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[merge_metrics.ml]: fix diff size for merge train MRs

Arvid Jakobsson requested to merge arvid@fix-diff-size-for-merge-train-mrs into master

What

We've being seeing errors since the introduction of merge trains on tezos/tezos like this:

https://gitlab.com/nomadic-labs/tezos-sw-metrics/-/jobs/7363570940

Seems like with merged results & merge trains, our old mechanism for calculating the size of MRs no longer work.

Indeed, we would take the two fields for each MR:

  • the head: mr.merge_request_sha, assuming this to be the tip of the MR
  • the merge commit: mr.merge_request_merge_commit_sha, assuming this to be the merge commit on master for the MR

Then, we would find out which of the merge commit's two parents is not the head that we can call base. Finally, the size of the MR is the size of the diff between base and head.

This seems to work fine before the introduction of merged result pipelines.

With merged result pipelines, the MR's head is not necessarily a parent of the merge commit. Consider tezos/tezos!14132 (merged).

Notice that the head does not figure in the parents.

This is since with

  • merged results MRs;
  • merge trains; and
  • marge-bot2,

the MR is no longer rebased just before merging. Hence, the last head of the MR is not a commit that will appear on master unless the MR was rebased on the target branch when merged.

The unknown parent 2f2a1469 that figures above is the HEAD of the MR as rebased on the tip of the target branch just before merging. This happens behind the scenes in this new setting: this rebased version of the branch is not pushed back to the MR.

Anyhow, we can side-step this fragile mechanism for working out which two commits to diff by using a new set of fields in the gitlab api under the key .diff_refs. It seems like .diff_refs.base_sha and .diff_refs.head_sha are exactly the two commits we want to compare. It seems they are not on master -- whatever, we just fetch them.

Edited by Arvid Jakobsson

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