Re: Online enabling of checksums

> On 07 Apr 2018, at 00:23, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2018-04-06 02:28:17 +0200, Daniel Gustafsson wrote:
>> Applying this makes the _cancel test pass, moving the failure instead to the
>> following _enable test (which matches what coypu and mylodon are seeing).
>
> FWIW, I'm somewhat annoyed that I'm now spending time debugging this to
> get the buildfarm green again.

Sorry about that, I’m a bit slow due to various $family situations at the
moment.

> I'm fairly certain that the bug here is a simple race condition in the
> test (not the main code!):

I wonder if it may perhaps be a case of both?

> It's
> exceedingly unsurprising that a 'pg_sleep(1)' is not a reliable way to
> make sure that a process has finished exiting. Then followup tests fail
> because the process is still running

I can reproduce the error when building with --disable-atomics, and it seems
that all the failing members either do that, lack atomic.h, lack atomics or a
combination. checksumhelper cancellation use pg_atomic_unlocked_test_flag() to
test if it’s running when asked to abort, something which seems unsafe to do in
semaphore simulation as it always returns true. If I for debugging synthesize
a flag test with testset/clear, the tests pass green (with the upstream patch
for pg_atomic_test_set_flag_impl() applied). Cancelling with semaphore sim is
thus doomed to never work IIUC. Or it’s a red herring.

As Magnus mentioned upstream, rewriting to not use an atomic flag is probably
the best option, once the current failure is understood.

> really? Let's just force the test take at least 6s purely from
> sleeping?

The test needs continuous reading in a session to try and trigger any bugs in
read access on the cluster during checksumming, is there a good way to do that
in the isolationtester? I have failed to find a good way to repeat a step like
that, but I might be missing something.

cheers ./daniel