Support rebasing forked repo against default branch in upstream repo

[dhood]

If we specify a particular branch on the ros2 repos to test for the CI jobs, we will rebase that branch against master.

There are cases when it would be useful to do the same when testing a given ros2.repos file, such as checking that PRs from external contributors will work correctly even after being rebased.

(There are probably also cases when we would not want to rebase)

[dirk-thomas]

The rebase always happens against the specified branch in the repos file. In the case where the repos file specifies a forked repo there is simply no knowledge about the upstream repo. So I don't see how this would be possible.

[dhood]

Couldn't we rebase the repo+branch in the custom ros2.repos (e.g. dhood/* + feature) against the repo+branch in the default ros2.repos (i.e. ros2/* + master)?

That is, mimic the process that happens when we accept these PRs.

[dirk-thomas]

Currently there is only a single repo file - not two. So yes, in theory that is possible. But that will require a significant effort in implementing the merging of two separate repo files (with all the corner cases, e.g. some repo might only be in one of the two files, what is the correct way of merging them?).

[dhood]

yep, good point about the edge cases. I am thinking about this for an improved process for testing external contributors' PRs (so more likely a 1:1 mapping). But perhaps one day PR testing will not require someone generating a custom repos file in the first place, so I think the more generic name for this ticket is good.