op_store: implement GC of unreachble operations and views

Since new operations and views may be added concurrently by another process,
there's a risk of data corruption. The keep_newer parameter is a mitigation
for this problem. It's set to preserve files modified within the last 2 weeks,
which is the default of "git gc". Still, a concurrent process may replace an
existing view which is about to be deleted by the gc process, and the view
file would be lost.

#12

CHANGELOG.md

@@ -14,6 +14,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * New `jj op abandon` command is added to clean up the operation history. If GC
   is implemented, Git refs and commit objects can be compacted.
 
+* `jj util gc` now removes unreachable operation and view objects.
+
 * `jj branch rename` will now warn if the renamed branch has a remote branch, since
   those will have to be manually renamed outside of `jj`.
 

cli/src/commands/util.rs

@@ -13,6 +13,8 @@
 // limitations under the License.
 
 use std::io::Write;
+use std::slice;
+use std::time::{Duration, SystemTime};
 
 use clap::Subcommand;
 use jj_lib::repo::Repo;
@@ -114,8 +116,14 @@ fn cmd_util_gc(
         ));
     }
     let workspace_command = command.workspace_helper(ui)?;
-    let store = workspace_command.repo().store();
-    store.gc().map_err(|err| user_error(err.to_string()))?;
+    // TODO: add command argument to specify the expiration time?
+    let keep_newer = SystemTime::now() - Duration::from_secs(14 * 86400);
+    let repo = workspace_command.repo();
+    repo.op_store()
+        .gc(slice::from_ref(repo.op_id()), keep_newer)?;
+    repo.store()
+        .gc()
+        .map_err(|err| user_error(err.to_string()))?;
     Ok(())
 }
 

lib/src/file_util.rs

@@ -104,6 +104,7 @@ pub fn persist_content_addressed_temp_file<P: AsRef<Path>>(
             Ok(file) => Ok(file),
             Err(PersistError { error, file: _ }) => {
                 if let Ok(existing_file) = File::open(new_path) {
+                    // TODO: Update mtime to help GC keep this file
                     Ok(existing_file)
                 } else {
                     Err(error)

lib/src/op_store.rs

@@ -17,6 +17,7 @@
 use std::collections::{BTreeMap, HashMap, HashSet};
 use std::fmt::{Debug, Error, Formatter};
 use std::iter;
+use std::time::SystemTime;
 
 use itertools::Itertools as _;
 use once_cell::sync::Lazy;
@@ -424,6 +425,15 @@ pub trait OpStore: Send + Sync + Debug {
         &self,
         prefix: &HexPrefix,
     ) -> OpStoreResult<PrefixResolution<OperationId>>;
+
+    /// Prunes unreachable operations and views.
+    ///
+    /// All operations and views reachable from the `head_ids` won't be
+    /// removed. In addition to that, objects created after `keep_newer` will be
+    /// preserved. This mitigates a risk of deleting new heads created
+    /// concurrently by another process.
+    // TODO: return stats?
+    fn gc(&self, head_ids: &[OperationId], keep_newer: SystemTime) -> OpStoreResult<()>;
 }
 
 #[cfg(test)]

lib/src/simple_op_store.rs

@@ -14,26 +14,28 @@
 
 #![allow(missing_docs)]
 
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, HashMap, HashSet};
 use std::fmt::Debug;
 use std::io::{ErrorKind, Write};
 use std::path::{Path, PathBuf};
+use std::time::SystemTime;
 use std::{fs, io};
 
+use itertools::Itertools as _;
 use prost::Message;
 use tempfile::NamedTempFile;
 use thiserror::Error;
 
 use crate::backend::{CommitId, MillisSinceEpoch, Timestamp};
 use crate::content_hash::blake2b_hash;
-use crate::file_util::{persist_content_addressed_temp_file, IoResultExt as _};
+use crate::file_util::{persist_content_addressed_temp_file, IoResultExt as _, PathError};
 use crate::merge::Merge;
 use crate::object_id::{HexPrefix, ObjectId, PrefixResolution};
 use crate::op_store::{
     OpStore, OpStoreError, OpStoreResult, Operation, OperationId, OperationMetadata, RefTarget,
     RemoteRef, RemoteRefState, RemoteView, View, ViewId, WorkspaceId,
 };
-use crate::{git, op_store};
+use crate::{dag_walk, git, op_store};
 
 // BLAKE2b-512 hash length in bytes
 const OPERATION_ID_LENGTH: usize = 64;
@@ -195,6 +197,89 @@ impl OpStore for SimpleOpStore {
             .context(&op_dir)
             .map_err(|err| OpStoreError::Other(err.into()))
     }
+
+    #[tracing::instrument(skip(self))]
+    fn gc(&self, head_ids: &[OperationId], keep_newer: SystemTime) -> OpStoreResult<()> {
+        let to_op_id = |entry: &fs::DirEntry| -> Option<OperationId> {
+            let name = entry.file_name().into_string().ok()?;
+            OperationId::try_from_hex(&name).ok()
+        };
+        let to_view_id = |entry: &fs::DirEntry| -> Option<ViewId> {
+            let name = entry.file_name().into_string().ok()?;
+            ViewId::try_from_hex(&name).ok()
+        };
+        let remove_file_if_not_new = |entry: &fs::DirEntry| -> Result<(), PathError> {
+            let path = entry.path();
+            // Check timestamp, but there's still TOCTOU problem if an existing
+            // file is renewed.
+            let metadata = entry.metadata().context(&path)?;
+            let mtime = metadata.modified().expect("unsupported platform?");
+            if mtime > keep_newer {
+                tracing::trace!(?path, "not removing");
+                Ok(())
+            } else {
+                tracing::trace!(?path, "removing");
+                fs::remove_file(&path).context(&path)
+            }
+        };
+
+        // Reachable objects are resolved without considering the keep_newer
+        // parameter. We could collect ancestors of the "new" operations here,
+        // but more files can be added anyway after that.
+        let read_op = |id: &OperationId| self.read_operation(id).map(|data| (id.clone(), data));
+        let reachable_ops: HashMap<OperationId, Operation> = dag_walk::dfs_ok(
+            head_ids.iter().map(read_op),
+            |(id, _)| id.clone(),
+            |(_, data)| data.parents.iter().map(read_op).collect_vec(),
+        )
+        .try_collect()?;
+        let reachable_views: HashSet<&ViewId> =
+            reachable_ops.values().map(|data| &data.view_id).collect();
+        tracing::info!(
+            reachable_op_count = reachable_ops.len(),
+            reachable_view_count = reachable_views.len(),
+            "collected reachable objects"
+        );
+
+        let prune_ops = || -> Result<(), PathError> {
+            let op_dir = self.path.join("operations");
+            for entry in op_dir.read_dir().context(&op_dir)? {
+                let entry = entry.context(&op_dir)?;
+                let Some(id) = to_op_id(&entry) else {
+                    tracing::trace!(?entry, "skipping invalid file name");
+                    continue;
+                };
+                if reachable_ops.contains_key(&id) {
+                    continue;
+                }
+                // If the operation was added after collecting reachable_views,
+                // its view mtime would also be renewed. So there's no need to
+                // update the reachable_views set to preserve the view.
+                remove_file_if_not_new(&entry)?;
+            }
+            Ok(())
+        };
+        prune_ops().map_err(|err| OpStoreError::Other(err.into()))?;
+
+        let prune_views = || -> Result<(), PathError> {
+            let view_dir = self.path.join("views");
+            for entry in view_dir.read_dir().context(&view_dir)? {
+                let entry = entry.context(&view_dir)?;
+                let Some(id) = to_view_id(&entry) else {
+                    tracing::trace!(?entry, "skipping invalid file name");
+                    continue;
+                };
+                if reachable_views.contains(&id) {
+                    continue;
+                }
+                remove_file_if_not_new(&entry)?;
+            }
+            Ok(())
+        };
+        prune_views().map_err(|err| OpStoreError::Other(err.into()))?;
+
+        Ok(())
+    }
 }
 
 fn io_to_read_error(err: std::io::Error, id: &impl ObjectId) -> OpStoreError {

lib/tests/test_operations.rs

@@ -15,6 +15,7 @@
 use std::path::Path;
 use std::slice;
 use std::sync::Arc;
+use std::time::SystemTime;
 
 use assert_matches::assert_matches;
 use itertools::Itertools as _;
@@ -30,6 +31,7 @@ fn list_dir(dir: &Path) -> Vec<String> {
     std::fs::read_dir(dir)
         .unwrap()
         .map(|entry| entry.unwrap().file_name().to_str().unwrap().to_owned())
+        .sorted()
         .collect()
 }
 
@@ -568,3 +570,81 @@ fn test_resolve_op_parents_children() {
         ))
     );
 }
+
+#[test]
+fn test_gc() {
+    let settings = stable_op_id_settings();
+    let test_repo = TestRepo::init();
+    let repo_0 = test_repo.repo;
+    let op_store = repo_0.op_store();
+    let op_dir = repo_0.repo_path().join("op_store").join("operations");
+    let view_dir = repo_0.repo_path().join("op_store").join("views");
+
+    // Set up operation graph:
+    //
+    //   F
+    //   E (empty)
+    // D |
+    // C |
+    // |/
+    // B
+    // A
+    // 0 (initial)
+    let empty_tx = |repo: &Arc<ReadonlyRepo>| repo.start_transaction(&settings);
+    let random_tx = |repo: &Arc<ReadonlyRepo>| {
+        let mut tx = repo.start_transaction(&settings);
+        write_random_commit(tx.mut_repo(), &settings);
+        tx
+    };
+    let repo_a = random_tx(&repo_0).commit("op A");
+    let repo_b = random_tx(&repo_a).commit("op B");
+    let repo_c = random_tx(&repo_b).commit("op C");
+    let repo_d = random_tx(&repo_c).commit("op D");
+    let repo_e = empty_tx(&repo_b).commit("op E");
+    let repo_f = random_tx(&repo_e).commit("op F");
+
+    // Sanity check for the original state
+    let mut expected_op_entries = list_dir(&op_dir);
+    let mut expected_view_entries = list_dir(&view_dir);
+    assert_eq!(expected_op_entries.len(), 7);
+    assert_eq!(expected_view_entries.len(), 6);
+
+    // No heads, but all kept by file modification time
+    op_store.gc(&[], SystemTime::UNIX_EPOCH).unwrap();
+    assert_eq!(list_dir(&op_dir), expected_op_entries);
+    assert_eq!(list_dir(&view_dir), expected_view_entries);
+
+    // All reachable from heads
+    let now = SystemTime::now();
+    let head_ids = [repo_d.op_id().clone(), repo_f.op_id().clone()];
+    op_store.gc(&head_ids, now).unwrap();
+    assert_eq!(list_dir(&op_dir), expected_op_entries);
+    assert_eq!(list_dir(&view_dir), expected_view_entries);
+
+    // E|F are no longer reachable, but E's view is still reachable
+    op_store.gc(slice::from_ref(repo_d.op_id()), now).unwrap();
+    expected_op_entries
+        .retain(|name| *name != repo_e.op_id().hex() && *name != repo_f.op_id().hex());
+    expected_view_entries.retain(|name| *name != repo_f.operation().view_id().hex());
+    assert_eq!(list_dir(&op_dir), expected_op_entries);
+    assert_eq!(list_dir(&view_dir), expected_view_entries);
+
+    // B|C|D are no longer reachable
+    op_store.gc(slice::from_ref(repo_a.op_id()), now).unwrap();
+    expected_op_entries.retain(|name| {
+        *name != repo_b.op_id().hex()
+            && *name != repo_c.op_id().hex()
+            && *name != repo_d.op_id().hex()
+    });
+    expected_view_entries.retain(|name| {
+        *name != repo_b.operation().view_id().hex()
+            && *name != repo_c.operation().view_id().hex()
+            && *name != repo_d.operation().view_id().hex()
+    });
+    assert_eq!(list_dir(&op_dir), expected_op_entries);
+    assert_eq!(list_dir(&view_dir), expected_view_entries);
+
+    // Sanity check for the last state
+    assert_eq!(expected_op_entries.len(), 2);
+    assert_eq!(expected_view_entries.len(), 2);
+}