Add shared file lock and concurrency test

src/utils/file_lock.py

@@ -22,3 +22,25 @@ def exclusive_lock(
     lock = portalocker.Lock(str(path), mode="a+b", timeout=timeout)
     with lock as fh:
         yield fh
+
+
+@contextmanager
+def shared_lock(
+    path: Path, timeout: Optional[float] = None
+) -> Generator[None, None, None]:
+    """Context manager that locks *path* with a shared lock.
+
+    The function opens the file in binary read/write mode and obtains a
+    shared lock using ``portalocker``. If ``timeout`` is provided, acquiring
+    the lock will wait for at most that many seconds before raising
+    ``portalocker.exceptions.LockException``.
+    """
+    path = Path(path)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.touch(exist_ok=True)
+    lock = portalocker.Lock(
+        str(path), mode="r+b", timeout=timeout, flags=portalocker.LockFlags.SHARED
+    )
+    with lock as fh:
+        fh.seek(0)
+        yield fh