Hi hackers,
I found a problem when doing the test shown below:
Time | Session A | Session B |
T1 | postgres=# create table test(a int); CREATE TABLE postgres=# insert into test values (1); INSERT 0 1 | |
T2 | postgres=# begin; BEGIN postgres=*# lock table test in access exclusive mode ; LOCK TABLE | |
T3 | | postgres=# begin; BEGIN postgres=*# lock table test in exclusive mode ; |
T4 | Case 1: postgres=*# lock table test in share row exclusive mode nowait; ERROR: could not obtain lock on relation "test" -------------------------------------------- Case 2: postgres=*# lock table test in share row exclusive mode; LOCK TABLE | |
At T4 moment in session A, (case 1) when executing SQL “lock table test in share row exclusive mode nowait;”, an error occurs with message “could not obtain lock on relation test";However, (case 2) when executing the SQL above without nowait, lock can be obtained successfully.
Digging into the source code, I find that in case 2 the lock was obtained in the function ProcSleep instead of LockAcquireExtended. Due to nowait logic processed before WaitOnLock->ProcSleep, acquiring lock failed in case 1. Can any changes be made so that the act of such lock granted occurs before WaitOnLock?
Providing a more universal case:
Transaction A already holds an n-mode lock on table test. If then transaction A requests an m-mode lock on table Test, m and n have the following constraints:
(lockMethodTable->conflictTab[n] & lockMethodTable->conflictTab[m]) == lockMethodTable->conflictTab[m]
Obviously, in this case, m<=n.
Should the m-mode lock be granted before WaitOnLock?
In the case of m=n (i.e. we already hold the lock), the m-mode lock is immediately granted in the LocalLock path, without the need of lock conflict check.
Based on the facts above, can we obtain a weaker lock (m<n) on the same object within the same transaction without doing lock conflict check?
Since m=n works, m<n should certainly work too.
I am attaching a patch here with which the problem in case 1 fixed.