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Yuya Nishihara
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docs/design/tracking-branches.md
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docs/design/tracking-branches.md
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# Remote/`@git` tracking branches
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This is a plan to implement more Git-like remote tracking branch UX.
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## Objective
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`jj` imports all remote branches to local branches by default. As described in
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[#1136], this doesn't interact nicely with Git if we have multiple Git remotes
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with a number of branches. The `git.auto-local-branch` config can mitigate this
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problem, but we'll get locally-deleted branches instead.
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The goal of this plan is to implement
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* proper support for tracking/non-tracking remote branches
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* logically consistent data model for importing/exporting Git refs
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[#1136]: https://github.com/martinvonz/jj/issues/1136
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## Current data model (as of jj 0.8.0)
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Under the current model, all remote branches are "tracking" branches, and
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remote changes are merged into the local counterparts.
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```
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branches
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[name]:
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local_target?
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remote_targets[remote]: target
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tags
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[name]: target
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git_refs
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["refs/heads/{name}"]: target # last-known local branches
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["refs/remotes/{remote}/{name}"]: target # last-known remote branches
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# (copied to remote_targets)
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["refs/tags/{name}"]: target # last-known tags
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git_head: target?
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```
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* Remote branches are stored in both `branches[name].remote_targets` and
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`git_refs["refs/remotes"]`. These two are kept in sync unless the branch is
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removed by `jj branch forget` command.
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* Pseudo `@git` remote branches are stored in `git_refs["refs/heads"]`.
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## Proposed data model
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We'll add a per-remote-branch `state` to distinguish non-tracking branches
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from tracking ones.
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```
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state = new # not merged in the local branch or tag
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| tracking # merged in the local branch or tag
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| forgotten # to be expunged on the next export
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# `ignored` state could be added if we want to manage it by view, not by
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# config file. target of ignored remote branch would be absent.
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```
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We'll add a per-remote view-like object to record the last known remote
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branches. It will replace `git_refs` and `branches[name].remote_targets` in
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the current model.
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```
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branches
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[name]: target
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tags
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[name]: target
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remotes
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["git"]:
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branches
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[name]: target, state # refs/heads/{name}
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tags
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[name]: target, state = tracking # refs/tags/{name}
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head: target?, state = TBD # refs/HEAD
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[remote]:
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branches
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[name]: target, state # refs/remotes/{remote}/{name}
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tags: (empty)
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head: (empty)
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```
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With the proposed data model, we can
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* naturally support remote branches which have no local counterparts
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* deduplicate `branches[name].remote_targets` and `git_refs["refs/remotes"]`
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* eliminate `git_` variables and methods from the view object
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The `git.auto-local-branch` config knob is applied when importing new remote
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branch. `jj branch` sub commands will be added to change the tracking state.
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```rust
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fn default_state_for_newly_imported_branch(config, remote) {
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if remote == "git" {
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State::Tracking
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} else if config["git.auto-local-branch"] {
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State::Tracking
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} else {
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State::New
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}
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}
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```
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A branch target to be merged is calculated based on the `state`.
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```rust
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fn target_in_merge_context(known_target, state) {
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match state {
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State::New => RefTarget::absent(),
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State::Tracking => known_target,
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State::Forgotten => RefTarget::absent(),
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}
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}
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```
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### Mapping to the current data model
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* New `remotes["git"].branches` corresponds to `git_refs["refs/heads"]`.
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* New `remotes["git"].tags` corresponds to `git_refs["refs/tags"]`.
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* New `remotes["git"].head` corresponds to `git_head`.
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* New `remotes[remote].branches` corresponds to
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`git_refs["refs/remotes/{remote}"]` and `branches[].remote_targets[remote]`.
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* If `git_refs["refs/remotes/{remote}"]` exists but `.remote_targets` doesn't,
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it means `state = forgotten` in new model.
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* `state = new|tracking` doesn't exist in the current model. It's determined
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by `git.auto-local-branch` config.
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## Common command behaviors
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In the following sections, a merge is expressed as `adds - removes`.
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In particular, a merge of local and remote targets is
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`[local, remote] - [known_remote]`.
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### fetch/import
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* `jj git fetch`
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1. Fetches remote changes to the backing Git repo.
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2. Import changes only for `remotes[remote].branches[glob]` (see below)
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* TODO: how about fetched `.tags`?
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* `jj git import`
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1. Calculates diff from the known `remotes` to the actual git repo.
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* `"refs/heads" - remotes["git"].branches`
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* `"refs/tags" - remotes["git"].tags`
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* `"HEAD" - remotes["git"].head` (unused)
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* `"refs/remotes/{remote}" - remotes[remote]`
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2. Merges diff in local `branches` and `tags` if `state` is `tracking`.
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* If the branch is new, the default `state` should be calculated.
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* If `state` is `forgotten`, the known branch is supposed to be removed,
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and the default `state` should be calculated.
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3. Updates `remotes` reflecting the import.
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* `absent` entries are removed from `remotes`.
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4. Abandons commits that are no longer referenced.
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### push/export
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* `jj git push`
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1. Calculates diff from the known `remotes[remote]` to the local changes.
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* `branches - remotes[remote].branches`
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* If `state` is `new|forgotten` (i.e. untracked), the known remote
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branch `target` is considered `absent`.
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* If `state` is `new|forgotten`, and if the local branch `target` is
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`absent`, the diff `[absent, remote] - absent` is noop. So it's not
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allowed to push deleted branch to untracked remote.
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* TODO: Copy Git's `--force-with-lease` behavior?
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* ~`tags`~ (not implemented, but should be the same as `branches`)
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2. Pushes diff to the remote Git repo (as well as remote tracking branches
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in the backing Git repo.)
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3. Sets `remotes[remote].branches[name].state = tracking`
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4. Import changes only for `remotes[remote].branches[glob]`
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* `jj git export`
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1. Calculates diff from the known `remotes["git"]` to the local changes
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and forgotten branches.
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* `branches - remotes["git"].branches` if `state` is `tracking`
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* If `remotes["git"].branches[name]` is `absent`, the default
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`state = tracking` applies.
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* If `state` is `forgotten` but local branch exists,
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`remotes["git"].branches[name]` is supposed to be removed, and
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the default `state = tracking` applies.
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* ~`tags`~ (not implemented, but should be the same as `branches`)
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* `absent - remotes[remote].branches` if `state` is `forgotten`
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2. Applies diff to the backing Git repo.
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3. Updates `remotes` reflecting the export.
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* `absent` entries are removed from `remotes`.
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### init/clone
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* `jj init`
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* Import, track, and merge per `git.auto_local_branch` config.
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* If `!git.auto_local_branch`, no `tracking` state will be set.
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* `jj git clone`
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* Import, track, and merge per `git.auto_local_branch` config.
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* The default branch will be tracked regardless of `git.auto_local_branch`
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config. (Because local branch is created for the default remote branch,
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it makes sense to track.)
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### branch
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* `jj branch set {name}`
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1. Sets local `branches[name]` entry.
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* `jj branch delete {name}`
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1. Removes local `branches[name]` entry.
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* `jj branch forget {name}`
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1. Removes local `branches[name]` entry if exists.
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2. Sets all `remotes[remote].branches[name].state = forgotten`.
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* `jj branch track {name}@{remote}` (new command)
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1. Merges `[local, remote] - [absent]` in local branch.
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* Same as "fetching/importing existing branch from untracked remote".
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2. Sets `remotes[remote].branches[name].state = tracking`.
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* `jj branch untrack {name}@{remote}` (new command)
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1. Sets `remotes[remote].branches[name].state = new`.
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* `jj branch list`
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* TODO: hide non-tracking branches by default? ...
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Note: desired behavior of `jj branch forget` is to
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* discard both local and remote branches (without actually removing branches
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at remotes)
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* not abandon commits which belongs to those branches (even if the branch is
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removed at a remote)
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## Command behavior examples
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### fetch/import
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* Fetching/importing new branch
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1. Decides new `state = new|tracking` based on `git.auto_local_branch`
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2. If new `state` is `tracking`, merges `[absent, new_remote] - [absent]`
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(i.e. creates local branch with `new_remote` target)
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3. Sets `remotes[remote].branches[name].state`
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* Fetching/importing existing branch from tracking remote
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1. Merges `[local, new_remote] - [known_remote]`
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* Fetching/importing existing branch from untracked remote
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1. Decides new `state = new|tracking` based on `git.auto_local_branch`
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2. If new `state` is `tracking`, merges `[local, new_remote] - [absent]`
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3. Sets `remotes[remote].branches[name].state`
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* Fetching/importing remotely-deleted branch from tracking remote
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1. Merges `[local, absent] - [known_remote]`
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2. Removes `remotes[remote].branches[name]` (`target` becomes `absent`)
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(i.e. the remote branch is no longer tracked)
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3. Abandons commits in the deleted branch
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* Fetching/importing remotely-deleted branch from untracked remote
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1. Decides new `state = new|tracking` based on `git.auto_local_branch`
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2. Noop anyway since `[local, absent] - [absent]` -> `local`
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* Fetching previously-forgotten branch from remote
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1. Decides new `state = new|tracking` based on `git.auto_local_branch`
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2. If new `state` is `tracking`, merges
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`[absent, new_remote] - [absent]` -> `new_remote`
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(The known `target` of forgotten remote branch is `absent`)
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3. Sets `remotes[remote].branches[name].state`
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* Fetching forgotten and remotely-deleted branch
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* Same as "remotely-deleted branch from untracked remote" since `forgotten`
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remote branch should never be `tracking`
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* Therefore, no local commits should be abandoned
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### push/export
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* Pushing/exporting new branch, remote doesn't exist
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1. Exports `[local, absent] - [absent]` -> `local`
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2. Sets `remotes[remote].branches[name].state = tracking`
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3. `import_refs()` merges `[local, local] - [absent]` -> `local` (noop)
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* Pushing/exporting new branch, untracked remote exists
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1. Exports `[local, remote] - [absent]`
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* Fails if `local` moved backwards or sideways
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2. Sets `remotes[remote].branches[name].state = tracking`
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3. `import_refs()` merges `[local, local] - [remote]` -> `local` (noop)
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* Pushing/exporting existing branch to tracking remote
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1. Exports `[local, remote] - [remote]` -> `local`
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* Fails if `local` moved backwards or sideways, and if `remote` is out of
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sync
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2. `import_refs()` merges `[local, local] - [remote]` -> `local` (noop)
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* Pushing/exporting existing branch to untracked remote
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* Same as "new branch"
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* Pushing/exporting deleted branch to tracking remote
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1. Exports `[absent, remote] - [remote]` -> `absent`
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* TODO: Fails if `remote` is out of sync?
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2. `import_refs()` merges `[absent, absent] - [remote]` -> `absent`
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3. Removes `remotes[remote].branches[name]` (`target` becomes `absent`)
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* Pushing/exporting deleted branch to untracked remote
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* Noop since `[absent, remote] - [absent]` -> `remote`
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* Perhaps, UI will report error
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* Pushing forgotten branch to untracked remote
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* Same as "deleted branch to untracked remote"
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* Exporting forgotten branch
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1. Local branch change is noop since `[absent, absent] - [absent]` -> `absent`
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2. Exports `forgotten` state to the backing Git repo:
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`[absent, known_remote] - [known_remote]` -> `absent`
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(This includes local branch in the pseudo `"git"` remote)
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3. Removes `remotes[remote].branches[name]` (`target` becomes `absent`)
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* Pushing previously-forgotten branch to remote
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* Same as "new branch, untracked remote exists"
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* The known `target` of forgotten remote branch is `absent`
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### `@git` remote
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* `jj branch untrack {name}@git`
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* Maybe rejected (to avoid confusion)?
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* Allowing this would mean different local branches of the same name coexist
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in jj and git.
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* `jj git fetch --remote git`
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* Maybe rejected (to avoid confusion)?
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* Conceptually, it's `git::import_refs()` only for local branches.
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* `jj git push --remote git`
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* Maybe rejected (to avoid confusion)?
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* Conceptually, it's `jj branch track` and `git::export_refs()` only for
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local branches.
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## Remaining issues
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* `git.auto_local_branch = false` by default to help Git interop?
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* https://github.com/martinvonz/jj/issues/1862
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* https://github.com/martinvonz/jj/issues/1278 pushing to tracked remote
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* Option could be added to push to all `tracking` remotes?
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* Track remote branch locally with different name
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* Local branch name could be stored per remote branch
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* Consider UI complexity
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* "private" state (suggested by @ilyagr)
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* "private" branches can be pushed to their own remote, but not to the
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upstream repo
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* This might be a state attached to a local branch (similar to Mercurial's
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"secret" phase)
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## References
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* https://github.com/martinvonz/jj/issues/1136
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* https://github.com/martinvonz/jj/issues/1666
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* https://github.com/martinvonz/jj/issues/1690
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* https://github.com/martinvonz/jj/issues/1734
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* https://github.com/martinvonz/jj/pull/1739
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