git: preserve HEAD when possible

Closes: jj-vcs#2210
isinyaaa · Oct 29, 2023 · 89ee21e · 89ee21e
1 parent 19a658e
commit 89ee21e
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Showing 2 changed files with 23 additions and 2 deletions.
diff --git a/cli/tests/test_checkout.rs b/cli/tests/test_checkout.rs
@@ -158,6 +158,23 @@ fn test_checkout_conflicting_change_ids() {
     "###);
 }
 
+#[test]
+fn test_checkout_non_empty_working_copy() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(repo_path.join("one"), "x").unwrap();
+
+    test_env.jj_cmd_ok(&repo_path, &["describe", "-m", "one"]);
+    let (_, stderr) = test_env.jj_cmd_ok(&repo_path, &["co", "@-"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Working copy now at: kkmpptxz fcdbbd73 (empty) (no description set)
+    Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
+    Added 0 files, modified 0 files, removed 1 files
+    "###)
+}
+
 fn get_log_output(test_env: &TestEnvironment, cwd: &Path) -> String {
     let template = r#"commit_id ++ " " ++ description"#;
     test_env.jj_cmd_success(cwd, &["log", "-T", template])

diff --git a/lib/src/git.rs b/lib/src/git.rs
@@ -809,11 +809,15 @@ pub fn reset_head(
 ) -> Result<(), git2::Error> {
     let first_parent_id = &wc_commit.parent_ids()[0];
     if first_parent_id != mut_repo.store().root_commit_id() {
+        let first_parent = RefTarget::normal(first_parent_id.clone());
+        let git_head = mut_repo.view().git_head();
         let new_git_commit_id = Oid::from_bytes(first_parent_id.as_bytes()).unwrap();
         let new_git_commit = git_repo.find_commit(new_git_commit_id)?;
-        git_repo.set_head_detached(new_git_commit_id)?;
+        if git_head != &first_parent {
+            git_repo.set_head_detached(new_git_commit_id)?;
+            mut_repo.set_git_head_target(RefTarget::normal(first_parent_id.clone()));
+        }
         git_repo.reset(new_git_commit.as_object(), git2::ResetType::Mixed, None)?;
-        mut_repo.set_git_head_target(RefTarget::normal(first_parent_id.clone()));
     } else {
         // Can't detach HEAD without a commit. Use placeholder ref to nullify the HEAD.
         // We can't set_head() an arbitrary unborn ref, so use reference_symbolic()