bind-gen: Support multiprocessing of Python on Windows, etc.

Supports Python's multiprocessing on platforms that do not
support 'fork'.

Bug: 839389
Change-Id: I2dbe28686c3d73badcbf6e07e403079a981f6b00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153166Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759952}

third_party/blink/renderer/bindings/scripts/bind_gen/__init__.py

@@ -33,11 +33,9 @@ def _setup_sys_path():
 
 _setup_sys_path()
 
-from . import style_format
 from .dictionary import generate_dictionaries
 from .enumeration import generate_enumerations
 from .interface import generate_interfaces
-from .path_manager import PathManager
 from .union import generate_unions
 
 
@@ -49,9 +47,9 @@ def init(root_src_dir, root_gen_dir, component_reldirs):
             "//out/Default/gen" in GN.
         component_reldirs: Pairs of component and output directory.
     """
-    style_format.init(root_src_dir)
 
-    PathManager.init(
+    from . import package_initializer
+    package_initializer.init(
         root_src_dir=root_src_dir,
         root_gen_dir=root_gen_dir,
         component_reldirs=component_reldirs)

third_party/blink/renderer/bindings/scripts/bind_gen/interface.py

@@ -44,6 +44,7 @@ from .codegen_utils import make_forward_declarations
 from .codegen_utils import make_header_include_directives
 from .codegen_utils import write_code_node_to_file
 from .mako_renderer import MakoRenderer
+from .package_initializer import package_initializer
 from .path_manager import PathManager
 
 
@@ -6426,6 +6427,12 @@ def generate_init_idl_interfaces(web_idl_database):
     write_code_node_to_file(source_node, path_manager.gen_path_to(source_path))
 
 
+def run_multiprocessing_task(args):
+    interface, package_initializer = args
+    package_initializer.init()
+    generate_interface(interface)
+
+
 def generate_interfaces(web_idl_database):
     # More processes do not mean better performance.  The default size was
     # chosen heuristically.
@@ -6437,7 +6444,9 @@ def generate_interfaces(web_idl_database):
     # Prior to Python3, Pool.map doesn't support user interrupts (e.g. Ctrl-C),
     # although Pool.map_async(...).get(...) does.
     timeout_in_sec = 3600  # Just enough long time
-    pool.map_async(generate_interface,
-                   web_idl_database.interfaces).get(timeout_in_sec)
+    pool.map_async(
+        run_multiprocessing_task,
+        map(lambda interface: (interface, package_initializer()),
+            web_idl_database.interfaces)).get(timeout_in_sec)
 
     generate_init_idl_interfaces(web_idl_database)

third_party/blink/renderer/bindings/scripts/bind_gen/package_initializer.py

+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+from . import style_format
+from .path_manager import PathManager
+
+
+def init(**kwargs):
+    """
+    Initializes this package.  See PackageInitializer.__init__ for details
+    about the arguments.
+    """
+    the_instance = PackageInitializer(**kwargs)
+    the_instance.init()
+    assert the_instance is PackageInitializer.the_instance()
+
+
+def package_initializer():
+    """
+    Returns the instance of PackageInitializer that actually initialized this
+    package.
+    """
+    the_instance = PackageInitializer.the_instance()
+    assert the_instance
+    return the_instance
+
+
+class PackageInitializer(object):
+    """
+    PackageInitializer is designed to support 'multiprocessing' package so that
+    users can initialize this package in another process with the same
+    settings.
+
+    When the 'start method' of 'multiprocessing' package is 'spawn', the global
+    environment (e.g. module variables, class variables, etc.) will not be
+    inherited.  See also https://docs.python.org/3/library/multiprocessing.html
+
+    PackageInitializer helps reproduce the same runtime environment of this
+    process in other processes.  PackageInitializer.init() initializes this
+    package in the same way as it was originally initialized iff the current
+    process' runtime environment has not yet been initialized.  In other words,
+    PackageInitializer.init() works with any start method of multiprocessing
+    package.
+    """
+
+    # The instance of PackageInitializer that actually initialized this
+    # package.
+    _the_instance = None
+
+    @classmethod
+    def the_instance(cls):
+        return cls._the_instance
+
+    def __init__(self, root_src_dir, root_gen_dir, component_reldirs):
+        """
+        Args:
+            root_src_dir: Project's root directory, which corresponds to "//"
+                in GN.
+            root_gen_dir: Root directory of generated files, which corresponds
+                to "//out/Default/gen" in GN.
+            component_reldirs: Pairs of component and output directory.
+        """
+
+        self._root_src_dir = root_src_dir
+        self._root_gen_dir = root_gen_dir
+        self._component_reldirs = component_reldirs
+
+    def init(self):
+        if PackageInitializer._the_instance:
+            return
+        PackageInitializer._the_instance = self
+
+        self._init()
+
+    def _init(self):
+        style_format.init(self._root_src_dir)
+
+        PathManager.init(
+            root_src_dir=self._root_src_dir,
+            root_gen_dir=self._root_gen_dir,
+            component_reldirs=self._component_reldirs)

third_party/blink/renderer/bindings/scripts/generate_bindings.pydeps

@@ -42,6 +42,7 @@ bind_gen/enumeration.py
 bind_gen/interface.py
 bind_gen/mako_renderer.py
 bind_gen/name_style.py
+bind_gen/package_initializer.py
 bind_gen/path_manager.py
 bind_gen/style_format.py
 bind_gen/union.py