ask: serialize concurrent CLI with repo lock and stale PID recovery

Add advisory lock under .git/hexai-ask.lock around Taskwarrior execution,
with metadata (PID and process basename) and Linux /proc comm checks to
remove orphan lock files when the recorded holder is gone or not ask.

Extract internal/filelock for shared flock helpers; stats uses it too.

Made-with: Cursor

1 files changed, 67 insertions, 0 deletions

diff --git a/internal/filelock/filelock_test.go b/internal/filelock/filelock_test.go
new file mode 100644
index 0000000..f1f5b65
--- /dev/null
+++ b/internal/filelock/filelock_test.go
@@ -0,0 +1,67 @@
+package filelock
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestTryExclusive_SecondDescriptorWouldBlock(t *testing.T) {
+	dir := t.TempDir()
+	p := filepath.Join(dir, "lock")
+	f1, err := os.OpenFile(p, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = f1.Close() })
+	f2, err := os.OpenFile(p, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = f2.Close() })
+	if err := TryExclusive(f1); err != nil {
+		t.Fatalf("first TryExclusive: %v", err)
+	}
+	err = TryExclusive(f2)
+	if !errors.Is(err, ErrWouldBlock) {
+		t.Fatalf("second TryExclusive = %v, want ErrWouldBlock", err)
+	}
+	if err := UnlockExclusive(f1); err != nil {
+		t.Fatal(err)
+	}
+	if err := TryExclusive(f2); err != nil {
+		t.Fatalf("after unlock: %v", err)
+	}
+	if err := UnlockExclusive(f2); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestAcquireExclusive_ContextCancelledWhileBlocked(t *testing.T) {
+	dir := t.TempDir()
+	p := filepath.Join(dir, "lock")
+	fHeld, err := os.OpenFile(p, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = fHeld.Close() })
+	if err := TryExclusive(fHeld); err != nil {
+		t.Fatal(err)
+	}
+	fWait, err := os.OpenFile(p, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = fWait.Close() })
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	_, err = AcquireExclusive(ctx, fWait)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("AcquireExclusive = %v, want context.Canceled", err)
+	}
+	if err := UnlockExclusive(fHeld); err != nil {
+		t.Fatal(err)
+	}
+}