Summary:
Ref T4327. I want to make change parsing testable; one thing which is blocking this is that the Git discovery process is still part of `PullLocal` daemon instead of being part of `DiscoveryEngine`. The unit test stuff which I want to use for change parsing relies on `DiscoveryEngine` to discover repositories during unit tests.
The major reason git discovery isn't part of `DiscoveryEngine` is that it relies on the messy "autoclose" logic, which we never implemented for Mercurial. Generally, I don't like how autoclose was implemented: it's complicated and gross and too hard to figure out and extend.
Instead, I want to do something more similar to what we do for pushes, which is cleaner overall. Basically this means remembering the old branch heads from the last time we parsed a repository, and figuring out what's new by comparing the old and new branch heads. This should give us several advantages:
- It should be simpler to understand than the autoclose stuff, which is pretty mind-numbing, at least for me.
- It will let us satisfy branch and tag queries cheaply (from the database) instead of having to go to the repository. We could also satisfy some ref-resolve queries from the database.
- It should be easier to extend to Mercurial.
This implements the basics -- pretty much a table to store the cursors, which we update only for Git for now.
Test Plan:
- Ran migration.
- Ran `bin/repository discover X --trace --verbose` on various repositories with branches and tags, before and after modifying pushes.
- Pushed commits to a git repo.
- Looked at database tables.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T4327
Differential Revision: https://secure.phabricator.com/D7982
