git: create no-gc ref by GitBackend

Since we now have an explicit method to import heads, it makes more sense to
manage no-gc refs by the backend.

lib/src/git.rs

@@ -26,7 +26,7 @@ use tempfile::NamedTempFile;
 use thiserror::Error;
 
 use crate::backend::{BackendError, CommitId, ObjectId};
-use crate::git_backend::{GitBackend, NO_GC_REF_NAMESPACE};
+use crate::git_backend::GitBackend;
 use crate::op_store::{BranchTarget, RefTarget, RefTargetOptionExt};
 use crate::repo::{MutableRepo, Repo};
 use crate::revset;
@@ -172,18 +172,6 @@ pub fn get_local_git_tracking_branch<'a>(view: &'a View, branch: &str) -> &'a Re
     view.get_git_ref(&format!("refs/heads/{branch}"))
 }
 
-fn prevent_gc(git_repo: &git2::Repository, id: &CommitId) -> Result<(), git2::Error> {
-    // If multiple processes do git::import_refs() in parallel, this can fail to
-    // acquire a lock file even with force=true.
-    git_repo.reference(
-        &format!("{}{}", NO_GC_REF_NAMESPACE, id.hex()),
-        Oid::from_bytes(id.as_bytes()).unwrap(),
-        true,
-        "used by jj",
-    )?;
-    Ok(())
-}
-
 /// Reflect changes made in the underlying Git repo in the Jujutsu repo.
 ///
 /// This function detects conflicts (if both Git and JJ modified a branch) and
@@ -255,9 +243,6 @@ pub fn import_some_refs(
             head_commits.push(commit);
         }
     }
-    for commit in &head_commits {
-        prevent_gc(git_repo, commit.id())?;
-    }
     mut_repo.add_heads(&head_commits);
 
     // Apply the change that happened in git since last time we imported refs.

lib/src/git_backend.rs

@@ -43,7 +43,7 @@ use crate::stacked_table::{
 const HASH_LENGTH: usize = 20;
 const CHANGE_ID_LENGTH: usize = 16;
 /// Ref namespace used only for preventing GC.
-pub const NO_GC_REF_NAMESPACE: &str = "refs/jj/keep/";
+const NO_GC_REF_NAMESPACE: &str = "refs/jj/keep/";
 const CONFLICT_SUFFIX: &str = ".jjconflict";
 
 #[derive(Debug, Error)]
@@ -233,6 +233,9 @@ impl GitBackend {
             &table_lock,
             &missing_head_ids,
         )?;
+        for &id in &missing_head_ids {
+            prevent_gc(&locked_repo, id)?;
+        }
         self.save_extra_metadata_table(mut_table, &table_lock)
     }
 }
@@ -371,6 +374,18 @@ fn create_no_gc_ref() -> String {
     format!("{NO_GC_REF_NAMESPACE}{}", hex::encode(random_bytes))
 }
 
+fn prevent_gc(git_repo: &git2::Repository, id: &CommitId) -> Result<(), BackendError> {
+    git_repo
+        .reference(
+            &format!("{NO_GC_REF_NAMESPACE}{}", id.hex()),
+            Oid::from_bytes(id.as_bytes()).unwrap(),
+            true,
+            "used by jj",
+        )
+        .map_err(|err| BackendError::Other(Box::new(err)))?;
+    Ok(())
+}
+
 fn validate_git_object_id(id: &impl ObjectId) -> Result<git2::Oid, BackendError> {
     if id.as_bytes().len() != HASH_LENGTH {
         return Err(BackendError::InvalidHashLength {
@@ -943,6 +958,14 @@ mod tests {
 
         // Import the head commit and its ancestors
         store.import_head_commits([&commit_id2]).unwrap();
+        // Ref should be created only for the head commit
+        let git_refs = store
+            .git_repo()
+            .references_glob("refs/jj/keep/*")
+            .unwrap()
+            .map(|git_ref| git_ref.unwrap().target().unwrap())
+            .collect_vec();
+        assert_eq!(git_refs, vec![git_commit_id2]);
 
         let commit = store.read_commit(&commit_id).unwrap();
         assert_eq!(&commit.change_id, &change_id);