Prevent race condition in genpy via named mutex (#2118)

Author: Chronial

com/win32com/client/gencache.py

@@ -23,6 +23,7 @@
 
 from __future__ import annotations
 
+import contextlib
 import glob
 import os
 import sys
@@ -34,6 +35,7 @@
 import pywintypes
 import win32com
 import win32com.client
+import win32event
 
 from . import CLSIDToClass
 
@@ -133,6 +135,22 @@ def _LoadDicts():
         f.close()
 
 
+@contextlib.contextmanager
+def ModuleMutex(module_name):
+    """Given the output of GetGeneratedFilename, acquire a named mutex for that module
+
+    This is required so that writes (generation) don't interfere with each other and with reads (import)
+    """
+    mutex = win32event.CreateMutex(None, False, module_name)
+    with contextlib.closing(mutex):
+        # acquire mutex
+        win32event.WaitForSingleObject(mutex, win32event.INFINITE)
+        try:
+            yield
+        finally:
+            win32event.ReleaseMutex(mutex)
+
+
 def GetGeneratedFileName(clsid, lcid, major, minor):
     """Given the clsid, lcid, major and  minor for a type lib, return
     the file name (no extension) providing this support.
@@ -258,7 +276,8 @@ class which wraps the COM object.
         if sub_mod is not None:
             sub_mod_name = mod.__name__ + "." + sub_mod
             try:
-                __import__(sub_mod_name)
+                with ModuleMutex(mod.__name__.split(".")[-1]):
+                    __import__(sub_mod_name)
             except ImportError:
                 info = typelibCLSID, lcid, major, minor
                 # Force the generation.  If this typelibrary has explicitly been added,
@@ -730,7 +749,8 @@ def GetGeneratedInfos():
 def _GetModule(fname):
     """Given the name of a module in the gen_py directory, import and return it."""
     mod_name = "win32com.gen_py.%s" % fname
-    mod = __import__(mod_name)
+    with ModuleMutex(fname):
+        __import__(mod_name)
     return sys.modules[mod_name]
 
 

com/win32com/client/genpy.py

@@ -19,9 +19,8 @@
 from itertools import chain
 
 import pythoncom
-import win32com
 
-from . import build
+from . import build, gencache
 
 makepy_version = "0.5.01"  # Written to generated file.
 
@@ -1017,35 +1016,15 @@ def open_writer(self, filename, encoding="utf-8"):
 
     def finish_writer(self, filename, f, worked):
         f.close()
-        try:
-            os.unlink(filename)
-        except OSError:
-            pass
         temp_filename = self.get_temp_filename(filename)
         if worked:
+            os.replace(temp_filename, filename)
+        else:
             try:
-                os.rename(temp_filename, filename)
+                os.unlink(filename)
+                os.unlink(temp_filename)
             except OSError:
-                # If we are really unlucky, another process may have written the
-                # file in between our calls to os.unlink and os.rename. So try
-                # again, but only once.
-                # There are still some race conditions, but they seem difficult to
-                # fix, and they probably occur much less frequently:
-                # * The os.rename failure could occur more than once if more than
-                #   two processes are involved.
-                # * In between os.unlink and os.rename, another process could try
-                #   to import the module, having seen that it already exists.
-                # * If another process starts a COM server while we are still
-                #   generating __init__.py, that process sees that the folder
-                #   already exists and assumes that __init__.py is already there
-                #   as well.
-                try:
-                    os.unlink(filename)
-                except OSError:
-                    pass
-                os.rename(temp_filename, filename)
-        else:
-            os.unlink(temp_filename)
+                pass
 
     def get_temp_filename(self, filename):
         return "%s.%d.temp" % (filename, os.getpid())
@@ -1327,7 +1306,8 @@ def generate_child(self, child, dir):
                     self.progress.Tick()
                     worked = True
                 finally:
-                    self.finish_writer(out_name, self.file, worked)
+                    with gencache.ModuleMutex(self.base_mod_name.split(".")[-1]):
+                        self.finish_writer(out_name, self.file, worked)
                     self.file = None
         finally:
             self.progress.Finished()

com/win32com/client/makepy.py

@@ -293,10 +293,11 @@ def GenerateFromTypeLibSpec(
     for typelib, info in typelibs:
         gen = genpy.Generator(typelib, info.dll, progress, bBuildHidden=bBuildHidden)
 
+        this_name = gencache.GetGeneratedFileName(
+            info.clsid, info.lcid, info.major, info.minor
+        )
+
         if file is None:
-            this_name = gencache.GetGeneratedFileName(
-                info.clsid, info.lcid, info.major, info.minor
-            )
             full_name = os.path.join(gencache.GetGeneratePath(), this_name)
             if bForDemand:
                 try:
@@ -327,7 +328,8 @@ def GenerateFromTypeLibSpec(
             worked = True
         finally:
             if file is None:
-                gen.finish_writer(outputName, fileUse, worked)
+                with gencache.ModuleMutex(this_name):
+                    gen.finish_writer(outputName, fileUse, worked)
         importlib.invalidate_caches()
         if bToGenDir:
             progress.SetDescription("Importing module")
@@ -372,7 +374,8 @@ def GenerateChildFromTypeLibSpec(
         gen.generate_child(child, dir_path_name)
         progress.SetDescription("Importing module")
         importlib.invalidate_caches()
-        __import__("win32com.gen_py." + dir_name + "." + child)
+        with gencache.ModuleMutex(dir_name):
+            __import__("win32com.gen_py." + dir_name + "." + child)
     progress.Close()