Hi, On Wed, Mar 25, 2026 at 3:06 PM Bharath Rupireddy < [email protected]> wrote:
> Hi, > > On Wed, Mar 25, 2026 at 2:15 PM SATYANARAYANA NARLAPURAM > <[email protected]> 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
v2-0001-lock-has-waiters-fast-path-fix.patch
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