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

[Barak Korren]

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?

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