git: cache the extra metadata table

Performance on repositories with many commits is limited somewhat by repeatedly
stating the tablestore directory to work out what the head is. By caching the
table rather than looking it up from disk on every request, we can much more
rapidly satisfy requests. 

This avoids the pathological case in #845 where jj operations take several
minutes to complete.

This patch doesn't change the normal flow of the write path: that will still
always call get_head() on the underlying TableStore, which will stat the
directory before writing out changes. It will however empty the cache when the
metadata has been written.

Fixes #845.

lib/src/git_backend.rs

@@ -16,7 +16,7 @@ use std::fmt::{Debug, Error, Formatter};
 use std::fs::File;
 use std::io::{Cursor, Read, Write};
 use std::path::Path;
-use std::sync::Mutex;
+use std::sync::{Arc, Mutex};
 
 use git2::Oid;
 use itertools::Itertools;
@@ -29,7 +29,7 @@ use crate::backend::{
     TreeId, TreeValue,
 };
 use crate::repo_path::{RepoPath, RepoPathComponent};
-use crate::stacked_table::{TableSegment, TableStore};
+use crate::stacked_table::{ReadonlyTable, TableSegment, TableStore};
 
 const HASH_LENGTH: usize = 20;
 /// Ref namespace used only for preventing GC.
@@ -50,6 +50,7 @@ pub struct GitBackend {
     root_commit_id: CommitId,
     empty_tree_id: TreeId,
     extra_metadata_store: TableStore,
+    cached_extra_metadata: Mutex<Option<Arc<ReadonlyTable>>>,
 }
 
 impl GitBackend {
@@ -61,6 +62,7 @@ impl GitBackend {
             root_commit_id,
             empty_tree_id,
             extra_metadata_store,
+            cached_extra_metadata: Mutex::new(None),
         }
     }
 
@@ -390,9 +392,17 @@ impl Backend for GitBackend {
             committer,
         };
 
-        let table = self.extra_metadata_store.get_head().map_err(|err| {
-            BackendError::Other(format!("Failed to read non-git metadata: {err}"))
-        })?;
+        let table = {
+            let mut locked_head = self.cached_extra_metadata.lock().unwrap();
+            match locked_head.take() {
+                Some(head) => Ok(head.clone()),
+                None => self.extra_metadata_store.get_head().map(|x| {
+                    *locked_head = Some(x.clone());
+                    x
+                }),
+            }
+        }
+        .map_err(|err| BackendError::Other(format!("Failed to read non-git metadata: {err}")))?;
         let maybe_extras = table.get_value(git_commit_id.as_bytes());
         if let Some(extras) = maybe_extras {
             deserialize_extras(&mut commit, extras);
@@ -452,6 +462,7 @@ impl Backend for GitBackend {
             .map_err(|err| {
                 BackendError::Other(format!("Failed to write non-git metadata: {err}"))
             })?;
+        *self.cached_extra_metadata.lock().unwrap() = None;
         Ok(id)
     }
 }