Re: "cancelling statement due to user request error" occurs but the transaction has committed.

On Wed, Sep 10, 2014 at 08:10:45PM -0400, Bruce Momjian wrote:
> On Tue, Jun 10, 2014 at 10:30:24AM -0400, Robert Haas wrote:
> > On Tue, Jun 10, 2014 at 10:18 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > > Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > >> I don't agree with this analysis. If the connection is closed after
> > >> the client sends a COMMIT and before it gets a response, then the
> > >> client must indeed be smart enough to figure out whether or not the
> > >> commit happened. But if the server sends a response, the client
> > >> should be able to rely on that response being correct. In this case,
> > >> an ERROR is getting sent but the transaction is getting committed;
> > >> yuck. I'm not sure whether the fix is right, but this definitely
> > >> seems like a bug.
> > >
> > > In general, the only way to avoid that sort of behavior for a post-commit
> > > error would be to PANIC ... and even then, the transaction got committed,
> > > which might not be the expectation of a client that got an error message,
> > > even if it said PANIC. So this whole area is a minefield, and the only
> > > attractive thing we can do is to try to reduce the number of errors that
> > > can get thrown post-commit. We already, for example, do not treat
> > > post-commit file unlink failures as ERROR, though we surely would prefer
> > > to do that.
> >
> > We could treated it as a lost-communication scenario. The appropriate
> > recovery actions from the client's point of view are identical.
> >
> > > So from this standpoint, redefining SIGINT as not throwing an error when
> > > we're in post-commit seems like a good idea. I'm not endorsing any
> > > details of the patch here, but the 20000-foot view seems generally sound.
> >
> > Cool, that makes sense to me also.
>
> Did we ever do anything about this?

I have researched this issue originally reported in June of 2014 and
implemented a patch to ignore cancel while we are completing a commit.
I am not clear if this is the proper place for this code, though a
disable_timeout() call on the line above suggests I am close. :-)
(The disable_timeout disables internal timeouts, but it doesn't disable
cancels coming from the client.)

The first patch is for testing and adds a sleep(5) to the end of the
TRUNCATE command, to give the tester time to press Control-C from psql,
and enables log_duration so the cancel is checked.

The second patch is the patch that disables cancel when we are in the
process of committing; before:

test=> CREATE TABLE test(x INT);
CREATE TABLE
test=> INSERT INTO test VALUES (3);
INSERT 0 1
test=> TRUNCATE test;
^CCancel request sent
--> ERROR: canceling statement due to user request
test=> SELECT * FROM test;
x
---
(0 rows)

and with both patches:

test=> CREATE TABLE test(x INT);
CREATE TABLE
test=> INSERT INTO test VALUES (3);
INSERT 0 1
test=> TRUNCATE test;
^CCancel request sent
--> TRUNCATE TABLE
test=> SELECT * FROM test;
x
---
(0 rows)

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +