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81 lines
3 KiB
Markdown
81 lines
3 KiB
Markdown
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# Git submodule storage
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## Objective
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Decide what approach(es) to Git submodule storage we should pursue.
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The decision will be recorded in [./git-submodules.md](./git-submodules.md).
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## Use cases to consider
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The submodule storage format should support the workflows specified in the
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[submodules roadmap](./git-submodules.md). It should be obvious how "Phase 1"
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requirements will be supported, and we should have an idea of how "Phases 2,3,X"
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might be supported.
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Notable use cases and workflows are noted below.
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### Fetching submodule commits
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Git's protocol is designed for communicating between copies of the same
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repository. Notably, a Git fetch calculates the list of required objects by
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performing reachability checks between the refs on the local and the remote
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side. We should expect that this will only work well if the submodule repository
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is stored as a local Git repository.
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Rolling our own Git fetch is too complex to be worth the effort.
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### "jj op restore" and operation log format
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We want `jj op restore` to restore to an "expected" state in the submodule.
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There is a potential distinction between running `jj op restore` in the
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superproject vs in the submodule, and the expected behavior may be different in
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each case, e.g. in the superproject, it might be enough to restore the submodule
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working copy, but in the submodule, refs also need to be restored.
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Currently, the operation log only references objects and refs in the
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superproject, so it is likely that proposed approaches will need to extend this
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format. It is also worth considering that submodules may be added, updated or
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removed in superproject commits, thus the list of submodules is likely to change
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over the repository's lifetime.
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### Nested submodules
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Git submodules may contain submodules themselves, so our chosen storage schemes
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should support that.
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We should consider limiting the recursion depth to avoid nasty edge cases (e.g.
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cyclical submodules.) that might surprise users.
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### Supporting future extensions
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There are certain extensions we may want to make in the future, but we don't
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have a timeline for them today. Proposed approaches should take these
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extensions into account (e.g. the approach should be theoretically extensible),
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but a full proposal for implementing them is not necessary.
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These extensions are:
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- Non-git subrepos
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- Colocated Git repos
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- Non-git backends
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## Possible approaches
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### Approach 1: Store Git submodules as full jj repos
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This would be somewhere in `.jj` but outside of `.jj/store`. We would then
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expose a "submodules" interface that gets hooked up to the relevant machinery
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(e.g. updating the working copy).
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TODO(chooglen): Discuss operation log
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TODO(chooglen): Discuss nested submodules
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### Approach 3: Store Git submodules as alternate jj repo backends
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This is Approach 3, but instead of storing the submodule in a Git backend,
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create a new backend that is backed by a full jj repo (like Approach 2), and
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store the Git submodule in its own jj repo backend.
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TODO(chooglen): Discuss operation log
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TODO(chooglen): Discuss nested submodules
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