doc: add plan for non-tracking branches support

jj-vcs#1136, jj-vcs#1666, jj-vcs#1734

docs/design/tracking-branches.md

@@ -0,0 +1,279 @@
+# Remote/`@git` tracking branches
+
+This is a plan to implement more Git-like remote tracking branch UX.
+
+## Objective
+
+`jj` imports all remote branches to local branches by default. As described in
+[#1136], this doesn't interact nicely with Git if we have multiple Git remotes
+with a number of branches. The `git.auto-local-branch` config can mitigate this
+problem, but we'll get locally-deleted branches instead.
+
+The goal of this plan is to implement
+* proper support for tracking/non-tracking remote branches
+* logically consistent data model for importing/exporting Git refs
+
+[#1136]: https://github.com/martinvonz/jj/issues/1136
+
+## Current data model (as of jj 0.8.0)
+
+Under the current model, all remote branches are "tracking" branches, and
+remote changes are merged into the local counterparts.
+
+```
+branches
+  [name]:
+    local_target?
+    remote_targets[remote]: target
+tags
+  [name]: target
+git_refs
+  ["refs/heads/{name}"]: target             # last-known local branches
+  ["refs/remotes/{remote}/{name}"]: target  # last-known remote branches
+                                            # (copied to remote_targets)
+  ["refs/tags/{name}"]: target              # last-known tags
+git_head: target?
+```
+
+* Remote branches are stored in both `branches[name].remote_targets` and
+  `git_refs["refs/remotes"]`. These two are kept in sync unless the branch is
+  removed by `jj branch forget` command.
+* Pseudo `@git` remote branches are stored in `git_refs["refs/heads"]`.
+
+## Proposed data model
+
+We'll add a per-remote-branch `state` to distinguish non-tracking branches
+from tracking ones.
+
+```
+state = new        # not merged in the local branch or tag
+      | tracking   # merged in the local branch or tag
+      | forgotten  # to be expunged on the next export
+# `ignored` state could be added if we want to manage it by view, not by
+# config file. target of ignored remote branch would be absent.
+```
+
+We'll add a per-remote view-like object to record the last known remote
+branches. It will replace `git_refs` and `branches[name].remote_targets` in
+the current model.
+
+```
+branches
+  [name]: target
+tags
+  [name]: target
+remotes
+  ["git"]:
+    branches
+      [name]: target, state                 # refs/heads/{name}
+    tags
+      [name]: target, state = tracking      # refs/tags/{name}
+    head: target?, state = TBD              # refs/HEAD
+  [remote]:
+    branches
+      [name]: target, state                 # refs/remotes/{remote}/{name}
+    tags: (empty)
+    head: (empty)
+```
+
+With the proposed data model, we can
+* naturally support remote branches which have no local counterparts
+* deduplicate `branches[name].remote_targets` and `git_refs["refs/remotes"]`
+* eliminate `git_` variables and methods from the view object
+
+The `git.auto-local-branch` config knob is applied when importing new remote
+branch.
+
+```rust
+fn default_state_for_newly_imported_branch(config, remote) {
+    if remote == "git" {
+        State::Tracking
+    } else if config["git.auto-local-branch"] {
+        State::Tracking
+    } else {
+        State::New
+    }
+}
+```
+
+A branch target to be merged is calculated based on the `state`.
+
+```rust
+fn target_in_merge_context(known_target, state) {
+    match state {
+        State::New => RefTarget::absent(),
+        State::Tracking => known_target,
+        State::Forgotten => RefTarget::absent(),
+    }
+}
+```
+
+### Mapping to the current data model
+
+* New `remotes["git"].branches` corresponds to `git_refs["refs/heads"]`.
+* New `remotes["git"].tags` corresponds to `git_refs["refs/tags"]`.
+* New `remotes["git"].head` corresponds to `git_head`.
+* New `remotes[remote].branches` corresponds to
+  `git_refs["refs/remotes/{remote}"]` and `branches[].remote_targets[remote]`.
+* If `git_refs["refs/remotes/{remote}"]` exists but `.remote_targets` doesn't,
+  it means `state = forgotten` in new model.
+* `state = new|tracking` doesn't exist in the current model. It's determined
+  by `git.auto-local-branch` config.
+
+## Common command behaviors
+
+### fetch/import
+
+* `jj git fetch`
+  1. Fetches remote changes to the backing Git repo.
+  2. Import changes only for `remotes[remote].branches[glob]` (see below)
+     * TODO: how about fetched `.tags`?
+
+* `jj git import`
+  1. Calculates diff from the known `remotes` to the actual git repo.
+     * `"refs/heads" - remotes["git"].branches`
+     * `"refs/tags" - remotes["git"].tags`
+     * `"HEAD" - remotes["git"].head` (unused)
+     * `"refs/remotes/{remote}" - remotes[remote]`
+  2. Merges them in local `branches` and `tags` if `state` is `tracking`.
+     * If the branch is new, the default `state` should be calculated.
+     * If `state` is `forgotten`, the known branch is supposed to be removed,
+       and the default `state` should be calculated.
+  3. Updates `remotes` reflecting the import.
+     * `absent` entries are removed from `remotes`.
+
+### push/export
+
+* `jj git push`
+  1. Calculates diff from the known `remotes` to the local changes.
+     * `branches - remotes[remote].branches`
+       * If `state` is `new|forgotten`, the known remote branch `target` is
+         considered `absent`.
+       * If `state` is `new|forgotten`, and if the local branch `target` is
+         `absent`, the diff `[absent, remote] - absent` is noop. So it's not
+         allowed to push deleted branch to untracked remote.
+     * ~`tags`~ (not implemented, but should be the same as `branches`)
+  2. Pushes them to the remote Git repo (as well as remote tracking branches
+     in the backing Git repo.)
+  3. Sets `remotes[remote].branches[name].state = tracking`
+  4. Import changes only for `remotes[remote].branches[glob]`
+
+* `jj git export`
+  1. Calculates diff from the known `remotes` to the local changes.
+     * `branches - remotes["git"].branches` if `state` is `tracking`
+       * If `remotes["git"].branches[name]` is `absent`, the default
+         `state = tracking` applies.
+       * If `state` is `forgotten` but local branch exists,
+         `remotes["git"].branches[name]` is supposed to be removed, and
+         the default `state = tracking` applies.
+     * ~`tags`~ (not implemented, but should be the same as `branches`)
+     * `absent - remotes[remote].branches` if `state` is `forgotten`
+  2. Applies them to the backing Git repo.
+  3. Updates `remotes` reflecting the export.
+     * `absent` entries are removed from `remotes`.
+
+### init/clone
+
+* `jj init`
+  * Import, track, and merge per `git.auto_local_branch` config.
+  * If `!git.auto_local_branch`, no `tracking` state will be set.
+  * https://github.com/martinvonz/jj/issues/1862
+
+* `jj git clone`
+  * Import, track, and merge per `git.auto_local_branch` config.
+  * The default branch will be tracked regardless of `git.auto_local_branch`
+    config. (Since local branch is created for the default remote branch,
+    it makes sense to track.)
+
+### branch
+
+* `jj branch set {name}`
+  1. Sets local `branches[name]` entry.
+* `jj branch delete {name}`
+  1. Removes local `branches[name]` entry.
+* `jj branch forget {name}`
+  1. Removes local `branches[name]` entry if exists.
+  2. Sets all `remotes[remote].branches[name].state = forgotten`.
+* `jj branch track {name}@{remote}` (new command)
+  1. Merges `[local, remote] - [absent]` in local branch.
+     (same as "fetching/importing existing branch from untracked remote")
+  2. Sets `remotes[remote].branches[name].state = tracking`.
+* `jj branch untrack {name}@{remote}` (new command)
+  1. Sets `remotes[remote].branches[name].state = new`.
+* `jj branch list`
+  * TODO: hide non-tracking branches by default? ...
+
+Note: desired behavior of `jj branch forget` is to
+* discard both local and remote branches (without actually removing branches
+  at remotes)
+* not abandon commits which belongs to those branches (even if the branch is
+  removed at a remote)
+
+## Command behavior examples
+
+### fetch/import
+
+* Fetching/importing new branch
+  1. Decides new `state = new|tracking` based on `git.auto_local_branch`
+  2. If new `state` is `tracking`, merges `[absent, new_remote] - [absent]`
+     (i.e. creates local branch with `new_remote` target)
+  3. Sets `remotes[remote].branches[name].state`
+* Fetching/importing existing branch from tracking remote
+  1. Merges `[local, new_remote] - [old_remote]`
+* Fetching/importing existing branch from untracked remote
+  1. Decides new `state = new|tracking` based on `git.auto_local_branch`
+  2. If new `state` is `tracking`, merges `[local, new_remote] - [absent]`
+  3. Sets `remotes[remote].branches[name].state`
+* Fetching/importing remotely-deleted branch from tracking remote
+  1. Merges `[local, absent] - [old_remote]`
+  2. Removes `remotes[remote].branches[name]` (`target` becomes `absent`)
+* Fetching/importing remotely-deleted branch from untracked remote
+  1. Decides new `state = new|tracking` based on `git.auto_local_branch`
+  2. Noop anyway since `[local, absent] - [absent]` -> `local`
+* Fetching previously-forgotten branch from remote
+  1. Decides new `state = new|tracking` based on `git.auto_local_branch`
+  2. Merges `[absent, new_remote] - [absent]` -> `new_remote`
+     (The known `target` of forgotten remote branch is `absent`)
+  3. Sets `remotes[remote].branches[name].state`
+
+### push/export
+
+* Pushing/exporting new branch, remote doesn't exist
+  1. Exports `[local, absent] - [absent]` -> `local`
+  2. Sets `remotes[remote].branches[name].state = tracking`
+  3. `import_refs()` merges `[local, local] - [absent]` -> `local` (noop)
+* Pushing/exporting new branch, untracked remote exists
+  1. Exports `[local, remote] - [absent]`
+     * Fails if `local` moved backwards or sideways
+  2. Sets `remotes[remote].branches[name].state = tracking`
+  3. `import_refs()` merges `[local, local] - [remote]` -> `local` (noop)
+* Pushing/exporting existing branch to tracking remote
+  1. Exports `[local, remote] - [remote]` -> `local`
+  2. `import_refs()` merges `[local, local] - [remote]` -> `local` (noop)
+* Pushing/exporting existing branch to untracked remote
+  * Same as new branch
+* Pushing/exporting deleted branch to tracking remote
+  1. Exports `[absent, remote] - [remote]` -> `absent`
+  2. `import_refs()` merges `[absent, absent] - [remote]` -> `absent`
+  3. Removes `remotes[remote].branches[name]` (`target` becomes `absent`)
+* Pushing/exporting deleted branch to untracked remote
+  * Noop since `[absent, remote] - [absent]` -> `remote`
+* Pushing forgotten branch to untracked remote
+  * Noop since `[absent, remote] - [absent]` -> `remote`
+* Exporting forgotten branch
+  1. Local branch change is noop since `[absent, absent] - [absent]` -> `absent`
+  2. Exports `forgotten` state to the backing Git repo:
+    `[absent, old_remote] - [old_remote]` -> `absent`
+    (This includes local branch in the pseudo `"git"` remote)
+  3. Removes `remotes[remote].branches[name]` (`target` becomes `absent`)
+* Pushing previously-forgotten branch to remote
+  * Same as new branch, untracked remote exists
+  * The known `target` of forgotten remote branch is `absent`
+
+## References
+
+* https://github.com/martinvonz/jj/issues/1136
+* https://github.com/martinvonz/jj/issues/1666
+* https://github.com/martinvonz/jj/issues/1690
+* https://github.com/martinvonz/jj/issues/1734
+* https://github.com/martinvonz/jj/pull/1739