[ovirt-devel] Gerrit parallel patch handling and CI (Or, why did my code fail post-merge)

[Vojtech Szocs]

----- Original Message -----
> From: "Barak Korren" <bkorren at redhat.com>
> To: "devel" <devel at ovirt.org>
> Sent: Sunday, November 20, 2016 3:07:56 PM
> Subject: [ovirt-devel] Gerrit parallel patch handling and CI (Or,	why did my code fail post-merge)
> 
> Hi there,
> 
> I would like to address a concernt that had been raised to us by
> multiple developers, and reach an agreement on how (and if)  to remedy
> it.
> 
> Lets assume the following situation:
> We have a Git repo in Gerrit with top commit C0 in master.
> On time t0 developers Alice and Bob push patches P1 and P2 respectively
> to master so that we end up with the following situation in git:
> C0 <= P1 (this is Alice`s patch)
> C0 <= P2 (this is Bob`s patch)
> 
> On time t1 CI runs for both patches checking the code as it looks for
> each patch. Lets assume CI is successful for both.
> 
> On time t2 Alice submits her patch and Gerrit merges it, resulting in
> the following situation in master:
> C0 <= P1
> 
> On time t2 Bob submits his patch. Gerrit, seeing master has changed,
> re-bases the patch and merges it, the resulting situation (If the
> rebase is successful) is:
> C0 <= P1 <= P2
> 
> This means that the resulting code was never tested in CI. This, in
> turn, causes various failures to show up post-merge despite having
> pre-merge CI run successfully.
> 
> This situation is a result of the way our repos are currently
> configured. Most repos ATM are configured with the "Rebase If
> Necessary" submit type. This means that Gerrit tries to automatically
> rebase patches as mentioned in t2 above.
> 
> We could, instead, configure the repos to use the "Fast Forward Only"
> submit type. In that case, when Bob submits on t2, Gerrit refuses to
> merge and asks Bob to rebase (While offering a convenient button to do
> it). When he does, a new patch set gets pushed, and subsequently
> checked by CI.
> 
> I recommend we switch all projects to use the "Fast Forward Only" submit
> type.
> 
> Thoughts? Concerns?

Question: after the patch is submitted in Gerrit (it's fully acked
and maintainer hits the "Submit" button), does Gerrit allow us to
run CI (e.g. `check-merged` script) *before* doing the actual merge?

[In other words, does Gerrit support gating merge based on CI script?]

That said, +1 on "Fast Forward Only" submit type.

> 
> --
> Barak Korren
> bkorren at redhat.com
> RHEV-CI Team
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