I just learned that attaching a git note is not enough to keep a
commit from being GC'd. I had read `git help gc` before but it was
quite misleading (I just sent a patch to clarify it). Since the git
note is not enough, we need to create some other reference. This patch
makes it so we write refs in `refs/jj/keep/` for every commit we
create. We will probably want to remove unnecessary refs (ancestors of
commits pointed to by other refs) once we have a `jj gc` command.
We store conflicts as blobs with JSON data and with a git note
pointing to them to prevent GC. These are stored in the git tree as
regular files. The only thing that distinguishes them is that their
filename ends with `.jjconflict`. Since they are referenced from the
tree, there's no need for the git note to prevent GC (which doesn't
work anyway, as I just learned), and we don't store any additional
data in the note either, so let's just remove it.
Returning the store's internal `git2::Repository` instance wrapped in
a `Mutex` makes it easy to run into deadlocks. Let's return a freshly
loaded repo instance instead.
This commit starts adding support for working with a Jujube repo's
underlyng Git repo (if there is one). It does so by adding a command
for pushing from the Git repo to a remote, so you can work with your
anonymous branches in Jujube and push to a remote Git repo without
having to switch repos and copy commit hashes.
For example, `jj git push origin main` will push to the "main" branch
on the remote called "origin". The remote name (such as "origin") is
resolved in that repo. Unlike most commands, it defaults to pushing
the working copy's parent, since it is probably a mistake to push a
working copy commit to a Git repo.
I plan to add more `jj git` subcommands later. There will probably be
at least a command (or several?) for making the Git repo's refs
available in the Jujube repo.
Before this commit, running Git's GC in a Git repo backing a Jujube
repo would risk deleting the conflict data we store as blobs in the
Git repo. This commit fixes that by adding a Git note pointing to the
conflict blob.
I wasn't able to add a test case for this because libgit2 doesn't
support gc [1]. Just testing that the ref is there doesn't seem very
useful.
[1] https://github.com/libgit2/libgit2/issues/3247
Very little code was saved by reusing `write_file()` and it made it
confusing (needed to provide an unused filename). Also, I'll soon want
access to the `locked_repo` variable in `write_conflict()`.
I have never run into this being a problem in practice, but this
change is a stepping stone for two things:
1. Using exponential backoff for other locks (in particular the
working copy).
2. Making the Git store write a ref for conflict objects, so they
don't get GC'd (I want to do that before even I start dogfooding).
I had tried to generate the protobuf code at build time many months
ago, but decided against it because it slowed down the build too
much. I didn't realize there was the
"cargo:rerun-if-changed=<filename>" feature that time. Given that that
exists, it seems like an obvious win to generate the source code at
build time.
I put the generated sources in `$OUT_DIR` (where [1] says they should
be), then include them in the `protos` module by using the `include!`
macro. The biggest problem with that is that I couldn't get IntelliJ
to understand it, even after enabling the experimental features
described in [2].
[1] https://doc.rust-lang.org/cargo/reference/build-script-examples.html#code-generation
[2] https://github.com/intellij-rust/intellij-rust/issues/1908#issuecomment-592773865
