In cache invalidation logic, we have the following comment:
/*
* Now that we have the lock, check for invalidation messages, so that we
* will update or flush any stale relcache entry before we try to use it.
* RangeVarGetRelid() specifically relies on us for this. We can skip
* this in the not-uncommon case that we already had the same type of lock
* being requested, since then no one else could have modified the
* relcache entry in an undesirable way. (In the case where our own xact
* modifies the rel, the relcache update happens via
* CommandCounterIncrement, not here.)
*/
if (res != LOCKACQUIRE_ALREADY_HELD) AcceptInvalidationMessages();
It is true after we hold the lock, nobody will further modify it but there
could be some left-over invalidation message we shall accept before we can
continue. This is can be demonstrated with the following invalidation
sequence:{ 1: inval A; 2: inval B; ...; 10: inval pg_class
}
After step 10, another session may encounter a lock and replays this sequence:
step 1: RelationBuildDesc(A), it heap_open(pg_class), pg_class lock not acquired yet, so it acquires the lock
and recursively replay the sequence, goto step 2.
step 2: RelationBuildDesc(B), it heap_open(pg_class), but this time we already have
LOCKACQUIRE_ALREADY_HELDwith pg_class, so we now access pg_class but it is wrong.
User may ends up with a "could not open file ..." error.
Is above sequence possible?
Regards,
Qingqing