git: allow conflicts in "HEAD@git"

Git's HEAD ref is similar to other refs and can logically have
conflicts just like the other refs in `git_refs`. As with the other
refs, it can happen if you run concurrent commands importing two
different updates from Git. So let's treat `git_head` the same as
`git_refs` by making it an `Option<RefTarget>`.

CHANGELOG.md

@@ -26,6 +26,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The `author`/`committer` templates now display both name and email. Use
   `author.name()`/`committer.name()` to extract the name.
 
+* Storage of the "HEAD@git" reference changed and can now have conflicts.
+  Operations written by a new `jj` binary will have a "HEAD@git" reference that
+  is not visible to older binaries.
+
 ### New features
 
 * The default log format now uses the committer timestamp instead of the author

lib/src/git.rs

@@ -75,7 +75,7 @@ pub fn import_refs(
         .flat_map(|old_target| old_target.adds())
         .collect_vec();
     if let Some(old_git_head) = mut_repo.view().git_head() {
-        old_git_heads.push(old_git_head);
+        old_git_heads.extend(old_git_head.adds());
     }
 
     let mut new_git_heads = HashSet::new();
@@ -90,7 +90,7 @@ pub fn import_refs(
         new_git_heads.insert(head_commit_id.clone());
         prevent_gc(git_repo, &head_commit_id);
         mut_repo.add_head(&head_commit);
-        mut_repo.set_git_head(head_commit_id);
+        mut_repo.set_git_head(RefTarget::Normal(head_commit_id));
     } else {
         mut_repo.clear_git_head();
     }

lib/src/legacy_thrift_op_store.rs

@@ -180,7 +180,7 @@ impl From<&simple_op_store_model::View> for View {
         view.git_head = thrift_view
             .git_head
             .as_ref()
-            .map(|head| CommitId::new(head.clone()));
+            .map(|head| RefTarget::Normal(CommitId::new(head.clone())));
 
         view
     }

lib/src/op_store.rs

@@ -199,7 +199,7 @@ content_hash! {
         /// The commit the Git HEAD points to.
         // TODO: Support multiple Git worktrees?
         // TODO: Do we want to store the current branch name too?
-        pub git_head: Option<CommitId>,
+        pub git_head: Option<RefTarget>,
         // The commit that *should be* checked out in the workspace. Note that the working copy
         // (.jj/working_copy/) has the source of truth about which commit *is* checked out (to be
         // precise: the commit to which we most recently completed an update to).

lib/src/proto_op_store.rs

@@ -224,7 +224,7 @@ fn view_to_proto(view: &View) -> crate::protos::op_store::View {
     }
 
     if let Some(git_head) = &view.git_head {
-        proto.git_head = git_head.to_bytes();
+        proto.git_head = Some(ref_target_to_proto(git_head));
     }
 
     proto
@@ -290,8 +290,11 @@ fn view_from_proto(proto: crate::protos::op_store::View) -> View {
         }
     }
 
-    if !proto.git_head.is_empty() {
-        view.git_head = Some(CommitId::new(proto.git_head));
+    #[allow(deprecated)]
+    if let Some(git_head) = proto.git_head.as_ref() {
+        view.git_head = Some(ref_target_from_proto(git_head.clone()));
+    } else if !proto.git_head_legacy.is_empty() {
+        view.git_head = Some(RefTarget::Normal(CommitId::new(proto.git_head_legacy)));
     }
 
     view

lib/src/protos/op_store.proto

@@ -67,7 +67,11 @@ message View {
   repeated Tag tags = 6;
   // Only a subset of the refs. For example, does not include refs/notes/.
   repeated GitRef git_refs = 3;
-  bytes git_head = 7;
+  // This field is just for historical reasons (before we had the RefTarget
+  // type). New Views have (only) the target field.
+  // TODO: Delete support for the old format.
+  bytes git_head_legacy = 7 [deprecated = true];
+  RefTarget git_head = 9;
 }
 
 message Operation {

lib/src/protos/op_store.rs

@@ -89,8 +89,14 @@ pub struct View {
     /// Only a subset of the refs. For example, does not include refs/notes/.
     #[prost(message, repeated, tag = "3")]
     pub git_refs: ::prost::alloc::vec::Vec<GitRef>,
+    /// This field is just for historical reasons (before we had the RefTarget
+    /// type). New Views have (only) the target field.
+    /// TODO: Delete support for the old format.
+    #[deprecated]
     #[prost(bytes = "vec", tag = "7")]
-    pub git_head: ::prost::alloc::vec::Vec<u8>,
+    pub git_head_legacy: ::prost::alloc::vec::Vec<u8>,
+    #[prost(message, optional, tag = "9")]
+    pub git_head: ::core::option::Option<RefTarget>,
 }
 #[allow(clippy::derive_partial_eq_without_eq)]
 #[derive(Clone, PartialEq, ::prost::Message)]

lib/src/repo.rs

@@ -980,8 +980,8 @@ impl MutableRepo {
         self.view_mut().remove_git_ref(name);
     }
 
-    pub fn set_git_head(&mut self, head_id: CommitId) {
-        self.view_mut().set_git_head(head_id);
+    pub fn set_git_head(&mut self, target: RefTarget) {
+        self.view_mut().set_git_head(target);
     }
 
     pub fn clear_git_head(&mut self) {

lib/src/revset.rs

@@ -2039,7 +2039,10 @@ pub fn evaluate_expression<'repo>(
             Ok(revset_for_commit_ids(repo, &commit_ids))
         }
         RevsetExpression::GitHead => {
-            let commit_ids = repo.view().git_head().into_iter().collect_vec();
+            let mut commit_ids = vec![];
+            if let Some(ref_target) = repo.view().git_head() {
+                commit_ids.extend(ref_target.adds());
+            }
             Ok(revset_for_commit_ids(repo, &commit_ids))
         }
         RevsetExpression::Filter(predicate) => Ok(Box::new(FilterRevset {

lib/src/simple_op_store.rs

@@ -204,7 +204,7 @@ mod tests {
                 "refs/heads/main".to_string() => git_refs_main_target,
                 "refs/heads/feature".to_string() => git_refs_feature_target
             },
-            git_head: Some(CommitId::from_hex("fff111")),
+            git_head: Some(RefTarget::Normal(CommitId::from_hex("fff111"))),
             wc_commit_ids: hashmap! {
                 WorkspaceId::default() => default_wc_commit_id,
                 WorkspaceId::new("test".to_string()) => test_wc_commit_id,
@@ -244,7 +244,7 @@ mod tests {
         // Test exact output so we detect regressions in compatibility
         assert_snapshot!(
             ViewId::new(blake2b_hash(&create_view()).to_vec()).hex(),
-            @"2a026b6a091219a3d8ca43d822984cf9be0c53438225d76a5ba5e6d3724fab15104579fb08fa949977c4357b1806d240bef28d958cbcd7d786962ac88c15df31"
+            @"7f47fa81494d7189cb1827b83b3f834662f0f61b4c4090298067e85cdc60f773bf639c4e6a3554a4e401650218ca240291ce591f45a1c501ade1d2b9f97e1a37"
         );
     }
 

lib/src/view.rs

@@ -84,8 +84,8 @@ impl View {
         &self.data.git_refs
     }
 
-    pub fn git_head(&self) -> Option<CommitId> {
-        self.data.git_head.clone()
+    pub fn git_head(&self) -> Option<&RefTarget> {
+        self.data.git_head.as_ref()
     }
 
     pub fn set_wc_commit(&mut self, workspace_id: WorkspaceId, commit_id: CommitId) {
@@ -244,8 +244,8 @@ impl View {
         self.data.git_refs.remove(name);
     }
 
-    pub fn set_git_head(&mut self, head_id: CommitId) {
-        self.data.git_head = Some(head_id);
+    pub fn set_git_head(&mut self, target: RefTarget) {
+        self.data.git_head = Some(target);
     }
 
     pub fn clear_git_head(&mut self) {

lib/tests/test_git.rs

@@ -161,7 +161,7 @@ fn test_import_refs() {
         view.git_refs().get("refs/tags/v1.0"),
         Some(RefTarget::Normal(jj_id(&commit5))).as_ref()
     );
-    assert_eq!(view.git_head(), Some(jj_id(&commit2)));
+    assert_eq!(view.git_head(), Some(&RefTarget::Normal(jj_id(&commit2))));
 }
 
 #[test]
@@ -432,7 +432,10 @@ fn test_import_refs_detached_head() {
     let expected_heads = hashset! { jj_id(&commit1) };
     assert_eq!(*repo.view().heads(), expected_heads);
     assert_eq!(repo.view().git_refs().len(), 0);
-    assert_eq!(repo.view().git_head(), Some(jj_id(&commit1)));
+    assert_eq!(
+        repo.view().git_head(),
+        Some(&RefTarget::Normal(jj_id(&commit1)))
+    );
 }
 
 #[test]

lib/tests/test_revset.rs

@@ -1324,7 +1324,7 @@ fn test_evaluate_expression_git_head(use_git: bool) {
         resolve_commit_ids(mut_repo.as_repo_ref(), "git_head()"),
         vec![]
     );
-    mut_repo.set_git_head(commit1.id().clone());
+    mut_repo.set_git_head(RefTarget::Normal(commit1.id().clone()));
     assert_eq!(
         resolve_commit_ids(mut_repo.as_repo_ref(), "git_head()"),
         vec![commit1.id().clone()]

lib/tests/test_view.rs

@@ -431,6 +431,41 @@ fn test_merge_views_git_refs() {
     );
 }
 
+#[test]
+fn test_merge_views_git_heads() {
+    // Tests merging of git heads (by performing concurrent operations). See
+    // test_refs.rs for tests of merging of individual ref targets.
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init(false);
+    let repo = &test_repo.repo;
+
+    let mut tx0 = repo.start_transaction(&settings, "test");
+    let tx0_head = write_random_commit(tx0.mut_repo(), &settings);
+    tx0.mut_repo()
+        .set_git_head(RefTarget::Normal(tx0_head.id().clone()));
+    let repo = tx0.commit();
+
+    let mut tx1 = repo.start_transaction(&settings, "test");
+    let tx1_head = write_random_commit(tx1.mut_repo(), &settings);
+    tx1.mut_repo()
+        .set_git_head(RefTarget::Normal(tx1_head.id().clone()));
+    tx1.commit();
+
+    let mut tx2 = repo.start_transaction(&settings, "test");
+    let tx2_head = write_random_commit(tx2.mut_repo(), &settings);
+    tx2.mut_repo()
+        .set_git_head(RefTarget::Normal(tx2_head.id().clone()));
+    tx2.commit();
+
+    let repo = repo.reload_at_head(&settings).unwrap();
+    let expected_git_head = RefTarget::Conflict {
+        removes: vec![tx0_head.id().clone()],
+        adds: vec![tx1_head.id().clone(), tx2_head.id().clone()],
+    };
+    // TODO: Should be equal
+    assert_ne!(repo.view().git_head(), Some(&expected_git_head));
+}
+
 fn commit_transactions(settings: &UserSettings, txs: Vec<Transaction>) -> Arc<ReadonlyRepo> {
     let repo_loader = txs[0].base_repo().loader();
     let mut op_ids = vec![];

src/cli_util.rs

@@ -33,7 +33,7 @@ use jujutsu_lib::git::{GitExportError, GitImportError};
 use jujutsu_lib::gitignore::GitIgnoreFile;
 use jujutsu_lib::matchers::{EverythingMatcher, Matcher, PrefixMatcher, Visit};
 use jujutsu_lib::op_heads_store::{self, OpHeadResolutionError, OpHeadsStore};
-use jujutsu_lib::op_store::{OpStore, OpStoreError, OperationId, WorkspaceId};
+use jujutsu_lib::op_store::{OpStore, OpStoreError, OperationId, RefTarget, WorkspaceId};
 use jujutsu_lib::operation::Operation;
 use jujutsu_lib::repo::{
     CheckOutCommitError, EditCommitError, MutableRepo, ReadonlyRepo, RepoLoader, RepoRef,
@@ -507,40 +507,42 @@ impl WorkspaceCommandHelper {
         let mut tx = self.start_transaction("import git refs").into_inner();
         git::import_refs(tx.mut_repo(), git_repo, &self.settings.git_settings())?;
         if tx.mut_repo().has_changes() {
-            let old_git_head = self.repo.view().git_head();
-            let new_git_head = tx.mut_repo().view().git_head();
+            let old_git_head = self.repo.view().git_head().cloned();
+            let new_git_head = tx.mut_repo().view().git_head().cloned();
             // If the Git HEAD has changed, abandon our old checkout and check out the new
             // Git HEAD.
-            if new_git_head != old_git_head && new_git_head.is_some() {
-                let workspace_id = self.workspace_id().to_owned();
-                let mut locked_working_copy = self.workspace.working_copy_mut().start_mutation();
-                if let Some(old_wc_commit_id) = self.repo.view().get_wc_commit_id(&workspace_id) {
+            match new_git_head {
+                Some(RefTarget::Normal(new_git_head_id)) if new_git_head != old_git_head => {
+                    let workspace_id = self.workspace_id().to_owned();
+                    let mut locked_working_copy =
+                        self.workspace.working_copy_mut().start_mutation();
+                    if let Some(old_wc_commit_id) = self.repo.view().get_wc_commit_id(&workspace_id)
+                    {
+                        tx.mut_repo()
+                            .record_abandoned_commit(old_wc_commit_id.clone());
+                    }
+                    let new_git_head_commit = tx.mut_repo().store().get_commit(&new_git_head_id)?;
                     tx.mut_repo()
-                        .record_abandoned_commit(old_wc_commit_id.clone());
+                        .check_out(workspace_id, &self.settings, &new_git_head_commit)?;
+                    // The working copy was presumably updated by the git command that updated
+                    // HEAD, so we just need to reset our working copy
+                    // state to it without updating working copy files.
+                    locked_working_copy.reset(&new_git_head_commit.tree())?;
+                    tx.mut_repo().rebase_descendants(&self.settings)?;
+                    self.repo = tx.commit();
+                    locked_working_copy.finish(self.repo.op_id().clone());
                 }
-                let new_checkout = self
-                    .repo
-                    .store()
-                    .get_commit(new_git_head.as_ref().unwrap())?;
-                tx.mut_repo()
-                    .check_out(workspace_id, &self.settings, &new_checkout)?;
-                // The working copy was presumably updated by the git command that updated HEAD,
-                // so we just need to reset our working copy state to it without updating
-                // working copy files.
-                locked_working_copy.reset(&new_checkout.tree())?;
-                tx.mut_repo().rebase_descendants(&self.settings)?;
-                self.repo = tx.commit();
-                locked_working_copy.finish(self.repo.op_id().clone());
-            } else {
-                let num_rebased = tx.mut_repo().rebase_descendants(&self.settings)?;
-                if num_rebased > 0 {
-                    writeln!(
-                        ui,
-                        "Rebased {num_rebased} descendant commits off of commits rewritten from \
-                         git"
-                    )?;
+                _ => {
+                    let num_rebased = tx.mut_repo().rebase_descendants(&self.settings)?;
+                    if num_rebased > 0 {
+                        writeln!(
+                            ui,
+                            "Rebased {num_rebased} descendant commits off of commits rewritten \
+                             from git"
+                        )?;
+                    }
+                    self.finish_transaction(ui, tx)?;
                 }
-                self.finish_transaction(ui, tx)?;
             }
         }
         Ok(())
@@ -567,7 +569,7 @@ impl WorkspaceCommandHelper {
                 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.reset(new_git_commit.as_object(), git2::ResetType::Mixed, None)?;
-                mut_repo.set_git_head(first_parent_id);
+                mut_repo.set_git_head(RefTarget::Normal(first_parent_id));
             }
         } else {
             // The workspace was removed (maybe the user undid the

src/commands/mod.rs

@@ -32,7 +32,7 @@ use jujutsu_lib::commit::Commit;
 use jujutsu_lib::dag_walk::topo_order_reverse;
 use jujutsu_lib::index::IndexEntry;
 use jujutsu_lib::matchers::EverythingMatcher;
-use jujutsu_lib::op_store::WorkspaceId;
+use jujutsu_lib::op_store::{RefTarget, WorkspaceId};
 use jujutsu_lib::repo::ReadonlyRepo;
 use jujutsu_lib::repo_path::RepoPath;
 use jujutsu_lib::revset::{RevsetAliasesMap, RevsetExpression};
@@ -985,7 +985,7 @@ fn cmd_init(ui: &mut Ui, command: &CommandHelper, args: &InitArgs) -> Result<(),
                 &git_repo,
                 &command.settings().git_settings(),
             )?;
-            if let Some(git_head_id) = tx.mut_repo().view().git_head() {
+            if let Some(RefTarget::Normal(git_head_id)) = tx.mut_repo().view().git_head().cloned() {
                 let git_head_commit = tx.mut_repo().store().get_commit(&git_head_id)?;
                 tx.check_out(&git_head_commit)?;
             }

src/templater.rs

@@ -362,10 +362,15 @@ impl TemplateProperty<Commit> for GitHeadProperty<'_> {
     type Output = String;
 
     fn extract(&self, context: &Commit) -> String {
-        if self.repo.view().git_head().as_ref() == Some(context.id()) {
-            "HEAD@git".to_string()
-        } else {
-            "".to_string()
+        match self.repo.view().git_head() {
+            Some(ref_target) if ref_target.has_add(context.id()) => {
+                if ref_target.is_conflict() {
+                    "HEAD@git?".to_string()
+                } else {
+                    "HEAD@git".to_string()
+                }
+            }
+            _ => "".to_string(),
         }
     }
 }