git: add hack to unset Git HEAD by using placeholder ref

As we can set HEAD to an arbitrary ref by using .reference_symbolic(), we don't have to manage a ref that can also be valid as a branch name. Fixes jj-vcs#1495
yuja · Oct 4, 2023 · 0c7e0f6 · 0c7e0f6
1 parent 7cfeb72
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diff --git a/cli/tests/test_git_colocated.rs b/cli/tests/test_git_colocated.rs
@@ -95,6 +95,138 @@ fn test_git_colocated() {
     );
 }
 
+#[test]
+fn test_git_colocated_unborn_branch() {
+    let test_env = TestEnvironment::default();
+    let workspace_root = test_env.env_root().join("repo");
+    let git_repo = git2::Repository::init(&workspace_root).unwrap();
+
+    let add_file_to_index = |name: &str, data: &str| {
+        std::fs::write(workspace_root.join(name), data).unwrap();
+        let mut index = git_repo.index().unwrap();
+        index.add_path(Path::new(name)).unwrap();
+        index.write().unwrap();
+    };
+    let checkout_index = || {
+        let mut index = git_repo.index().unwrap();
+        index.read(true).unwrap(); // discard in-memory cache
+        git_repo.checkout_index(Some(&mut index), None).unwrap();
+    };
+
+    // Initially, HEAD isn't set.
+    test_env.jj_cmd_success(&workspace_root, &["init", "--git-repo", "."]);
+    assert!(git_repo.head().is_err());
+    assert_eq!(
+        git_repo.find_reference("HEAD").unwrap().symbolic_target(),
+        Some("refs/heads/master")
+    );
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_root), @r###"
+    @  230dd059e1b059aefc0da06a2e5a7dbf22362f22
+    ◉  0000000000000000000000000000000000000000
+    "###);
+
+    // Stage some change, and check out root. This shouldn't clobber the HEAD.
+    add_file_to_index("file0", "");
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&workspace_root, &["checkout", "root()"]), @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
+    "###);
+    assert!(git_repo.head().is_err());
+    assert_eq!(
+        git_repo.find_reference("HEAD").unwrap().symbolic_target(),
+        Some("refs/heads/master")
+    );
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_root), @r###"
+    @  fcdbbd731496cae17161cd6be9b6cf1f759655a8
+    │ ◉  1de814dbef9641cc6c5c80d2689b80778edcce09
+    ├─╯
+    ◉  0000000000000000000000000000000000000000
+    "###);
+    // Staged change shouldn't persist.
+    checkout_index();
+    insta::assert_snapshot!(test_env.jj_cmd_success(&workspace_root, &["status"]), @r###"
+    The working copy is clean
+    Working copy : kkmpptxz fcdbbd73 (empty) (no description set)
+    Parent commit: zzzzzzzz 00000000 (empty) (no description set)
+    "###);
+
+    // Stage some change, and create new HEAD. This shouldn't move the default
+    // branch.
+    add_file_to_index("file1", "");
+    insta::assert_snapshot!(test_env.jj_cmd_success(&workspace_root, &["new"]), @r###"
+    Working copy now at: royxmykx 76c60bf0 (empty) (no description set)
+    Parent commit      : kkmpptxz f8d5bc77 (no description set)
+    "###);
+    assert!(git_repo.head().unwrap().symbolic_target().is_none());
+    insta::assert_snapshot!(
+        git_repo.head().unwrap().peel_to_commit().unwrap().id().to_string(),
+        @"f8d5bc772d1147351fd6e8cea52a4f935d3b31e7"
+    );
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_root), @r###"
+    @  76c60bf0a66dcbe74d74d58c23848d96f9e86e84
+    ◉  f8d5bc772d1147351fd6e8cea52a4f935d3b31e7 HEAD@git
+    │ ◉  1de814dbef9641cc6c5c80d2689b80778edcce09
+    ├─╯
+    ◉  0000000000000000000000000000000000000000
+    "###);
+    // Staged change shouldn't persist.
+    checkout_index();
+    insta::assert_snapshot!(test_env.jj_cmd_success(&workspace_root, &["status"]), @r###"
+    The working copy is clean
+    Working copy : royxmykx 76c60bf0 (empty) (no description set)
+    Parent commit: kkmpptxz f8d5bc77 (no description set)
+    "###);
+
+    // Assign the default branch. The branch is no longer "unborn".
+    test_env.jj_cmd_success(&workspace_root, &["branch", "set", "-r@-", "master"]);
+
+    // Stage some change, and check out root again. This should unset the HEAD.
+    // https://github.com/martinvonz/jj/issues/1495
+    add_file_to_index("file2", "");
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&workspace_root, &["checkout", "root()"]), @r###"
+    Working copy now at: znkkpsqq 10dd328b (empty) (no description set)
+    Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
+    Added 0 files, modified 0 files, removed 2 files
+    "###);
+    assert!(git_repo.head().is_err());
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_root), @r###"
+    @  10dd328bb906e15890e55047740eab2812a3b2f7
+    │ ◉  2c576a57d2e6e8494616629cfdbb8fe5e3fea73b
+    │ ◉  f8d5bc772d1147351fd6e8cea52a4f935d3b31e7 master
+    ├─╯
+    │ ◉  1de814dbef9641cc6c5c80d2689b80778edcce09
+    ├─╯
+    ◉  0000000000000000000000000000000000000000
+    "###);
+    // Staged change shouldn't persist.
+    checkout_index();
+    insta::assert_snapshot!(test_env.jj_cmd_success(&workspace_root, &["status"]), @r###"
+    The working copy is clean
+    Working copy : znkkpsqq 10dd328b (empty) (no description set)
+    Parent commit: zzzzzzzz 00000000 (empty) (no description set)
+    "###);
+
+    // New snapshot and commit can be created after the HEAD got unset.
+    std::fs::write(workspace_root.join("file3"), "").unwrap();
+    insta::assert_snapshot!(test_env.jj_cmd_success(&workspace_root, &["new"]), @r###"
+    Working copy now at: wqnwkozp cab23370 (empty) (no description set)
+    Parent commit      : znkkpsqq 8f5b2638 (no description set)
+    "###);
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_root), @r###"
+    @  cab233704a5c0b21bde070943055f22142fb2043
+    ◉  8f5b263819457712a2937428b9c58a2a84afbb1c HEAD@git
+    │ ◉  2c576a57d2e6e8494616629cfdbb8fe5e3fea73b
+    │ ◉  f8d5bc772d1147351fd6e8cea52a4f935d3b31e7 master
+    ├─╯
+    │ ◉  1de814dbef9641cc6c5c80d2689b80778edcce09
+    ├─╯
+    ◉  0000000000000000000000000000000000000000
+    "###);
+}
+
 #[test]
 fn test_git_colocated_export_branches_on_snapshot() {
     // Checks that we export branches that were changed only because the working

diff --git a/lib/src/git.rs b/lib/src/git.rs
@@ -36,6 +36,8 @@ use crate::view::{RefName, View};
 
 /// Reserved remote name for the backing Git repo.
 pub const REMOTE_NAME_FOR_LOCAL_GIT_REPO: &str = "git";
+/// Ref name used as a placeholder to unset HEAD without a commit.
+const UNBORN_ROOT_REF_NAME: &str = "refs/jj/root";
 
 #[derive(Error, Debug)]
 pub enum GitImportError {
@@ -449,7 +451,6 @@ pub fn export_refs(
 ) -> Result<Vec<FailedRefExport>, GitExportError> {
     export_some_refs(mut_repo, git_repo, |_| true)
 }
-
 pub fn export_some_refs(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,
@@ -690,6 +691,25 @@ pub fn reset_head(
         git_repo.set_head_detached(new_git_commit_id)?;
         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()
+        // instead. Git CLI appears to deal with that. It would be nice if Git CLI
+        // couldn't create a commit without setting a valid branch name.
+        if mut_repo.git_head().is_present() {
+            match git_repo.find_reference(UNBORN_ROOT_REF_NAME) {
+                Ok(mut git_repo_ref) => git_repo_ref.delete()?,
+                Err(err) if err.code() == git2::ErrorCode::NotFound => {}
+                Err(err) => return Err(err),
+            }
+            git_repo.reference_symbolic("HEAD", UNBORN_ROOT_REF_NAME, true, "unset HEAD by jj")?;
+        }
+        // git_reset() of libgit2 requires a commit object. Do that manually.
+        let mut index = git_repo.index()?;
+        index.clear()?; // or read empty tree
+        index.write()?;
+        git_repo.cleanup_state()?;
+        mut_repo.set_git_head_target(RefTarget::absent());
     }
     Ok(())
 }

diff --git a/lib/tests/test_git.rs b/lib/tests/test_git.rs
@@ -35,6 +35,7 @@ use jj_lib::op_store::{BranchTarget, RefTarget};
 use jj_lib::repo::{MutableRepo, ReadonlyRepo, Repo};
 use jj_lib::settings::{GitSettings, UserSettings};
 use jj_lib::view::RefName;
+use jj_lib::workspace::Workspace;
 use maplit::{btreemap, hashset};
 use tempfile::TempDir;
 use testutils::{
@@ -1572,6 +1573,64 @@ fn test_export_reexport_transitions() {
     );
 }
 
+#[test]
+fn test_reset_head_to_root() {
+    // Create colocated workspace
+    let settings = testutils::user_settings();
+    let temp_dir = testutils::new_temp_dir();
+    let workspace_root = temp_dir.path().join("repo");
+    let git_repo = git2::Repository::init(&workspace_root).unwrap();
+    let (_workspace, repo) =
+        Workspace::init_external_git(&settings, &workspace_root, &workspace_root.join(".git"))
+            .unwrap();
+
+    let mut tx = repo.start_transaction(&settings, "test");
+    let mut_repo = tx.mut_repo();
+
+    let root_commit_id = repo.store().root_commit_id();
+    let tree_id = repo.store().empty_merged_tree_id();
+    let commit1 = mut_repo
+        .new_commit(&settings, vec![root_commit_id.clone()], tree_id.clone())
+        .write()
+        .unwrap();
+    let commit2 = mut_repo
+        .new_commit(&settings, vec![commit1.id().clone()], tree_id.clone())
+        .write()
+        .unwrap();
+
+    // Set Git HEAD to commit2's parent (i.e. commit1)
+    git::reset_head(tx.mut_repo(), &git_repo, &commit2).unwrap();
+    assert!(git_repo.head().is_ok());
+    assert_eq!(
+        tx.mut_repo().git_head(),
+        RefTarget::normal(commit1.id().clone())
+    );
+
+    // Set Git HEAD back to root
+    git::reset_head(tx.mut_repo(), &git_repo, &commit1).unwrap();
+    assert!(git_repo.head().is_err());
+    assert!(tx.mut_repo().git_head().is_absent());
+
+    // Move placeholder ref as if new commit were created by git
+    git_repo
+        .reference("refs/jj/root", git_id(&commit1), false, "")
+        .unwrap();
+    git::reset_head(tx.mut_repo(), &git_repo, &commit2).unwrap();
+    assert!(git_repo.head().is_ok());
+    assert_eq!(
+        tx.mut_repo().git_head(),
+        RefTarget::normal(commit1.id().clone())
+    );
+    assert!(git_repo.find_reference("refs/jj/root").is_ok());
+
+    // Set Git HEAD back to root
+    git::reset_head(tx.mut_repo(), &git_repo, &commit1).unwrap();
+    assert!(git_repo.head().is_err());
+    assert!(tx.mut_repo().git_head().is_absent());
+    // The placeholder ref should be deleted
+    assert!(git_repo.find_reference("refs/jj/root").is_err());
+}
+
 #[test]
 fn test_init() {
     let settings = testutils::user_settings();