Date: Tue, 3 Oct 2017 16:07:14 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>,
	Andreas Seltenreich <seltenreich@gmx.de>,
	pgsql-hackers@postgresql.org,
	Michael Paquier <michael.paquier@gmail.com>
Subject: Re: [sqlsmith] stuck spinlock in pg_stat_get_wal_receiver
 after OOM
Message-ID: <20171003140714.ozcduitwplj3ropj@alvherre.pgsql>
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Fixed the pstrdup problem by replacing with strlcpy() to stack-allocated
variables (rather than palloc + memcpy as proposed in Michael's patch).

About the other problems:

Tom Lane wrote:

> I took a quick look through walreceiver.c, and there are a couple of
> obvious problems of the same ilk in WalReceiverMain, which is doing this:
> 
> 	walrcv->lastMsgSendTime = walrcv->lastMsgReceiptTime = walrcv->latestWalEndTime = GetCurrentTimestamp();
> 
> (ie, a potential kernel call) inside a spinlock.  But there seems no
> real problem with just collecting the timestamp before we enter that
> critical section.

Done that way.

> I also don't especially like the fact that just above there it reaches
> elog(PANIC) with the lock still held, though at least that's a case that
> should never happen.

Fixed by releasing spinlock just before elog.

> Further down, it's doing a pfree() inside the spinlock, apparently
> for no other reason than to save one "if (tmp_conninfo)".

Fixed.

> I don't especially like the Asserts inside spinlocks, either.  Personally,
> I think if those conditions are worth testing then they're worth testing
> for real (in production).  Variables that are manipulated by multiple
> processes are way more likely to assume unexpected states than local
> variables.

I didn't change these.  It doesn't look to me that these asserts are
worth very much as production code.

> I'm also rather befuddled by the fact that this code sets and clears
> walrcv->latch outside the critical sections.  If that field is used
> by any other process, surely that's completely unsafe.  If it isn't,
> why is it being kept in shared memory?

I think the latch is only used locally.  Seems that it was only put in
shmem to avoid a separate variable ...

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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