backend: pass Index and keep_newer timestamp parameters to gc()

GitBackend::gc() will need to check if a commit is reachable from any
historical operations. This could be calculated from the view and commit
objects, but the Index will do a better job.

cli/examples/custom-backend/main.rs

@@ -15,6 +15,7 @@
 use std::any::Any;
 use std::io::Read;
 use std::path::Path;
+use std::time::SystemTime;
 
 use async_trait::async_trait;
 use jj_cli::cli_util::{CliRunner, CommandError, CommandHelper};
@@ -24,6 +25,7 @@ use jj_lib::backend::{
     Conflict, ConflictId, FileId, SigningFn, SymlinkId, Tree, TreeId,
 };
 use jj_lib::git_backend::GitBackend;
+use jj_lib::index::Index;
 use jj_lib::repo::StoreFactories;
 use jj_lib::repo_path::RepoPath;
 use jj_lib::settings::UserSettings;
@@ -171,7 +173,7 @@ impl Backend for JitBackend {
         self.inner.write_commit(contents, sign_with)
     }
 
-    fn gc(&self) -> BackendResult<()> {
-        self.inner.gc()
+    fn gc(&self, index: &dyn Index, keep_newer: SystemTime) -> BackendResult<()> {
+        self.inner.gc(index, keep_newer)
     }
 }

cli/src/commands/util.rs

@@ -136,7 +136,7 @@ fn cmd_util_gc(
     let repo = workspace_command.repo();
     repo.op_store()
         .gc(slice::from_ref(repo.op_id()), keep_newer)?;
-    repo.store().gc()?;
+    repo.store().gc(repo.index(), keep_newer)?;
     Ok(())
 }
 

lib/src/backend.rs

@@ -19,12 +19,14 @@ use std::collections::BTreeMap;
 use std::fmt::Debug;
 use std::io::Read;
 use std::result::Result;
+use std::time::SystemTime;
 use std::vec::Vec;
 
 use async_trait::async_trait;
 use thiserror::Error;
 
 use crate::content_hash::ContentHash;
+use crate::index::Index;
 use crate::merge::Merge;
 use crate::object_id::{id_type, ObjectId};
 use crate::repo_path::{RepoPath, RepoPathComponent, RepoPathComponentBuf};
@@ -450,6 +452,9 @@ pub trait Backend: Send + Sync + Debug {
     ) -> BackendResult<(CommitId, Commit)>;
 
     /// Perform garbage collection.
-    // TODO: pass in the set of commits to keep here
-    fn gc(&self) -> BackendResult<()>;
+    ///
+    /// All commits found in the `index` won't be removed. In addition to that,
+    /// objects created after `keep_newer` will be preserved. This mitigates a
+    /// risk of deleting new commits created concurrently by another process.
+    fn gc(&self, index: &dyn Index, keep_newer: SystemTime) -> BackendResult<()>;
 }

lib/src/git_backend.rs

@@ -21,6 +21,7 @@ use std::io::{Cursor, Read};
 use std::path::{Path, PathBuf};
 use std::process::{Command, ExitStatus};
 use std::sync::{Arc, Mutex, MutexGuard};
+use std::time::SystemTime;
 use std::{fs, io, str};
 
 use async_trait::async_trait;
@@ -37,6 +38,7 @@ use crate::backend::{
     TreeValue,
 };
 use crate::file_util::{IoResultExt as _, PathError};
+use crate::index::Index;
 use crate::lock::FileLock;
 use crate::merge::{Merge, MergeBuilder};
 use crate::object_id::ObjectId;
@@ -1081,7 +1083,8 @@ impl Backend for GitBackend {
         Ok((id, contents))
     }
 
-    fn gc(&self) -> BackendResult<()> {
+    fn gc(&self, _index: &dyn Index, _keep_newer: SystemTime) -> BackendResult<()> {
+        // TODO: pass in keep_newer to "git gc" command
         run_git_gc(self.git_repo_path()).map_err(|err| BackendError::Other(err.into()))
     }
 }

lib/src/local_backend.rs

@@ -20,6 +20,7 @@ use std::fs;
 use std::fs::File;
 use std::io::{Read, Write};
 use std::path::{Path, PathBuf};
+use std::time::SystemTime;
 
 use async_trait::async_trait;
 use blake2::{Blake2b512, Digest};
@@ -33,6 +34,7 @@ use crate::backend::{
 };
 use crate::content_hash::blake2b_hash;
 use crate::file_util::persist_content_addressed_temp_file;
+use crate::index::Index;
 use crate::merge::MergeBuilder;
 use crate::object_id::ObjectId;
 use crate::repo_path::{RepoPath, RepoPathComponentBuf};
@@ -299,7 +301,7 @@ impl Backend for LocalBackend {
         Ok((id, commit))
     }
 
-    fn gc(&self) -> BackendResult<()> {
+    fn gc(&self, _index: &dyn Index, _keep_newer: SystemTime) -> BackendResult<()> {
         Ok(())
     }
 }

lib/src/store.rs

@@ -19,6 +19,7 @@ use std::collections::HashMap;
 use std::fmt::{Debug, Formatter};
 use std::io::Read;
 use std::sync::{Arc, RwLock};
+use std::time::SystemTime;
 
 use pollster::FutureExt;
 
@@ -27,6 +28,7 @@ use crate::backend::{
     SymlinkId, TreeId,
 };
 use crate::commit::Commit;
+use crate::index::Index;
 use crate::merge::{Merge, MergedTreeValue};
 use crate::merged_tree::MergedTree;
 use crate::repo_path::{RepoPath, RepoPathBuf};
@@ -266,7 +268,7 @@ impl Store {
         TreeBuilder::new(self.clone(), base_tree_id)
     }
 
-    pub fn gc(&self) -> BackendResult<()> {
-        self.backend.gc()
+    pub fn gc(&self, index: &dyn Index, keep_newer: SystemTime) -> BackendResult<()> {
+        self.backend.gc(index, keep_newer)
     }
 }

lib/testutils/src/test_backend.rs

@@ -18,12 +18,14 @@ use std::fmt::{Debug, Error, Formatter};
 use std::io::{Cursor, Read};
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex, MutexGuard, OnceLock};
+use std::time::SystemTime;
 
 use async_trait::async_trait;
 use jj_lib::backend::{
     make_root_commit, Backend, BackendError, BackendResult, ChangeId, Commit, CommitId, Conflict,
     ConflictId, FileId, SecureSig, SigningFn, SymlinkId, Tree, TreeId,
 };
+use jj_lib::index::Index;
 use jj_lib::object_id::ObjectId;
 use jj_lib::repo_path::{RepoPath, RepoPathBuf};
 
@@ -298,7 +300,7 @@ impl Backend for TestBackend {
         Ok((id, contents))
     }
 
-    fn gc(&self) -> BackendResult<()> {
+    fn gc(&self, _index: &dyn Index, _keep_newer: SystemTime) -> BackendResult<()> {
         Ok(())
     }
 }