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, 45 insertions, 0 deletions

diff --git a/internal/filelock/filelock.go b/internal/filelock/filelock.go
new file mode 100644
index 0000000..192c3ca
--- /dev/null
+++ b/internal/filelock/filelock.go
@@ -0,0 +1,45 @@
+// Package filelock provides advisory exclusive locks on open files.
+package filelock
+
+import (
+	"context"
+	"errors"
+	"os"
+	"time"
+)
+
+// ErrWouldBlock indicates a non-blocking lock attempt could not acquire the lock.
+var ErrWouldBlock = errors.New("filelock: would block")
+
+// TryExclusive attempts a non-blocking exclusive advisory lock on f.
+func TryExclusive(f *os.File) error {
+	return tryLockExclusive(f.Fd())
+}
+
+// UnlockExclusive releases the advisory lock held on f.
+func UnlockExclusive(f *os.File) error {
+	return unlockExclusive(f.Fd())
+}
+
+// AcquireExclusive spins with TryExclusive until the lock is acquired, ctx is done, or a non-would-block error occurs.
+func AcquireExclusive(ctx context.Context, f *os.File) (func() error, error) {
+	fd := f.Fd()
+	retryTimer := time.NewTimer(5 * time.Millisecond)
+	defer retryTimer.Stop()
+	for {
+		err := tryLockExclusive(fd)
+		if err == nil {
+			return func() error { return unlockExclusive(fd) }, nil
+		}
+		if errors.Is(err, ErrWouldBlock) {
+			retryTimer.Reset(5 * time.Millisecond)
+			select {
+			case <-ctx.Done():
+				return nil, ctx.Err()
+			case <-retryTimer.C:
+			}
+			continue
+		}
+		return nil, err
+	}
+}