Fix rebase API race condition - #173
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Prior to this fix marge-bot would behave as follows, when trying to
rebase a MR using GitLab's rebase API.
1. Make API request to rebase.
2. Immediately refetch MR info.
3. Observe that the MR can still be rebased, immediately retry.
4. Due to a race condition GitLab rebases again, this time
skipping pipelines.
5. Comment on the MR "Someone skipped the queue! Will have to
try again...".
6. Check the MR info again, see that the MR can be merged.
7. Merge the MR without actually waiting for the tests to pass.
This commit addresses 2 root causes with the following measures.
1. Sleep a bit after rebasing a branch to avoid the rebase race
condition. Polling is also done every 5 seconds vs the previous
case of every 1 second.
2. To prevent the comment "Someone skipped the queue! [...]", the
bot does not compare locally computed SHA with GitLab's SHA after
a rebase. This check was bogous from the start, because commits
(including rebase'd commits) include a timestamp in them so it
is unlikely that two different rebases result in the same SHA.
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This follows my observations raised on #160 (comment). I tried to change as little as possible, because I'm not that familiar with Python nor the intricacies of GitLab's API.
The fix is described in detail in kgilden@89207cb.