Add basic patching of fcntl module

CHANGES.md

@@ -6,6 +6,9 @@ The released versions correspond to PyPi releases.
 ### Changes
 * `os.listdir`, `os.scandir` and `pathlib.Path.listdir` now return the
   directory list in a random order (see [#638](../../issues/638))
+* the `fcntl` module under Unix is now mocked, e.g. all functions have no 
+  effect (this may be changed in the future if needed,
+  see [#645](../../issues/645))  
 
 ### Fixes
 * fixed handling of alternative path separator in `os.path.split`,

pyfakefs/fake_filesystem.py

@@ -4864,6 +4864,49 @@ def __getattr__(self, name):
         return getattr(self._io_module, name)
 
 
+if sys.platform != 'win32':
+    import fcntl
+
+    class FakeFcntlModule:
+        """Replaces the fcntl module. Only valid under Linux/MacOS,
+        currently just mocks the functionality away.
+        """
+
+        @staticmethod
+        def dir() -> List[str]:
+            """Return the list of patched function names. Used for patching
+            functions imported from the module.
+            """
+            return ['fcntl', 'ioctl', 'flock', 'lockf']
+
+        def __init__(self, filesystem: FakeFilesystem):
+            """
+            Args:
+                filesystem: FakeFilesystem used to provide file system
+                information (currently not used).
+            """
+            self.filesystem = filesystem
+            self._fcntl_module = fcntl
+
+        def fcntl(self, fd: int, cmd: int, arg: int = 0) -> Union[int, bytes]:
+            return 0
+
+        def ioctl(self, fd: int, request: int, arg: int = 0,
+                  mutate_flag: bool = True) -> Union[int, bytes]:
+            return 0
+
+        def flock(self, fd: int, operation: int) -> None:
+            pass
+
+        def lockf(self, fd: int, cmd: int, len: int = 0,
+                  start: int = 0, whence=0) -> Any:
+            pass
+
+        def __getattr__(self, name):
+            """Forwards any unfaked calls to the standard fcntl module."""
+            return getattr(self._fcntl_module, name)
+
+
 class FakeFileWrapper:
     """Wrapper for a stream object for use by a FakeFile object.
 

pyfakefs/fake_filesystem_unittest.py

@@ -391,7 +391,8 @@ class Patcher:
 
     IS_WINDOWS = sys.platform in ('win32', 'cygwin')
 
-    SKIPNAMES = {'os', 'path', 'io', 'genericpath', OS_MODULE, PATH_MODULE}
+    SKIPNAMES = {'os', 'path', 'io', 'genericpath', 'fcntl',
+                 OS_MODULE, PATH_MODULE}
 
     # hold values from last call - if changed, the cache has to be invalidated
     PATCHED_MODULE_NAMES: Set[str] = set()
@@ -537,6 +538,9 @@ def _init_fake_module_classes(self) -> None:
         if IS_PYPY:
             # in PyPy io.open, the module is referenced as _io
             self._fake_module_classes['_io'] = fake_filesystem.FakeIoModule
+        if sys.platform != 'win32':
+            self._fake_module_classes[
+                'fcntl'] = fake_filesystem.FakeFcntlModule
 
         # class modules maps class names against a list of modules they can
         # be contained in - this allows for alternative modules like