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Re: BUG #6379: SQL Function Causes Back-end Crash

From: Paul Ramsey <pramsey(at)cleverelephant(dot)ca>
To: Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #6379: SQL Function Causes Back-end Crash
Date: 2012-01-04 20:35:33
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugspgsql-hackers
Further notes, from Andrew (RhodiumToad) on IRC about the cause of this crasher:

[12:03pm] RhodiumToad: what happens is this
[12:04pm] RhodiumToad: postquel_start know this statement doesn't
return the result, so it supplies None_Receiver as the dest-receiver
for the query
[12:04pm] RhodiumToad: however, it knows it's a plannedStmt, so it
fires up the full executor to run it
[12:05pm] RhodiumToad: and the executor allocates a new destreceiver
in its own memory context, replaces es->qd->dest with it,
[12:05pm] RhodiumToad: (the new destreceiver is the one that writes
tuples to the created table)
[12:06pm] RhodiumToad: then at executorEnd (called from postquel_end),
executor shutdown closes the new rel, _and then frees the executor's
memory context, including the destreceiver it created
[12:07pm] RhodiumToad: postquel_end doesn't know that its setting of
->dest was clobbered, so it goes to try and destroy it again, and gets
garbage (if assertions are on)
[12:07pm] RhodiumToad: if assertions weren't on, then the rDestroy
call is harmless
[12:07pm] RhodiumToad: well, mostly harmless
[12:07pm] RhodiumToad: sneaky one, that
[12:09pm] RhodiumToad: you can confirm it by tracing through that
second call to postquel_end and confirming that it's the call to
ExecutorEnd that stomps the content of qd->dest
[12:12pm] pramsey: confirmed, the pass through ExecutorEnd has
clobbered the value so there's garbage when it arrives at line 638
[12:14pm] RhodiumToad: if you trace through ExecutorEnd itself, it
should be the FreeExecutorState that does it
[12:15pm] RhodiumToad: wonder how far back this bug goes
[12:16pm] RhodiumToad: actually not very far
[12:17pm] RhodiumToad: older versions just figured that qd->dest was
always None_Receiver and therefore did not need an rDestroy call
[12:17pm] RhodiumToad: (which is a no-op for None_Receiver)
[12:17pm] pramsey: kills my 8.4
[12:17pm] RhodiumToad: so this is broken in 8.4+
[12:17pm] pramsey: ah
[12:18pm] RhodiumToad: 8.4 introduced the lazy-eval of selects in sql functions
[12:19pm] RhodiumToad: prior to that they were always run immediately
to completion
[12:19pm] RhodiumToad: that requires juggling the destreceiver a bit,
hence the bug
[12:20pm] RhodiumToad: btw, the first statement of the function
shouldn't be needed
[12:21pm] RhodiumToad: just  ... as $f$ create table foo as select 1
as x; $f$;  should be enough to break it
[12:31pm] RhodiumToad: there's no trivial fix

On Wed, Jan 4, 2012 at 11:32 AM, Paul Ramsey <pramsey(at)cleverelephant(dot)ca> wrote:
> One extra detail, my PostgreSQL is compiled with --enable-cassert.
> This is required to set off the killer function.
>> On Wed, Jan 04, 2012 at 07:17:17PM +0000, pramsey(at)cleverelephant(dot)ca wrote:
>>> The following bug has been logged on the website:
>>> Bug reference:      6379
>>> Logged by:          Paul Ramsey
>>> Email address:      pramsey(at)cleverelephant(dot)ca
>>> PostgreSQL version: 9.1.2
>>> Operating system:   OSX 10.6.8
>>> Description:
>>> AS $$
>>>   DROP TABLE if EXISTS foo;
>>>   CREATE TABLE foo AS SELECT * FROM pg_class LIMIT 1;
>>> $$ LANGUAGE 'sql';
>>> SELECT kill_backend();

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