() returns false for that plan
> forever, so ri_FetchPreparedPlan() will never SPI_freeplan() it: on the
> next invalidation it sets entry->plan = NULL and the plan is orphaned in
> CacheMemoryContext for the life of the backend.
>
> A PG_TRY/PG_FINALLY around the execute, or tying the decrement to
> resource-owner or subtransaction cleanup, would fix this.
>
>
> 2. Entries are never removed when the refcount drops to zero on a plan
> that is still valid.
>
> ri_PreparedPlanExecutionFinished() only does HASH_REMOVE inside
>
> if (entry->refcount == 0 && !SPI_plan_is_valid(plan))
>
> which is the uncommon case. Normally the entry stays behind with
> refcount 0 forever. I think the removal should happen whenever the
> count reaches 0, independently of plan validity.
>
>
> 3. Keying the hash table on the raw SPIPlanPtr looks fragile.
>
> Because entries outlive the plans they describe (point 2), the table
> accumulates entries keyed on pointers that have since been freed. That
> would be harmless if addresses were never reused, but they are. Driving
> 40 plan invalidations through ALTER TABLE, I only ever saw three
> distinct plan addresses, cycling:
>
> entries=1 plan=0x60f5822cf900
> entries=2 plan=0x60f5822ce8d0
> entries=3 plan=0x60f5822ce0b0
> ... then those same three addresses repeatedly, entries stuck at 3
>
> So a freshly created plan routinely lands on an address that already has
> an entry and inherits whatever refcount it was left holding.
>
> I want to be careful not to overstate this: in every path I could
> actually reach, the inherited value was 0, and I could not turn this
> into a demonstrable failure. So treat it as a design concern rather
> than a proven bug. But combined with point 1, which does leave counts
> above zero, a new plan could start life pinned and never be freed -- or
> a count could reach zero while an outer reentrant frame still holds the
> plan, which is the use-after-free this patch exists to prevent. Storing
> the refcount on the RI_QueryHashEntry that already owns the plan would
> sidestep the question entirely.
>
>
> 4. entry->refcount-- is unguarded.
>
> It is a uint32, so a stray extra Finished() call (or a stale entry per
> point 3) wraps it to 4294967295 rather than tripping anything. An
> Assert(entry->refcount > 0) before the decrement would catch that in
> cassert builds.
>
>
> 5. The ri_InitHashTables() call in ri_PreparedPlanExecutionStarted().
>
> if (!ri_query_plan_cache_executing_refcount)
> ri_InitHashTables();
>
> ri_InitHashTables() unconditionally recreates all four hash tables and
> re-runs both CacheRegisterSyscacheCallback() calls. If this branch were
> ever taken with the other caches already populated it would orphan
> ri_constraint_cache, ri_query_cache and ri_compare_cache, and register
> duplicate syscache callbacks against a limited pool. In practice
> ri_PerformCheck() is only reached after the caches exist, so the branch
> looks unreachable -- which argues for an Assert instead, or for
> splitting the refcount table's initialisation out.
>
>
> 6. Style points
>
> - "// Remove the entry" needs to be a /* */ comment.
> - "RI_QueryPlanCacheExecutingRefCountEntry* entry" should be
> "... *entry" (three occurrences).
> - "bool found" is declared in ri_PreparedPlanExecutionFinished() and
> ri_PreparedPlanCanRelease() but never read; both test !entry.
> - Several lines run to 103-131 columns.
> - The three new functions have no comment headers, unlike their
> neighbours in this file.
> - "this call can free the plan..." should start with a capital.
>
> All pgindent/typedefs.list territory rather than anything substantive.
>
>
> == Test patch ==
>
> 1. The identifiers are in Spanish -- crash_reentrancia_tabla_
> autoreferencial, nombre, padre_id, crash_reentrancia_segunda_tabla,
> valor. The rest of the tree is English, so these will need renaming.
>
> 2. The test leans on overflowing the shared invalidation queue with 1000
> temp table create/drops. It does reproduce reliably here (7.2s), but
> nothing makes it fail loudly if that stops being enough -- it would just
> start passing on an unfixed backend. Is there a way to make the
> invalidation deterministic?
>
> 3. A crash test in the isolation schedule takes down the whole cluster
> when it fails, aborting the rest of the schedule. I do not know the
> project's preference here, but it may be worth asking whether this
> belongs in src/test/isolation or as a TAP test.
>
> 4. The expected output only shows that s1_delete completed; it does not
> check the resulting table contents. Asserting the surviving rows would
> turn "did not crash" into "cascaded correctly".
>
> 5. Four of the whitespace errors above are in this file.
>
>
> == Regression testing ==
>
> Full "meson test" with the patch: 359 ok, 49 skipped, 1 failed.
>
> The failure was recovery/027_stream_regress, with
>
> TRAP: failed Assert("plan->magic == _SPI_PLAN_MAGIC"),
> File: "../src/backend/executor/spi.c", Line: 1951
> client backend was terminated by signal 6: Aborted
> DETAIL: Failed process was running: UPDATE temporal_mltrng
> SET valid_at = datemultirange(daterange('2016
> -02-01','2016-03-01'))
> WHERE id = '[5,6)' AND ...
>
> I initially assumed the patch had caused this, since it is the same
> use-after-free shape the patch is about. It has not. Clean master with
> no patch applied reproduces the identical assertion. Counts over 15
> runs each:
>
> master : 5/15 runs hit the assert (33%)
> patched : 9/15 runs hit the assert (60%)
>
> Fisher exact two-tailed p = 0.27, so the difference is not significant
> at these sample sizes and I am not claiming the patch makes it worse --
> only that it does not fix it and that the failure is pre-existing. I
> will report that one separately rather than tangle it up with this
> thread.
>
> Aside from 027_stream_regress, nothing regressed.
>
>
> == Summary ==
>
> The crash is real, easy to trigger, and the patch fixes it. I would
> call the direction sound but the bookkeeping not ready: point 1 is a
> straightforward leak on a common error path, and point 3 makes me uneasy
> about the choice of hash key.
>
> Marking this Waiting on Author. Happy to retest a v4.
>
> On the wider question about whether a BEFORE DELETE trigger should be
> deleting rows at all -- I do not have the standing to argue that either
> way. But a backend that segfaults seems worth closing regardless of
> whether the usage is advisable, and if the consensus is that it should
> not be allowed, an explicit error would still need this same reentrancy
> information to detect the situation.
>
> Regards,
> Trakshan Mishra
>