shallow: Support shallow git repositories

cli/src/command_error.rs

@@ -438,20 +438,11 @@ impl From<git2::Error> for CommandError {
 impl From<GitImportError> for CommandError {
     fn from(err: GitImportError) -> Self {
         let hint = match &err {
-            GitImportError::MissingHeadTarget { .. }
-            | GitImportError::MissingRefAncestor { .. } => Some(
-                "\
-Is this Git repository a shallow or partial clone (cloned with the --depth or --filter \
-                 argument)?
-jj currently does not support shallow/partial clones. To use jj with this \
-                 repository, try
-unshallowing the repository (https://stackoverflow.com/q/6802145) or re-cloning with the full
-repository contents."
-                    .to_string(),
-            ),
             GitImportError::RemoteReservedForLocalGitRepo => {
                 Some("Run `jj git remote rename` to give different name.".to_string())
             }
+            GitImportError::MissingHeadTarget { .. } => None,
+            GitImportError::MissingRefAncestor { .. } => None,
             GitImportError::InternalBackend(_) => None,
             GitImportError::InternalGitError(_) => None,
             GitImportError::UnexpectedBackend => None,

lib/src/git_backend.rs

@@ -519,6 +519,7 @@ fn commit_from_git_without_root_parent(
     id: &CommitId,
     git_object: &gix::Object,
     uses_tree_conflict_format: bool,
+    is_shallow: bool,
 ) -> BackendResult<Commit> {
     let commit = git_object
         .try_to_commit_ref()
@@ -537,10 +538,15 @@ fn commit_from_git_without_root_parent(
             .map(|b| b.reverse_bits())
             .collect(),
     );
-    let parents = commit
-        .parents()
-        .map(|oid| CommitId::from_bytes(oid.as_bytes()))
-        .collect_vec();
+    // shallow commits don't have parents their parents actually fetched, so we discard them here
+    let parents = if is_shallow {
+        vec![]
+    } else {
+        commit
+            .parents()
+            .map(|oid| CommitId::from_bytes(oid.as_bytes()))
+            .collect_vec()
+    };
     let tree_id = TreeId::from_bytes(commit.tree().as_bytes());
     // If this commit is a conflict, we'll update the root tree later, when we read
     // the extra metadata.
@@ -859,6 +865,8 @@ fn import_extra_metadata_entries_from_heads(
     head_ids: &HashSet<&CommitId>,
     uses_tree_conflict_format: bool,
 ) -> BackendResult<()> {
+    let shallow_commits = git_repo.shallow_commits().ok().flatten();
+
     let mut work_ids = head_ids
         .iter()
         .filter(|&id| mut_table.get_value(id.as_bytes()).is_none())
@@ -868,11 +876,18 @@ fn import_extra_metadata_entries_from_heads(
         let git_object = git_repo
             .find_object(validate_git_object_id(&id)?)
             .map_err(|err| map_not_found_err(err, &id))?;
+        let is_shallow = shallow_commits
+            .as_ref()
+            .is_some_and(|shallow| shallow.contains(&git_object.id));
         // TODO(#1624): Should we read the root tree here and check if it has a
         // `.jjconflict-...` entries? That could happen if the user used `git` to e.g.
         // change the description of a commit with tree-level conflicts.
-        let commit =
-            commit_from_git_without_root_parent(&id, &git_object, uses_tree_conflict_format)?;
+        let commit = commit_from_git_without_root_parent(
+            &id,
+            &git_object,
+            uses_tree_conflict_format,
+            is_shallow,
+        )?;
         mut_table.add_entry(id.to_bytes(), serialize_extras(&commit));
         work_ids.extend(
             commit
@@ -1142,7 +1157,12 @@ impl Backend for GitBackend {
             let git_object = locked_repo
                 .find_object(git_commit_id)
                 .map_err(|err| map_not_found_err(err, id))?;
-            commit_from_git_without_root_parent(id, &git_object, false)?
+            let is_shallow = locked_repo
+                .shallow_commits()
+                .ok()
+                .flatten()
+                .is_some_and(|shallow| shallow.contains(&git_object.id));
+            commit_from_git_without_root_parent(id, &git_object, false, is_shallow)?
         };
         if commit.parents.is_empty() {
             commit.parents.push(self.root_commit_id.clone());
@@ -2151,8 +2171,8 @@ mod tests {
         author Someone <[email protected]> 0 +0000
         committer Someone <[email protected]> 0 +0000
         gpgsig test sig
-         
-         
+
+
          hash=9ad9526c3b2103c41a229f2f3c82d107a0ecd902f476a855f0e1dd5f7bef1430663de12749b73e293a877113895a8a2a0f29da4bbc5a5f9a19c3523fb0e53518
 
         initial