chore(worker): Improve git operation perf (#791)

Author: brendan-kellam

CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added endpoint for searching commit history for a git repository. [#625](https://github.com/sourcebot-dev/sourcebot/pull/625)
 - Added `pushedAt` field to the Repo table to track when a repository last was committed to across all branches. [#790](https://github.com/sourcebot-dev/sourcebot/pull/790)
 
+### Changed
+- Added commit graph generation to improve performance for commit traversal operations. [#791](https://github.com/sourcebot-dev/sourcebot/pull/791)
+
 ## [4.10.17] - 2026-01-23
 
 ### Fixed

docs/docs/connections/local-repos.mdx

@@ -55,7 +55,6 @@ To get Sourcebot to index these repositories:
 
 ## Examples
 
-
 <AccordionGroup>
     <Accordion title="Sync individual repo">
         ```json
@@ -76,6 +75,22 @@ To get Sourcebot to index these repositories:
     </Accordion>
 </AccordionGroup>
 
+## Optimizing git operations
+
+Sourcebot performs a number of operations that require traversing a repository's entire commit history (e.g., `git rev-list --count HEAD`). These operations can be slow in repositories with a large number of commits.
+
+Typically, a [commit graph](https://git-scm.com/docs/commit-graph) is generated to speed up these operations (see [#791](https://github.com/sourcebot-dev/sourcebot/pull/791)). However, since local repositories are treated as read-only, Sourcebot **will not** generate a commit graph for them.
+
+A commit graph can be manually generated by running the following command in the repository's root directory:
+```sh
+git commit-graph write --reachable
+```
+
+The commit graph can be updated when fetching with `--write-commit-graph`:
+```sh
+git fetch --write-commit-graph
+```
+
 ## Schema reference
 
 <Accordion title="Reference">

packages/backend/src/git.ts

@@ -82,6 +82,12 @@ export const cloneRepository = async (
             keys: ["remote.origin.url"],
             signal,
         });
+
+        // @note: operations that need to iterate over a lot of commits (e.g., rev-list --count)
+        // can be slow on larger repositories. Commit graphs are a acceleration structure that
+        // speed up these operations.
+        // @see: https://git-scm.com/docs/commit-graph
+        await writeCommitGraph({ path, signal });
     } catch (error: unknown) {
         const baseLog = `Failed to clone repository: ${path}`;
 
@@ -121,7 +127,10 @@ export const fetchRepository = async (
             cloneUrl,
             "+refs/heads/*:refs/heads/*",
             "--prune",
-            "--progress"
+            "--progress",
+            // On fetch, ensure the commit graph is up to date.
+            // @see: https://git-scm.com/docs/commit-graph
+            "--write-commit-graph"
         ]);
 
         // Update HEAD to match the remote's default branch. This handles the case where the remote's
@@ -405,3 +414,27 @@ export const getLatestCommitTimestamp = async ({
         return undefined;
     }
 }
+
+/**
+ * Writes or updates the commit-graph file for the repository.
+ * This pre-computes commit metadata to speed up operations like
+ * rev-list --count, log, and merge-base.
+ */
+export const writeCommitGraph = async ({
+    path,
+    onProgress,
+    signal,
+}: {
+    path: string,
+    onProgress?: onProgressFn,
+    signal?: AbortSignal,
+}): Promise<void> => {
+    const git = createGitClientForPath(path, onProgress, signal);
+
+    try {
+        await git.raw(['commit-graph', 'write', '--reachable']);
+    } catch (error) {
+        // Don't throw an exception here since this is just a performance optimization.
+        logger.debug(`Failed to write commit-graph for ${path}:`, error);
+    }
+}