Fix multithreaded Windows builds when a thread has a file open for write

Python 2 enables handle inheritance by default for child processes. It wasn't
until Python 3.4 that it was disabled. This causes problems because if a Python
action is writing to a file and a child process is spawned at that exact
moment, builds fail because of sharing issues.

1 files changed, 34 insertions, 25 deletions

diff --git a/src/engine/SCons/Platform/win32.py b/src/engine/SCons/Platform/win32.py
index 0a3b199..c0cf03b 100644
--- a/src/engine/SCons/Platform/win32.py
+++ b/src/engine/SCons/Platform/win32.py
@@ -51,10 +51,6 @@ try:
     import msvcrt
     import win32api
     import win32con
-
-    msvcrt.get_osfhandle
-    win32api.SetHandleInformation
-    win32con.HANDLE_FLAG_INHERIT
 except ImportError:
     parallel_msg = \
         "you do not seem to have the pywin32 extensions installed;\n" + \
@@ -66,28 +62,41 @@ except AttributeError:
 else:
     parallel_msg = None
 
-    _builtin_open = open
-
-    def _scons_open(*args, **kw):
-        fp = _builtin_open(*args, **kw)
-        win32api.SetHandleInformation(msvcrt.get_osfhandle(fp.fileno()),
-                                      win32con.HANDLE_FLAG_INHERIT,
-                                      0)
-        return fp
-
-    open = _scons_open
-
     if sys.version_info.major == 2:
-        _builtin_file = file
-        class _scons_file(_builtin_file):
-            def __init__(self, *args, **kw):
-                _builtin_file.__init__(self, *args, **kw)
-                win32api.SetHandleInformation(msvcrt.get_osfhandle(self.fileno()),
-                 win32con.HANDLE_FLAG_INHERIT, 0)
-        file = _scons_file
-    else:
-        # No longer needed for python 3.4 and above. Files are opened non sharable
-        pass
+        import __builtin__
+
+        _builtin_file = __builtin__.file
+        _builtin_open = __builtin__.open
+
+        def _scons_fixup_mode(mode):
+            """Adjust 'mode' to mark handle as non-inheritable.
+
+            SCons is multithreaded, so allowing handles to be inherited by
+            children opens us up to races, where (e.g.) processes spawned by
+            the Taskmaster may inherit and retain references to files opened
+            by other threads. This may lead to sharing violations and,
+            ultimately, build failures.
+
+            By including 'N' as part of fopen's 'mode' parameter, all file
+            handles returned from these functions are atomically marked as
+            non-inheritable.
+            """
+            if not mode:
+                # Python's default is 'r'.
+                # https://docs.python.org/2/library/functions.html#open
+                mode = 'rN'
+            elif 'N' not in mode:
+                mode += 'N'
+            return mode
+
+        def _scons_file(name, mode=None, *args, **kwargs):
+            return _builtin_file(name, _scons_fixup_mode(mode), *args, **kwargs)
+
+        def _scons_open(name, mode=None, *args, **kwargs):
+            return _builtin_open(name, _scons_fixup_mode(mode), *args, **kwargs)
+
+        __builtin__.file = _scons_file
+        __builtin__.open = _scons_open