git: on import, add GC-preventing refs to all seen refs

To prevent git's GC from breaking a repo, we already add a git ref to
commits we create in the git backend. However, we don't add refs to
commits we import from git. This fixes that.

Closes #815.

CHANGELOG.md

@@ -105,6 +105,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   later removed if you checked out another commit. You can now use `git` to
   populate the submodule directory and `jj` will leave it alone.
 
+* Git's GC could remove commits that were referenced from jj in some cases. We
+  are now better at adding Git refs to prevent that.
+  [#815](https://github.com/martinvonz/jj/issues/815)
+
 ### Contributors
 
 Thanks to the people who made this release happen!

lib/src/git.rs

@@ -22,6 +22,7 @@ use thiserror::Error;
 
 use crate::backend::CommitId;
 use crate::commit::Commit;
+use crate::git_backend::NO_GC_REF_NAMESPACE;
 use crate::op_store;
 use crate::op_store::RefTarget;
 use crate::repo::MutableRepo;
@@ -50,6 +51,17 @@ fn parse_git_ref(ref_name: &str) -> Option<RefName> {
     }
 }
 
+fn prevent_gc(git_repo: &git2::Repository, id: &CommitId) {
+    git_repo
+        .reference(
+            &format!("{}{}", NO_GC_REF_NAMESPACE, id.hex()),
+            Oid::from_bytes(id.as_bytes()).unwrap(),
+            true,
+            "used by jj",
+        )
+        .unwrap();
+}
+
 /// Reflect changes made in the underlying Git repo in the Jujutsu repo.
 pub fn import_refs(
     mut_repo: &mut MutableRepo,
@@ -75,6 +87,7 @@ pub fn import_refs(
         let head_commit_id = CommitId::from_bytes(head_git_commit.id().as_bytes());
         let head_commit = store.get_commit(&head_commit_id).unwrap();
         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);
     } else {
@@ -112,6 +125,7 @@ pub fn import_refs(
         let old_target = existing_git_refs.remove(&full_name);
         let new_target = Some(RefTarget::Normal(id.clone()));
         if new_target != old_target {
+            prevent_gc(git_repo, &id);
             mut_repo.set_git_ref(full_name.clone(), RefTarget::Normal(id.clone()));
             let commit = store.get_commit(&id).unwrap();
             mut_repo.add_head(&commit);

lib/src/git_backend.rs

@@ -33,7 +33,7 @@ use crate::stacked_table::{TableSegment, TableStore};
 
 const HASH_LENGTH: usize = 20;
 /// Ref namespace used only for preventing GC.
-const NO_GC_REF_NAMESPACE: &str = "refs/jj/keep/";
+pub const NO_GC_REF_NAMESPACE: &str = "refs/jj/keep/";
 const CONFLICT_SUFFIX: &str = ".jjconflict";
 
 impl From<git2::Error> for BackendError {