From b672c9aaf33f6931537bc7fc55b47b4a9b38385d Mon Sep 17 00:00:00 2001
From: mika kuns <mika.kuns@fb-tuning.de>
Date: Tue, 9 Jun 2026 09:55:39 +0200
Subject: [PATCH] fix(git): serialize concurrent worktree add to prevent
 commondir race

Parallel task starts called 'git worktree add' simultaneously; git's shared
.git/worktrees metadata mutation isn't concurrency-safe and one add failed with
'failed to read .git/worktrees/<other>/commondir'. Serialize adds behind a
process-wide gate plus a bounded retry on the transient error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 src/ClaudeDo.Data/Git/GitService.cs | 33 +++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/src/ClaudeDo.Data/Git/GitService.cs b/src/ClaudeDo.Data/Git/GitService.cs
index 1b43c45..7ba2cf9 100644
--- a/src/ClaudeDo.Data/Git/GitService.cs
+++ b/src/ClaudeDo.Data/Git/GitService.cs
@@ -7,6 +7,11 @@ public sealed record MergePreview(bool Supported, bool Clean, IReadOnlyList<stri
 
 public sealed class GitService
 {
+    // git mutates shared .git/worktrees/ metadata during `worktree add`; concurrent adds
+    // race and fail with "failed to read .git/worktrees/<other>/commondir". Serialize them
+    // process-wide so parallel task starts don't collide.
+    private static readonly SemaphoreSlim WorktreeAddGate = new(1, 1);
+
     public async Task<bool> IsGitRepoAsync(string dir, CancellationToken ct = default)
     {
         var (exitCode, _, _) = await RunGitAsync(dir, ["rev-parse", "--git-dir"], ct);
@@ -23,10 +28,30 @@ public sealed class GitService
 
     public async Task WorktreeAddAsync(string repoDir, string branchName, string worktreePath, string baseCommit, CancellationToken ct = default)
     {
-        var (exitCode, _, stderr) = await RunGitAsync(repoDir,
-            ["worktree", "add", "-b", branchName, worktreePath, baseCommit], ct);
-        if (exitCode != 0)
-            throw new InvalidOperationException($"git worktree add failed (exit {exitCode}): {stderr}");
+        await WorktreeAddGate.WaitAsync(ct);
+        try
+        {
+            const int maxAttempts = 3;
+            for (var attempt = 1; ; attempt++)
+            {
+                var (exitCode, _, stderr) = await RunGitAsync(repoDir,
+                    ["worktree", "add", "-b", branchName, worktreePath, baseCommit], ct);
+                if (exitCode == 0)
+                    return;
+
+                // Transient races leave a half-written worktree metadata dir; retry briefly.
+                var transient = stderr.Contains("commondir", StringComparison.OrdinalIgnoreCase)
+                    || stderr.Contains("failed to read", StringComparison.OrdinalIgnoreCase);
+                if (!transient || attempt >= maxAttempts)
+                    throw new InvalidOperationException($"git worktree add failed (exit {exitCode}): {stderr}");
+
+                await Task.Delay(150 * attempt, ct);
+            }
+        }
+        finally
+        {
+            WorktreeAddGate.Release();
+        }
     }
 
     public async Task<string> GetStatusPorcelainAsync(string workingDirectory, CancellationToken ct = default)