Re: pg_rewind race condition just after promotion

On 08/12/2020 06:45, Kyotaro Horiguchi wrote:
> At Mon, 7 Dec 2020 20:13:25 +0200, Heikki Linnakangas <hlinnaka@iki.fi> wrote in
>> I think we should fix this properly. I'm not sure if it can lead to a
>> broken cluster, but at least it can cause pg_rewind to fail
>> unnecessarily and in a user-unfriendly way. But this is actually
>> pretty simple to fix. pg_rewind looks at the control file to find out
>> the timeline the server is on. When promotion happens, the startup
>> process updates minRecoveryPoint and minRecoveryPointTLI fields in the
>> control file. We just need to read it from there. Patch attached.
> 
> Looks fine to me.  A bit concerned about making sourceHistory
> needlessly file-local but on the other hand unifying sourceHistory and
> targetHistory looks better.

Looking closer, findCommonAncestorTimeline() was freeing sourceHistory, 
which was pretty horrible when it's a file-local variable. I changed it 
so that both the source and target histories are passed to 
findCommonAncestorTimeline() as arguments. That seems more clear.

> For the test part, that change doesn't necessariry catch the failure
> of the current version, but I *believe* the prevous code is the result
> of an actual failure in the past so the test probablistically (or
> dependently on platforms?) hits the failure if it happned.

Right. I think the current test coverage is good enough. We've been 
bitten by this a few times by now, when we've forgotten to add the 
manual checkpoint commands to new tests, and the buildfarm has caught it 
pretty quickly.

>> I think we should also backpatch this. Back in 2015, we decided that
>> we can live with this, but it's always been a bit bogus, and seems
>> simple enough to fix.
> 
> I don't think this changes any successful behavior and it just saves
> the failure case so +1 for back-patching.

Thanks for the review! New patch version attached.

- Heikki