snapshot of https://github.com/zulip/zulip.git @ 48c69911c7dd4e03b20552eb3bd77ad8151c1357 (2026-09-15)

zerver/lib/context_managers.py

@@ -0,0 +1,50 @@
+"""
+Context managers, i.e. things you can use with the 'with' statement.
+"""
+
+import fcntl
+from collections.abc import Iterator
+from contextlib import contextmanager
+from typing import IO, Any
+
+
+@contextmanager
+def flock(lockfile: int | IO[Any], shared: bool = False) -> Iterator[None]:
+    """Lock a file object using flock(2) for the duration of a 'with' statement.
+
+    If shared is True, use a LOCK_SH lock, otherwise LOCK_EX."""
+
+    fcntl.flock(lockfile, fcntl.LOCK_SH if shared else fcntl.LOCK_EX)
+    try:
+        yield
+    finally:
+        fcntl.flock(lockfile, fcntl.LOCK_UN)
+
+
+@contextmanager
+def lockfile(filename: str, shared: bool = False) -> Iterator[None]:
+    """Lock a file using flock(2) for the duration of a 'with' statement.
+
+    If shared is True, use a LOCK_SH lock, otherwise LOCK_EX.
+
+    The file is given by name and will be created if it does not exist."""
+    with open(filename, "w") as lock, flock(lock, shared=shared):
+        yield
+
+
+@contextmanager
+def lockfile_nonblocking(filename: str) -> Iterator[bool]:  # nocoverage
+    """Lock a file using flock(2) for the duration of a 'with' statement.
+
+    Doesn't block, yields False immediately if the lock can't be acquired."""
+    with open(filename, "w") as f:
+        lock_acquired = False
+        try:
+            fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            lock_acquired = True
+            yield lock_acquired
+        except BlockingIOError:
+            yield False
+        finally:
+            if lock_acquired:
+                fcntl.flock(f, fcntl.LOCK_UN)