Topic branches that lag behind their base branch don't build reproducibly with mergebasebranch build option

intrigeri wrote on #12633-note_36:

I think there’s a bug in the handling of the base branch merge or something related: https://jenkins.tails.boum.org/view/Tails_ISO/job/reproducibly_build_Tails_ISO_bugfix-14796-topicons-position/8/artifact/build-artifacts/diffoscope.html shows differing (merge) commit IDs, while the build logs pretend we’re merging the exact same commit (“at commit …”). Neither of these two commit IDs are in the official tails.git, hence my guess that a different merge commit ID is created for some reason unknown to me. I don’t know what’s going on here but it seems fishy. Please investigate and if this is Someone Else’s Problem™ file a ticket for them, otherwise fix it. I suspect it would help debugging if we passed --verbose to this git merge, and/or displayed the ID of the resulting merge commit, or similar.

I think I’ve got it: two otherwise identical merge commits done at different times get different IDs. So if I got it right:

• base branches are not affected by this bug, which is nice
• any topic branch is affected by this bug only if it is lagging behind its base branch; that’s a rather common case so it feels wrong to report such confusing info to the reviewer; it’s easy to workaround (merge base branch,& push) but then you have to wait about 2h20min for the next results, IMO our already painful (some would say “bureaucratic”) reviewe’n’merge process really does not need to be made more cumbersome.

I’ve played locally with using faketime to workaround this problem and it seems to work fine: faketime -f "YYYY-MM-DD hh:mm:ss" git merge --no-edit $whatever always creates the same merge commit ID. Note that -f is important otherwise the clock keeps running and if for some reason things get slow, then the commit can be created after the specified timestamp.

In passing, too bad git merge has no --date option, while git commit does.

Feature Branch: bugfix/14946-mergebasebranch-breaks-reproducibility

Parent Task: #5630 (closed)

Related issues

• Related to #12633 (closed)
• Blocks #14924 (closed)

Original created by @intrigeri on 14946 (Redmine)