Re: LockHasWaiters() crashes on fast-path locks

Hi,

On Wed, Mar 25, 2026 at 3:06 PM Bharath Rupireddy <
bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:

> Hi,
>
> On Wed, Mar 25, 2026 at 2:15 PM SATYANARAYANA NARLAPURAM
> <satyanarlapuram(at)gmail(dot)com> wrote:
> >
> > Hi Hackers,
> >
> > LockHasWaiters() assumes that the LOCALLOCK's lock and proclock pointers
> are populated, but this is not the case for locks acquired via the
> fast-path optimization. Weak locks (< ShareUpdateExclusiveLock) on
> relations may not be stored in the shared lock hash table, and the
> LOCALLOCK entry is left with lock = NULL and proclock = NULL in such a case.
> >
> > If LockHasWaiters() is called for such a lock, it dereferences those
> NULL pointers when it reads proclock->holdMask and lock->waitMask, causing
> a segfault.
> >
> > The only existing caller is lazy_truncate_heap() in VACUUM, which
> queries LockHasWaitersRelation(rel, AccessExclusiveLock). Since
> AccessExclusiveLock is the strongest lock level, it is never fast-pathed,
> so the bug has never been triggered in practice. However, any new caller
> that passes a weak lock mode, for example, checking whether a DDL is
> waiting on an AccessShareLock will crash. The fix is to transfer the lock
> to the main lock table before we access them.
> >
> > Attached a patch to address this issue.
>
> Nice find! It would be good to add a test case (perhaps in an existing
> test extension even though we may not commit it; it can act as a
> demo).
>

Please refer the patches in the thread [2] below for a repro / use case.

>
> I see that this type of lock transfer is happening for prepared
> statements (see AtPrepare_Locks [1]). However, I see the proposed
> patch relying on lock == NULL for detecting whether the lock was
> acquired using fast-path. Although this looks correct because if the
> lock or proclock pointers are NULL, this identifies that the lock was
> taken using fast-path. But for consistency purposes, can we have the
> same check as that of AtPrepare_Locks?

Thank you for the review and code pointer, this is addressed now in v2
patch, attached.

[2]
https://www.postgresql.org/message-id/CAHg%2BQDfdoR%3D7iqEAvLW9qtzV0Sx1wp2FuALeamqcCdiVEmMF-Q%40mail.gmail.com

Thanks,
Satya