Horton

Re: Relations being opened without any lock whatever

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2018-10-01T13:29:18Z
Lists: pgsql-hackers 
Amit Langote <Langote_Amit_f8@lab.ntt.co.jp> writes:
> On 2018/10/01 4:20, Tom Lane wrote:
>> Running the regression tests with the patch I showed in
>> https://www.postgresql.org/message-id/16565.1538327894@sss.pgh.pa.us

> Maybe you've noticed but the relation_open calls coming from bootstrap.c
> all pass NoLock which trigger the WARNING:

Yeah, I'd missed noticing that at the time I posted that patch, but I
sure noticed after changing the WARNING to an Assert ;-)

> Do we need to do something about that, like teaching boot_openrel() and
> gettype() in bootstrap.c to pass AccessShareLock instead of NoLock?

No, bootstrap mode has no need for locking.  I think the right fix is
just to skip the check:
 
+	/*
+	 * If we didn't get the lock ourselves, assert that caller holds one,
+	 * except in bootstrap mode where no locks are used.
+	 */
+	Assert(lockmode != NoLock ||
+		   IsBootstrapProcessingMode() ||
+		   CheckRelationLockedByMe(r, AccessShareLock, true));

It's possible that at some point we'd decide to make bootstrap mode
do locking the same as normal mode, but that's not a change I want
to make as part of this patch.

			regards, tom lane

Commits

  1. Fix tuple_data_split() to not open a relation without any lock.

  2. Lock relation used to generate fresh data for RMV.

  3. Fix ALTER COLUMN TYPE to not open a relation without any lock.