git: add hack to unset Git HEAD by using random ref name

I considered using the working-copy commit id to generate a unique ref, but the
placeholder ref can be "born" by committing changes by git, and the ref name can
conflict later by editing the original (hidden) commit. So I think it's better
to generate a random name instead.

Fixes jj-vcs#1495

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 somehow 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

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 namespace used as a placeholder to unset HEAD without a commit.
+const UNBORN_REF_NAMESPACE: &str = "refs/jj/unborn/";
 
 #[derive(Error, Debug)]
 pub enum GitImportError {
@@ -690,6 +692,22 @@ 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 random 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() {
+            let random_bytes: [u8; 16] = rand::random();
+            let name = format!("{UNBORN_REF_NAMESPACE}{}", hex::encode(random_bytes));
+            git_repo.reference_symbolic("HEAD", &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(())
 }