backend: let backend choose root change id

Our internal backend at Google uses a 32-byte change id, so I'd like
to make the backend able to decide the length. To start with, let's
make the backend able to decide what the root change id should
be. That's consistent with how we already let the backend decide what
the root commit id should be.

examples/custom-backend/main.rs

@@ -19,7 +19,8 @@ use git2::Repository;
 use jujutsu::cli_util::{CliRunner, CommandError, CommandHelper};
 use jujutsu::ui::Ui;
 use jujutsu_lib::backend::{
-    Backend, BackendResult, Commit, CommitId, Conflict, ConflictId, FileId, SymlinkId, Tree, TreeId,
+    Backend, BackendResult, ChangeId, Commit, CommitId, Conflict, ConflictId, FileId, SymlinkId,
+    Tree, TreeId,
 };
 use jujutsu_lib::git_backend::GitBackend;
 use jujutsu_lib::repo::StoreFactories;
@@ -120,6 +121,10 @@ impl Backend for JitBackend {
         self.inner.root_commit_id()
     }
 
+    fn root_change_id(&self) -> &ChangeId {
+        self.inner.root_change_id()
+    }
+
     fn empty_tree_id(&self) -> &TreeId {
         self.inner.empty_tree_id()
     }

lib/src/backend.rs

@@ -18,7 +18,6 @@ use std::io::Read;
 use std::result::Result;
 use std::vec::Vec;
 
-use once_cell::sync::Lazy;
 use thiserror::Error;
 
 use crate::content_hash::ContentHash;
@@ -350,13 +349,7 @@ fn iter_half_bytes(bytes: &[u8]) -> impl ExactSizeIterator<Item = u8> + '_ {
     })
 }
 
-pub fn root_change_id() -> &'static ChangeId {
-    static ROOT_CHANGE_ID: Lazy<ChangeId> =
-        Lazy::new(|| ChangeId::new(vec![0; CHANGE_ID_HASH_LENGTH]));
-    &ROOT_CHANGE_ID
-}
-
-pub fn make_root_commit(empty_tree_id: TreeId) -> Commit {
+pub fn make_root_commit(root_change_id: ChangeId, empty_tree_id: TreeId) -> Commit {
     let timestamp = Timestamp {
         timestamp: MillisSinceEpoch(0),
         tz_offset: 0,
@@ -366,12 +359,11 @@ pub fn make_root_commit(empty_tree_id: TreeId) -> Commit {
         email: String::new(),
         timestamp,
     };
-    let change_id = root_change_id().to_owned();
     Commit {
         parents: vec![],
         predecessors: vec![],
         root_tree: empty_tree_id,
-        change_id,
+        change_id: root_change_id,
         description: String::new(),
         author: signature.clone(),
         committer: signature,
@@ -398,6 +390,8 @@ pub trait Backend: Send + Sync + Debug {
 
     fn root_commit_id(&self) -> &CommitId;
 
+    fn root_change_id(&self) -> &ChangeId;
+
     fn empty_tree_id(&self) -> &TreeId;
 
     fn read_tree(&self, path: &RepoPath, id: &TreeId) -> BackendResult<Tree>;

lib/src/git_backend.rs

@@ -25,7 +25,7 @@ use prost::Message;
 use crate::backend::{
     make_root_commit, Backend, BackendError, BackendResult, ChangeId, Commit, CommitId, Conflict,
     ConflictId, ConflictPart, FileId, MillisSinceEpoch, ObjectId, Signature, SymlinkId, Timestamp,
-    Tree, TreeId, TreeValue,
+    Tree, TreeId, TreeValue, CHANGE_ID_HASH_LENGTH,
 };
 use crate::repo_path::{RepoPath, RepoPathComponent};
 use crate::stacked_table::{ReadonlyTable, TableSegment, TableStore};
@@ -38,6 +38,7 @@ const CONFLICT_SUFFIX: &str = ".jjconflict";
 pub struct GitBackend {
     repo: Mutex<git2::Repository>,
     root_commit_id: CommitId,
+    root_change_id: ChangeId,
     empty_tree_id: TreeId,
     extra_metadata_store: TableStore,
     cached_extra_metadata: Mutex<Option<Arc<ReadonlyTable>>>,
@@ -46,10 +47,12 @@ pub struct GitBackend {
 impl GitBackend {
     fn new(repo: git2::Repository, extra_metadata_store: TableStore) -> Self {
         let root_commit_id = CommitId::from_bytes(&[0; HASH_LENGTH]);
+        let root_change_id = ChangeId::from_bytes(&[0; CHANGE_ID_HASH_LENGTH]);
         let empty_tree_id = TreeId::from_hex("4b825dc642cb6eb9a060e54bf8d69288fbee4904");
         GitBackend {
             repo: Mutex::new(repo),
             root_commit_id,
+            root_change_id,
             empty_tree_id,
             extra_metadata_store,
             cached_extra_metadata: Mutex::new(None),
@@ -250,6 +253,10 @@ impl Backend for GitBackend {
         &self.root_commit_id
     }
 
+    fn root_change_id(&self) -> &ChangeId {
+        &self.root_change_id
+    }
+
     fn empty_tree_id(&self) -> &TreeId {
         &self.empty_tree_id
     }
@@ -382,7 +389,10 @@ impl Backend for GitBackend {
 
     fn read_commit(&self, id: &CommitId) -> BackendResult<Commit> {
         if *id == self.root_commit_id {
-            return Ok(make_root_commit(self.empty_tree_id.clone()));
+            return Ok(make_root_commit(
+                self.root_change_id().clone(),
+                self.empty_tree_id.clone(),
+            ));
         }
         let git_commit_id = validate_git_object_id(id)?;
 

lib/src/local_backend.rs

@@ -25,7 +25,7 @@ use tempfile::{NamedTempFile, PersistError};
 use crate::backend::{
     make_root_commit, Backend, BackendError, BackendResult, ChangeId, Commit, CommitId, Conflict,
     ConflictId, ConflictPart, FileId, MillisSinceEpoch, ObjectId, Signature, SymlinkId, Timestamp,
-    Tree, TreeId, TreeValue,
+    Tree, TreeId, TreeValue, CHANGE_ID_HASH_LENGTH,
 };
 use crate::content_hash::blake2b_hash;
 use crate::file_util::persist_content_addressed_temp_file;
@@ -69,6 +69,7 @@ fn map_not_found_err(err: std::io::Error, id: &impl ObjectId) -> BackendError {
 pub struct LocalBackend {
     path: PathBuf,
     root_commit_id: CommitId,
+    root_change_id: ChangeId,
     empty_tree_id: TreeId,
 }
 
@@ -89,10 +90,12 @@ impl LocalBackend {
 
     pub fn load(store_path: &Path) -> Self {
         let root_commit_id = CommitId::from_bytes(&[0; 64]);
+        let root_change_id = ChangeId::from_bytes(&[0; CHANGE_ID_HASH_LENGTH]);
         let empty_tree_id = TreeId::from_hex("482ae5a29fbe856c7272f2071b8b0f0359ee2d89ff392b8a900643fbd0836eccd067b8bf41909e206c90d45d6e7d8b6686b93ecaee5fe1a9060d87b672101310");
         LocalBackend {
             path: store_path.to_path_buf(),
             root_commit_id,
+            root_change_id,
             empty_tree_id,
         }
     }
@@ -185,6 +188,10 @@ impl Backend for LocalBackend {
         &self.root_commit_id
     }
 
+    fn root_change_id(&self) -> &ChangeId {
+        &self.root_change_id
+    }
+
     fn empty_tree_id(&self) -> &TreeId {
         &self.empty_tree_id
     }
@@ -231,7 +238,10 @@ impl Backend for LocalBackend {
 
     fn read_commit(&self, id: &CommitId) -> BackendResult<Commit> {
         if *id == self.root_commit_id {
-            return Ok(make_root_commit(self.empty_tree_id.clone()));
+            return Ok(make_root_commit(
+                self.root_change_id().clone(),
+                self.empty_tree_id.clone(),
+            ));
         }
 
         let path = self.commit_path(id);

lib/src/repo.rs

@@ -294,7 +294,7 @@ impl ReadonlyRepo {
 
     pub fn shortest_unique_id_prefix_len(&self, target_id_bytes: &[u8]) -> usize {
         let root_commit_id = self.store().root_commit_id();
-        let root_change_id = backend::root_change_id();
+        let root_change_id = self.store().root_change_id();
         if target_id_bytes == root_commit_id.as_bytes()
             || target_id_bytes == root_change_id.as_bytes()
         {

lib/src/store.rs

@@ -18,7 +18,7 @@ use std::sync::{Arc, RwLock};
 
 use crate::backend;
 use crate::backend::{
-    Backend, BackendResult, CommitId, Conflict, ConflictId, FileId, SymlinkId, TreeId,
+    Backend, BackendResult, ChangeId, CommitId, Conflict, ConflictId, FileId, SymlinkId, TreeId,
 };
 use crate::commit::Commit;
 use crate::repo_path::RepoPath;
@@ -59,6 +59,10 @@ impl Store {
         self.backend.root_commit_id()
     }
 
+    pub fn root_change_id(&self) -> &ChangeId {
+        self.backend.root_change_id()
+    }
+
     pub fn root_commit(self: &Arc<Self>) -> Commit {
         self.get_commit(self.backend.root_commit_id()).unwrap()
     }