Add topological merge bot

[JornVernee]

This PR adds a bot that does a topological merge of the branches in a repo.

The branches can declare a dependencies file, which lists the branches that they depend on. This bot will crawl the branches, collect the dependencies for each branch, and topologically sort them based on their dependencies. Following that it will attempt to merge each dependency into the dependent in this order (this is mainly done so that we get less merges/failures if one of the root merges fails).

Branches that do not declare a dependency file implicitly depend on the master branch. Therefore the list of branches that the bot considers is passed in during configuration.

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Aside from that, it also fixes a minor problem with Repository::clone on Windows.

Progress

• Change must not contain extraneous whitespace
• Change must be properly reviewed

Approvers

• Maurizio Cimadamore (mcimadamore - no project role) Note! Review applies to 4a8f361
• Erik Helin (ehelin - Reviewer)

[bridgekeeper]

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[mlbridge]

Webrevs

• 05: Full - Incremental (62d1e15)
• 04: Full - Incremental (2099a07)
• 03: Full - Incremental (d232a87)
• 02: Full - Incremental (4ffe0b0)
• 01: Full - Incremental (7df299e)
• 00: Full (4a8f361)

[edvbld]

Hi Jorn, thanks for the contribution! 👏

Just a quick suggestion after a brief look - how about moving Edge to its own file, Edge.java and also move tsort into its own class, TopologicalSort (in TopologicalSort.java)? That way you should have a fairly easy time to add a new test class, TopologicalSortTests, with some unit tests for the topological sort

[mcimadamore]

While the code looks generally ok, I'd like to understand a bit more what it does (e.g. what the various API method translate to when it comes to Git, and what the actual behavior is in case of merge conflicts). Also there are some improvements on the topo sort handling.

[mcimadamore]

from my understanding, git merge --abort is just fine here. This new command is the same - except for the --hard part?

[mcimadamore]

I see - you also wanna recover from a failed push? [in which case the changeset is there and there is no further changes in the working tree]

[JornVernee]

Afaik merge --abort reverts any working tree changes, like conflicting file. reset --hard actually resets the branch to a certain commit. We need the hard reset when the push fails, but the merge commit has already succeeded (i.e. there is nothing to abort).

[mcimadamore]

looks ok

[edvbld]

This is starting to look very nice, just a few more comments!

[edvbld]

Looks good now, thanks for hanging in there! 👏

[openjdk]

@JornVernee This change can now be integrated. The commit message will be:

Add topological merge bot

Reviewed-by: mcimadamore, ehelin

• If you would like to add a summary, use the /summary command.
• To list additional contributors, use the /contributor command.

Since the source branch of this PR was last updated there have been 4 commits pushed to the master branch:

• c999004: Batch commit, add and files methods in HgRepository
• 1f95176: 91: Avoid discarding pending items of different types
• b46b8e5: jcheck "repository" property in [general] section not read from .jcheck/conf
• 417c6f5: Avoid running PullRequestBot WorkItems for the same PR concurrently

Since there are no conflicts, your changes will automatically be rebased on top of the above commits when integrating. If you prefer to do this manually, please merge master into your branch first.

• To integrate this PR with the above commit message, type /integrate in a new comment.

[JornVernee]

/integrate

[openjdk]

@JornVernee The following commits have been pushed to master since your change was applied:

• c999004
• 1f95176
• b46b8e5
• 417c6f5

Your commit was automatically rebased without conflicts.
Pushed as commit 8501faf.