Draft for import --reset BRANCH.

lib/src/git.rs

@@ -36,7 +36,7 @@ pub enum GitImportError {
     InternalGitError(#[from] git2::Error),
 }
 
-fn parse_git_ref(ref_name: &str) -> Option<RefName> {
+pub fn parse_git_ref(ref_name: &str) -> Option<RefName> {
     if let Some(branch_name) = ref_name.strip_prefix("refs/heads/") {
         Some(RefName::LocalBranch(branch_name.to_string()))
     } else if let Some(remote_and_branch) = ref_name.strip_prefix("refs/remotes/") {
@@ -620,13 +620,6 @@ pub fn fetch(
     // Apart from `jj branch forget`, jj doesn't provide commands to manipulate
     // remote-tracking branches, and local git branches don't affect fetch
     // behaviors. So, it's unnecessary to export anything else.
-    //
-    // TODO: Create a command the user can use to reset jj's
-    // branch state to the git repo's state. In this case, `jj branch forget`
-    // doesn't work as it tries to delete the latter. One possible name is `jj
-    // git import --reset BRANCH`.
-    // TODO: Once the command described above exists, it should be mentioned in `jj
-    // help branch forget`.
     let nonempty_branches: HashSet<_> = mut_repo
         .view()
         .branches()

src/commands/branch.rs

@@ -79,6 +79,15 @@ pub struct BranchListArgs {
 ///
 /// A forgotten branch will not impact remotes on future pushes. It will be
 /// recreated on future pulls if it still exists in the remote.
+///
+/// When exporting the forgetting of a branch in colocated repositories or
+/// during a `jj git export`, jj will delete the corresponding git repo branch
+/// if it exists at the expected position.
+///
+/// If you'd like to instead override jj's branch with the git repo's branch,
+/// you can use `jj git import --reset`. That command can also be used in rare
+/// cases where a race condition causes the sync between git branches and jj
+/// branches to fail because of conflicts.
 #[derive(clap::Args, Clone, Debug)]
 pub struct BranchForgetArgs {
     /// The branches to forget.

src/commands/git.rs

@@ -10,7 +10,9 @@ use std::time::Instant;
 use clap::{ArgGroup, Subcommand};
 use itertools::Itertools;
 use jj_lib::backend::{CommitId, ObjectId, TreeValue};
-use jj_lib::git::{self, parse_gitmodules, GitFetchError, GitPushError, GitRefUpdate};
+use jj_lib::git::{
+    self, parse_git_ref, parse_gitmodules, GitFetchError, GitPushError, GitRefUpdate,
+};
 use jj_lib::git_backend::GitBackend;
 use jj_lib::op_store::{BranchTarget, RefTarget};
 use jj_lib::refs::{classify_branch_push_action, BranchPushAction, BranchPushUpdate};
@@ -19,7 +21,7 @@ use jj_lib::repo_path::RepoPath;
 use jj_lib::revset::{self, RevsetIteratorExt as _};
 use jj_lib::settings::{ConfigResultExt as _, UserSettings};
 use jj_lib::store::Store;
-use jj_lib::view::View;
+use jj_lib::view::{RefName, View};
 use jj_lib::workspace::Workspace;
 use maplit::hashset;
 
@@ -153,7 +155,30 @@ pub struct GitPushArgs {
 
 /// Update repo with changes made in the underlying Git repo
 #[derive(clap::Args, Clone, Debug)]
-pub struct GitImportArgs {}
+pub struct GitImportArgs {
+    /// Destroy jj's record of BRANCH and then, if possible, restore it from the
+    /// git repo (can be repeated)
+    ///
+    /// The state of BRANCH and its remote-tracking branches that was previously
+    /// recorded in jj is lost. If the git repo has corresponding branches or
+    /// remote-tracking branches, that state will then be imported into jj.
+    ///
+    /// Branches other than BRANCH are not imported when --reset is specified.
+    ///
+    /// This command can be used for recovery if the remote-tracking branches or
+    /// git-tracking branches become conflicted (this can happen, for example,
+    /// due to a data race).
+    ///
+    /// If you'd like to destroy git's record of the BRANCH as well as jj's, do
+    /// `jj git import --reset BRANCH && jj branch forget BRANCH && jj git
+    /// export`. This should work even if a plain `jj branch forget BRANCH
+    /// && jj git export` doesn't work due to conflicts.
+    //
+    // We could provide an operation that only destroys jj's record of BRANCH, but it would have
+    // confusingly different behavior in colocated and non-colocated repos.
+    #[arg(long, value_name = "BRANCH")]
+    reset: Vec<String>,
+}
 
 /// Update the underlying Git repo with changes made in the repo
 #[derive(clap::Args, Clone, Debug)]
@@ -958,13 +983,39 @@ fn classify_branch_update(
 fn cmd_git_import(
     ui: &mut Ui,
     command: &CommandHelper,
-    _args: &GitImportArgs,
+    args: &GitImportArgs,
 ) -> Result<(), CommandError> {
     let mut workspace_command = command.workspace_helper(ui)?;
-    let repo = workspace_command.repo();
+    let repo = workspace_command.repo().clone();
     let git_repo = get_git_repo(repo.store())?;
-    let mut tx = workspace_command.start_transaction("import git refs");
-    git::import_refs(tx.mut_repo(), &git_repo, &command.settings().git_settings())?;
+    let mut tx;
+    if args.reset.is_empty() {
+        tx = workspace_command.start_transaction("import git refs");
+        git::import_refs(tx.mut_repo(), &git_repo, &command.settings().git_settings())?;
+    } else {
+        tx = workspace_command.start_transaction("git import --reset");
+        for branch in args.reset.iter() {
+            tx.mut_repo().remove_branch(branch);
+        }
+        let git_ref_filter = |ref_name: &RefName| match ref_name {
+            RefName::LocalBranch(branch) => args.reset.contains(branch),
+            RefName::RemoteBranch { branch, .. } => args.reset.contains(branch),
+            _ => false,
+        };
+        for git_tracking_ref in repo.view().git_refs().keys() {
+            if let Some(ref_name) = parse_git_ref(git_tracking_ref) {
+                if git_ref_filter(&ref_name) {
+                    tx.mut_repo().remove_git_ref(git_tracking_ref);
+                }
+            }
+        }
+        git::import_some_refs(
+            tx.mut_repo(),
+            &git_repo,
+            &command.settings().git_settings(),
+            git_ref_filter,
+        )?;
+    }
     tx.finish(ui)?;
     Ok(())
 }

tests/test_git_import_export.rs

@@ -263,6 +263,83 @@ fn test_git_import_move_export_with_default_undo() {
     "###);
 }
 
+#[test]
+fn test_git_import_resert_conflcited_git_tracking() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+    let git_repo = git2::Repository::open(repo_path.join(".jj/repo/store/git")).unwrap();
+
+    test_env.jj_cmd_success(&repo_path, &["branch", "create", "br"]);
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m=a"]);
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
+    br: 812679adcf38 a
+    "###);
+    // Export a branch `br` when it's at `a`
+    insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["git", "export"]), @"");
+    let opid_with_git_tracking_at_a = current_operation_id(&test_env, &repo_path);
+
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m=b"]);
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
+    br: b4a6b8c586dd b
+      @git (ahead by 1 commits, behind by 1 commits): 812679adcf38 a
+    "###);
+    // Export a branch `br` when it's at `b`
+    insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["git", "export"]), @"");
+
+    // Now, the git repo is at b. We create a conflict by importing at a point of
+    // time where jj thinks the branch is at a.
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &["--at-op", &opid_with_git_tracking_at_a, "git", "import"],
+    );
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
+    Concurrent modification detected, resolving automatically.
+    br: b4a6b8c586dd b
+    "###);
+
+    // Now we are stuck, export is broken. Forgetting the branch only forgets the
+    // local branch, and exporting does not work nor fix the git-tracking branches.
+    test_env.jj_cmd_success(&repo_path, &["branch", "forget", "br"]);
+    insta::assert_debug_snapshot!(get_git_repo_refs(&git_repo), @r###"
+    [
+        (
+            "refs/heads/br",
+            CommitId(
+                "b4a6b8c586ddf9e89264f164bbcf5c47219a5e5a",
+            ),
+        ),
+    ]
+    "###);
+    insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["git", "export"]), @"");
+    // Same as before
+    insta::assert_debug_snapshot!(get_git_repo_refs(&git_repo), @"[]");
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @"");
+
+    // import --reset removes the conflict
+    test_env.jj_cmd_success(&repo_path, &["git", "import", "--reset=br"]);
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @"");
+
+    // After import --reset, `branch forget` works properly
+    test_env.jj_cmd_success(&repo_path, &["branch", "forget", "br"]);
+    insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["git", "export"]), @"");
+    // Same as before
+    insta::assert_debug_snapshot!(get_git_repo_refs(&git_repo), @r###"
+    [
+        (
+            "refs/heads/a",
+            CommitId(
+                "230dd059e1b059aefc0da06a2e5a7dbf22362f22",
+            ),
+        ),
+    ]
+    "###);
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
+    a: 230dd059e1b0 (no description set)
+    "###);
+}
+
 fn get_branch_output(test_env: &TestEnvironment, repo_path: &Path) -> String {
     test_env.jj_cmd_success(repo_path, &["branch", "list"])
 }