Limit repository creation to one process

pre_commit/file_lock.py

@@ -0,0 +1,53 @@
+import contextlib
+import errno
+
+
+try:  # pragma: no cover (windows)
+    import msvcrt
+
+    # https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/locking
+
+    # on windows we lock "regions" of files, we don't care about the actual
+    # byte region so we'll just pick *some* number here.
+    _region = 0xffff
+
+    @contextlib.contextmanager
+    def _locked(fileno):
+        while True:
+            try:
+                msvcrt.locking(fileno, msvcrt.LK_LOCK, _region)
+            except OSError as e:
+                # Locking violation. Returned when the _LK_LOCK or _LK_RLCK
+                # flag is specified and the file cannot be locked after 10
+                # attempts.
+                if e.errno != errno.EDEADLOCK:
+                    raise
+            else:
+                break
+
+        try:
+            yield
+        finally:
+            # From cursory testing, it seems to get unlocked when the file is
+            # closed so this may not be necessary.
+            # The documentation however states:
+            # "Regions should be locked only briefly and should be unlocked
+            # before closing a file or exiting the program."
+            msvcrt.locking(fileno, msvcrt.LK_UNLCK, _region)
+except ImportError:  # pragma: no cover (posix)
+    import fcntl
+
+    @contextlib.contextmanager
+    def _locked(fileno):
+        fcntl.flock(fileno, fcntl.LOCK_EX)
+        try:
+            yield
+        finally:
+            fcntl.flock(fileno, fcntl.LOCK_UN)
+
+
+@contextlib.contextmanager
+def lock(path):
+    with open(path, 'a+') as f:
+        with _locked(f.fileno()):
+            yield