mojo/public/tools/mojom/mojom_parser.py - chromium/src - Git at Google

 #!/usr/bin/env python
 # 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.
 """Parses mojom IDL files.

 This script parses one or more input mojom files and produces corresponding
 module files fully describing the definitions contained within each mojom. The
 module data is pickled and can be easily consumed by other tools to, e.g.,
 generate usable language bindings.
 """

 import argparse
 import builtins
 import codecs
 import errno
 import json
 import logging
 import multiprocessing
 import os
 import os.path
 import sys
 import traceback
 from collections import defaultdict

 from mojom.generate import module
 from mojom.generate import translate
 from mojom.parse import parser
 from mojom.parse import conditional_features


 # Disable this for easier debugging.
 # In Python 2, subprocesses just hang when exceptions are thrown :(.
 _ENABLE_MULTIPROCESSING = sys.version_info[0] > 2

 if sys.version_info < (3, 4):
   _MULTIPROCESSING_USES_FORK = sys.platform.startswith('linux')
 else:
   # https://docs.python.org/3/library/multiprocessing.html#:~:text=bpo-33725
   if __name__ == '__main__' and sys.platform == 'darwin':
     multiprocessing.set_start_method('fork')
   _MULTIPROCESSING_USES_FORK = multiprocessing.get_start_method() == 'fork'


 def _ResolveRelativeImportPath(path, roots):
   """Attempts to resolve a relative import path against a set of possible roots.

   Args:
     path: The relative import path to resolve.
     roots: A list of absolute paths which will be checked in descending length
         order for a match against path.

   Returns:
     A normalized absolute path combining one of the roots with the input path if
     and only if such a file exists.

   Raises:
     ValueError: The path could not be resolved against any of the given roots.
   """
   for root in reversed(sorted(roots, key=len)):
     abs_path = os.path.join(root, path)
     if os.path.isfile(abs_path):
       return os.path.normcase(os.path.normpath(abs_path))

   raise ValueError('"%s" does not exist in any of %s' % (path, roots))


 def _RebaseAbsolutePath(path, roots):
   """Rewrites an absolute file path as relative to an absolute directory path in
   roots.

   Args:
     path: The absolute path of an existing file.
     roots: A list of absolute directory paths. The given path argument must fall
         within one of these directories.

   Returns:
     A path equivalent to the input path, but relative to one of the provided
     roots. If the input path falls within multiple roots, the longest root is
     chosen (and thus the shortest relative path is returned).

     Paths returned by this method always use forward slashes as a separator to
     mirror mojom import syntax.

   Raises:
     ValueError if the given path does not fall within any of the listed roots.
   """
   assert os.path.isabs(path)
   assert os.path.isfile(path)
   assert all(map(os.path.isabs, roots))

   sorted_roots = list(reversed(sorted(roots, key=len)))

   def try_rebase_path(path, root):
     head, rebased_path = os.path.split(path)
     while head != root:
       head, tail = os.path.split(head)
       if not tail:
         return None
       rebased_path = os.path.join(tail, rebased_path)
     return rebased_path

   for root in sorted_roots:
     relative_path = try_rebase_path(path, root)
     if relative_path:
       # TODO(crbug.com/953884): Use pathlib for this kind of thing once we're
       # fully migrated to Python 3.
       return relative_path.replace('\\', '/')

   raise ValueError('%s does not fall within any of %s' % (path, sorted_roots))


 def _GetModuleFilename(mojom_filename):
   return mojom_filename + '-module'


 def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
                        dependencies, loaded_modules, module_metadata):
   """Recursively ensures that a module and its dependencies are loaded.

   Args:
     mojom_abspath: An absolute file path pointing to a mojom file to load.
     module_path: The relative path used to identify mojom_abspath.
     abs_paths: A mapping from module paths to absolute file paths for all
         inputs given to this execution of the script.
     asts: A map from each input mojom's absolute path to its parsed AST.
     dependencies: A mapping of which input mojoms depend on each other, indexed
         by absolute file path.
     loaded_modules: A mapping of all modules loaded so far, including non-input
         modules that were pulled in as transitive dependencies of the inputs.
     module_metadata: Metadata to be attached to every module loaded by this
         helper.

   Returns:
     None

     On return, loaded_modules will be populated with the loaded input mojom's
     Module as well as the Modules of all of its transitive dependencies."""

   if mojom_abspath in loaded_modules:
     # Already done.
     return

   for dep_abspath, dep_path in sorted(dependencies[mojom_abspath]):
     if dep_abspath not in loaded_modules:
       _EnsureInputLoaded(dep_abspath, dep_path, abs_paths, asts, dependencies,
                          loaded_modules, module_metadata)

   imports = {}
   for imp in asts[mojom_abspath].import_list:
     path = imp.import_filename
     imports[path] = loaded_modules[abs_paths[path]]
   loaded_modules[mojom_abspath] = translate.OrderedModule(
       asts[mojom_abspath], module_path, imports)
   loaded_modules[mojom_abspath].metadata = dict(module_metadata)


 def _CollectAllowedImportsFromBuildMetadata(build_metadata_filename):
   allowed_imports = set()
   processed_deps = set()

   def collect(metadata_filename):
     processed_deps.add(metadata_filename)
     with open(metadata_filename) as f:
       metadata = json.load(f)
       allowed_imports.update(
           map(os.path.normcase, map(os.path.normpath, metadata['sources'])))
       for dep_metadata in metadata['deps']:
         if dep_metadata not in processed_deps:
           collect(dep_metadata)

   collect(build_metadata_filename)
   return allowed_imports


 # multiprocessing helper.
 def _ParseAstHelper(mojom_abspath, enabled_features):
   with codecs.open(mojom_abspath, encoding='utf-8') as f:
     ast = parser.Parse(f.read(), mojom_abspath)
     conditional_features.RemoveDisabledDefinitions(ast, enabled_features)
     return mojom_abspath, ast


 # multiprocessing helper.
 def _SerializeHelper(mojom_abspath, mojom_path):
   module_path = os.path.join(_SerializeHelper.output_root_path,
                              _GetModuleFilename(mojom_path))
   module_dir = os.path.dirname(module_path)
   if not os.path.exists(module_dir):
     try:
       # Python 2 doesn't support exist_ok on makedirs(), so we just ignore
       # that failure if it happens. It's possible during build due to races
       # among build steps with module outputs in the same directory.
       os.makedirs(module_dir)
     except OSError as e:
       if e.errno != errno.EEXIST:
         raise
   with open(module_path, 'wb') as f:
     _SerializeHelper.loaded_modules[mojom_abspath].Dump(f)


 class _ExceptionWrapper:
   def __init__(self):
     # Do not capture exception object to ensure pickling works.
     self.formatted_trace = traceback.format_exc()


 class _FuncWrapper:
   """Marshals exceptions and spreads args."""

   def __init__(self, func):
     self._func = func

   def __call__(self, args):
     # multiprocessing does not gracefully handle excptions.
     # https://crbug.com/1219044
     try:
       return self._func(*args)
     except:  # pylint: disable=bare-except
       return _ExceptionWrapper()


 def _Shard(target_func, arg_list, processes=None):
   arg_list = list(arg_list)
   if processes is None:
     processes = multiprocessing.cpu_count()
   # Seems optimal to have each process perform at least 2 tasks.
   processes = min(processes, len(arg_list) // 2)

   if sys.platform == 'win32':
     # TODO(crbug.com/1190269) - we can't use more than 56
     # cores on Windows or Python3 may hang.
     processes = min(processes, 56)

   # Don't spin up processes unless there is enough work to merit doing so.
   if not _ENABLE_MULTIPROCESSING or processes < 2:
     for arg_tuple in arg_list:
       yield target_func(*arg_tuple)
     return

   pool = multiprocessing.Pool(processes=processes)
   try:
     wrapped_func = _FuncWrapper(target_func)
     for result in pool.imap_unordered(wrapped_func, arg_list):
       if isinstance(result, _ExceptionWrapper):
         sys.stderr.write(result.formatted_trace)
         sys.exit(1)
       yield result
   finally:
     pool.close()
     pool.join()  # Needed on Windows to avoid WindowsError during terminate.
     pool.terminate()


 def _ParseMojoms(mojom_files,
                  input_root_paths,
                  output_root_path,
                  enabled_features,
                  module_metadata,
                  allowed_imports=None):
   """Parses a set of mojom files and produces serialized module outputs.

   Args:
     mojom_files: A list of mojom files to process. Paths must be absolute paths
         which fall within one of the input or output root paths.
     input_root_paths: A list of absolute filesystem paths which may be used to
         resolve relative mojom file paths.
     output_root_path: An absolute filesystem path which will service as the root
         for all emitted artifacts. Artifacts produced from a given mojom file
         are based on the mojom's relative path, rebased onto this path.
         Additionally, the script expects this root to contain already-generated
         modules for any transitive dependencies not listed in mojom_files.
     enabled_features: A list of enabled feature names, controlling which AST
         nodes are filtered by [EnableIf] attributes.
     module_metadata: A list of 2-tuples representing metadata key-value pairs to
         attach to each compiled module output.

   Returns:
     None.

     Upon completion, a mojom-module file will be saved for each input mojom.
   """
   assert input_root_paths
   assert output_root_path

   loaded_mojom_asts = {}
   loaded_modules = {}
   input_dependencies = defaultdict(set)
   mojom_files_to_parse = dict((os.path.normcase(abs_path),
                                _RebaseAbsolutePath(abs_path, input_root_paths))
                               for abs_path in mojom_files)
   abs_paths = dict(
       (path, abs_path) for abs_path, path in mojom_files_to_parse.items())

   logging.info('Parsing %d .mojom into ASTs', len(mojom_files_to_parse))
   map_args = ((mojom_abspath, enabled_features)
               for mojom_abspath in mojom_files_to_parse)
   for mojom_abspath, ast in _Shard(_ParseAstHelper, map_args):
     loaded_mojom_asts[mojom_abspath] = ast

   logging.info('Processing dependencies')
   for mojom_abspath, ast in sorted(loaded_mojom_asts.items()):
     invalid_imports = []
     for imp in ast.import_list:
       import_abspath = _ResolveRelativeImportPath(imp.import_filename,
                                                   input_root_paths)
       if allowed_imports and import_abspath not in allowed_imports:
         invalid_imports.append(imp.import_filename)

       abs_paths[imp.import_filename] = import_abspath
       if import_abspath in mojom_files_to_parse:
         # This import is in the input list, so we're going to translate it
         # into a module below; however it's also a dependency of another input
         # module. We retain record of dependencies to help with input
         # processing later.
         input_dependencies[mojom_abspath].add(
             (import_abspath, imp.import_filename))
       elif import_abspath not in loaded_modules:
         # We have an import that isn't being parsed right now. It must already
         # be parsed and have a module file sitting in a corresponding output
         # location.
         module_path = _GetModuleFilename(imp.import_filename)
         module_abspath = _ResolveRelativeImportPath(module_path,
                                                     [output_root_path])
         with open(module_abspath, 'rb') as module_file:
           loaded_modules[import_abspath] = module.Module.Load(module_file)

     if invalid_imports:
       raise ValueError(
           '\nThe file %s imports the following files not allowed by build '
           'dependencies:\n\n%s\n' % (mojom_abspath, '\n'.join(invalid_imports)))
   logging.info('Loaded %d modules from dependencies', len(loaded_modules))

   # At this point all transitive imports not listed as inputs have been loaded
   # and we have a complete dependency tree of the unprocessed inputs. Now we can
   # load all the inputs, resolving dependencies among them recursively as we go.
   logging.info('Ensuring inputs are loaded')
   num_existing_modules_loaded = len(loaded_modules)
   for mojom_abspath, mojom_path in mojom_files_to_parse.items():
     _EnsureInputLoaded(mojom_abspath, mojom_path, abs_paths, loaded_mojom_asts,
                        input_dependencies, loaded_modules, module_metadata)
   assert (num_existing_modules_loaded +
           len(mojom_files_to_parse) == len(loaded_modules))

   # Now we have fully translated modules for every input and every transitive
   # dependency. We can dump the modules to disk for other tools to use.
   logging.info('Serializing %d modules', len(mojom_files_to_parse))

   # Windows does not use fork() for multiprocessing, so we'd need to pass
   # loaded_module via IPC rather than via globals. Doing so is slower than not
   # using multiprocessing.
   _SerializeHelper.loaded_modules = loaded_modules
   _SerializeHelper.output_root_path = output_root_path
   # Doesn't seem to help past 4. Perhaps IO bound here?
   processes = 4 if _MULTIPROCESSING_USES_FORK else 0
   map_args = mojom_files_to_parse.items()
   for _ in _Shard(_SerializeHelper, map_args, processes=processes):
     pass


 def Run(command_line):
   debug_logging = os.environ.get('MOJOM_PARSER_DEBUG', '0') != '0'
   logging.basicConfig(level=logging.DEBUG if debug_logging else logging.WARNING,
                       format='%(levelname).1s %(relativeCreated)6d %(message)s')
   logging.info('Started (%s)', os.path.basename(sys.argv[0]))

   arg_parser = argparse.ArgumentParser(
       description="""
 Parses one or more mojom files and produces corresponding module outputs fully
 describing the definitions therein. The output is exhaustive, stable, and
 sufficient for another tool to consume and emit e.g. usable language
 bindings based on the original mojoms.""",
       epilog="""
 Note that each transitive import dependency reachable from the input mojoms must
 either also be listed as an input or must have its corresponding compiled module
 already present in the provided output root.""")

   arg_parser.add_argument(
       '--input-root',
       default=[],
       action='append',
       metavar='ROOT',
       dest='input_root_paths',
       help='Adds ROOT to the set of root paths against which relative input '
       'paths should be resolved. Provided root paths are always searched '
       'in order from longest absolute path to shortest.')
   arg_parser.add_argument(
       '--output-root',
       action='store',
       required=True,
       dest='output_root_path',
       metavar='ROOT',
       help='Use ROOT as the root path in which the parser should emit compiled '
       'modules for each processed input mojom. The path of emitted module is '
       'based on the relative input path, rebased onto this root. Note that '
       'ROOT is also searched for existing modules of any transitive imports '
       'which were not included in the set of inputs.')
   arg_parser.add_argument(
       '--mojoms',
       nargs='+',
       dest='mojom_files',
       default=[],
       metavar='MOJOM_FILE',
       help='Input mojom filename(s). Each filename must be either an absolute '
       'path which falls within one of the given input or output roots, or a '
       'relative path the parser will attempt to resolve using each of those '
       'roots in unspecified order.')
   arg_parser.add_argument(
       '--mojom-file-list',
       action='store',
       metavar='LIST_FILENAME',
       help='Input file whose contents are a list of mojoms to process. This '
       'may be provided in lieu of --mojoms to avoid hitting command line '
       'length limtations')
   arg_parser.add_argument(
       '--enable-feature',
       dest='enabled_features',
       default=[],
       action='append',
       metavar='FEATURE',
       help='Enables a named feature when parsing the given mojoms. Features '
       'are identified by arbitrary string values. Specifying this flag with a '
       'given FEATURE name will cause the parser to process any syntax elements '
       'tagged with an [EnableIf=FEATURE] attribute. If this flag is not '
       'provided for a given FEATURE, such tagged elements are discarded by the '
       'parser and will not be present in the compiled output.')
   arg_parser.add_argument(
       '--check-imports',
       dest='build_metadata_filename',
       action='store',
       metavar='METADATA_FILENAME',
       help='Instructs the parser to check imports against a set of allowed '
       'imports. Allowed imports are based on build metadata within '
       'METADATA_FILENAME. This is a JSON file with a `sources` key listing '
       'paths to the set of input mojom files being processed by this parser '
       'run, and a `deps` key listing paths to metadata files for any '
       'dependencies of these inputs. This feature can be used to implement '
       'build-time dependency checking for mojom imports, where each build '
       'metadata file corresponds to a build target in the dependency graph of '
       'a typical build system.')
   arg_parser.add_argument(
       '--add-module-metadata',
       dest='module_metadata',
       default=[],
       action='append',
       metavar='KEY=VALUE',
       help='Adds a metadata key-value pair to the output module. This can be '
       'used by build toolchains to augment parsed mojom modules with product-'
       'specific metadata for later extraction and use by custom bindings '
       'generators.')

   args, _ = arg_parser.parse_known_args(command_line)
   if args.mojom_file_list:
     with open(args.mojom_file_list) as f:
       args.mojom_files.extend(f.read().split())

   if not args.mojom_files:
     raise ValueError(
         'Must list at least one mojom file via --mojoms or --mojom-file-list')

   mojom_files = list(map(os.path.abspath, args.mojom_files))
   input_roots = list(map(os.path.abspath, args.input_root_paths))
   output_root = os.path.abspath(args.output_root_path)

   if args.build_metadata_filename:
     allowed_imports = _CollectAllowedImportsFromBuildMetadata(
         args.build_metadata_filename)
   else:
     allowed_imports = None

   module_metadata = list(
       map(lambda kvp: tuple(kvp.split('=')), args.module_metadata))
   _ParseMojoms(mojom_files, input_roots, output_root, args.enabled_features,
                module_metadata, allowed_imports)
   logging.info('Finished')
   # Exit without running GC, which can save multiple seconds due the large
   # number of object created.
   os._exit(0)


 if __name__ == '__main__':
   Run(sys.argv[1:])
	#!/usr/bin/env python
	# 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.
	"""Parses mojom IDL files.

	This script parses one or more input mojom files and produces corresponding
	module files fully describing the definitions contained within each mojom. The
	module data is pickled and can be easily consumed by other tools to, e.g.,
	generate usable language bindings.
	"""

	import argparse
	import builtins
	import codecs
	import errno
	import json
	import logging
	import multiprocessing
	import os
	import os.path
	import sys
	import traceback
	from collections import defaultdict

	from mojom.generate import module
	from mojom.generate import translate
	from mojom.parse import parser
	from mojom.parse import conditional_features


	# Disable this for easier debugging.
	# In Python 2, subprocesses just hang when exceptions are thrown :(.
	_ENABLE_MULTIPROCESSING = sys.version_info[0] > 2

	if sys.version_info < (3, 4):
	_MULTIPROCESSING_USES_FORK = sys.platform.startswith('linux')
	else:
	# https://docs.python.org/3/library/multiprocessing.html#:~:text=bpo-33725
	if __name__ == '__main__' and sys.platform == 'darwin':
	multiprocessing.set_start_method('fork')
	_MULTIPROCESSING_USES_FORK = multiprocessing.get_start_method() == 'fork'


	def _ResolveRelativeImportPath(path, roots):
	"""Attempts to resolve a relative import path against a set of possible roots.

	Args:
	path: The relative import path to resolve.
	roots: A list of absolute paths which will be checked in descending length
	order for a match against path.

	Returns:
	A normalized absolute path combining one of the roots with the input path if
	and only if such a file exists.

	Raises:
	ValueError: The path could not be resolved against any of the given roots.
	"""
	for root in reversed(sorted(roots, key=len)):
	abs_path = os.path.join(root, path)
	if os.path.isfile(abs_path):
	return os.path.normcase(os.path.normpath(abs_path))

	raise ValueError('"%s" does not exist in any of %s' % (path, roots))


	def _RebaseAbsolutePath(path, roots):
	"""Rewrites an absolute file path as relative to an absolute directory path in
	roots.

	Args:
	path: The absolute path of an existing file.
	roots: A list of absolute directory paths. The given path argument must fall
	within one of these directories.

	Returns:
	A path equivalent to the input path, but relative to one of the provided
	roots. If the input path falls within multiple roots, the longest root is
	chosen (and thus the shortest relative path is returned).

	Paths returned by this method always use forward slashes as a separator to
	mirror mojom import syntax.

	Raises:
	ValueError if the given path does not fall within any of the listed roots.
	"""
	assert os.path.isabs(path)
	assert os.path.isfile(path)
	assert all(map(os.path.isabs, roots))

	sorted_roots = list(reversed(sorted(roots, key=len)))

	def try_rebase_path(path, root):
	head, rebased_path = os.path.split(path)
	while head != root:
	head, tail = os.path.split(head)
	if not tail:
	return None
	rebased_path = os.path.join(tail, rebased_path)
	return rebased_path

	for root in sorted_roots:
	relative_path = try_rebase_path(path, root)
	if relative_path:
	# TODO(crbug.com/953884): Use pathlib for this kind of thing once we're
	# fully migrated to Python 3.
	return relative_path.replace('\\', '/')

	raise ValueError('%s does not fall within any of %s' % (path, sorted_roots))


	def _GetModuleFilename(mojom_filename):
	return mojom_filename + '-module'


	def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
	dependencies, loaded_modules, module_metadata):
	"""Recursively ensures that a module and its dependencies are loaded.

	Args:
	mojom_abspath: An absolute file path pointing to a mojom file to load.
	module_path: The relative path used to identify mojom_abspath.
	abs_paths: A mapping from module paths to absolute file paths for all
	inputs given to this execution of the script.
	asts: A map from each input mojom's absolute path to its parsed AST.
	dependencies: A mapping of which input mojoms depend on each other, indexed
	by absolute file path.
	loaded_modules: A mapping of all modules loaded so far, including non-input
	modules that were pulled in as transitive dependencies of the inputs.
	module_metadata: Metadata to be attached to every module loaded by this
	helper.

	Returns:
	None

	On return, loaded_modules will be populated with the loaded input mojom's
	Module as well as the Modules of all of its transitive dependencies."""

	if mojom_abspath in loaded_modules:
	# Already done.
	return

	for dep_abspath, dep_path in sorted(dependencies[mojom_abspath]):
	if dep_abspath not in loaded_modules:
	_EnsureInputLoaded(dep_abspath, dep_path, abs_paths, asts, dependencies,
	loaded_modules, module_metadata)

	imports = {}
	for imp in asts[mojom_abspath].import_list:
	path = imp.import_filename
	imports[path] = loaded_modules[abs_paths[path]]
	loaded_modules[mojom_abspath] = translate.OrderedModule(
	asts[mojom_abspath], module_path, imports)
	loaded_modules[mojom_abspath].metadata = dict(module_metadata)


	def _CollectAllowedImportsFromBuildMetadata(build_metadata_filename):
	allowed_imports = set()
	processed_deps = set()

	def collect(metadata_filename):
	processed_deps.add(metadata_filename)
	with open(metadata_filename) as f:
	metadata = json.load(f)
	allowed_imports.update(
	map(os.path.normcase, map(os.path.normpath, metadata['sources'])))
	for dep_metadata in metadata['deps']:
	if dep_metadata not in processed_deps:
	collect(dep_metadata)

	collect(build_metadata_filename)
	return allowed_imports


	# multiprocessing helper.
	def _ParseAstHelper(mojom_abspath, enabled_features):
	with codecs.open(mojom_abspath, encoding='utf-8') as f:
	ast = parser.Parse(f.read(), mojom_abspath)
	conditional_features.RemoveDisabledDefinitions(ast, enabled_features)
	return mojom_abspath, ast


	# multiprocessing helper.
	def _SerializeHelper(mojom_abspath, mojom_path):
	module_path = os.path.join(_SerializeHelper.output_root_path,
	_GetModuleFilename(mojom_path))
	module_dir = os.path.dirname(module_path)
	if not os.path.exists(module_dir):
	try:
	# Python 2 doesn't support exist_ok on makedirs(), so we just ignore
	# that failure if it happens. It's possible during build due to races
	# among build steps with module outputs in the same directory.
	os.makedirs(module_dir)
	except OSError as e:
	if e.errno != errno.EEXIST:
	raise
	with open(module_path, 'wb') as f:
	_SerializeHelper.loaded_modules[mojom_abspath].Dump(f)


	class _ExceptionWrapper:
	def __init__(self):
	# Do not capture exception object to ensure pickling works.
	self.formatted_trace = traceback.format_exc()


	class _FuncWrapper:
	"""Marshals exceptions and spreads args."""

	def __init__(self, func):
	self._func = func

	def __call__(self, args):
	# multiprocessing does not gracefully handle excptions.
	# https://crbug.com/1219044
	try:
	return self._func(*args)
	except: # pylint: disable=bare-except
	return _ExceptionWrapper()


	def _Shard(target_func, arg_list, processes=None):
	arg_list = list(arg_list)
	if processes is None:
	processes = multiprocessing.cpu_count()
	# Seems optimal to have each process perform at least 2 tasks.
	processes = min(processes, len(arg_list) // 2)

	if sys.platform == 'win32':
	# TODO(crbug.com/1190269) - we can't use more than 56
	# cores on Windows or Python3 may hang.
	processes = min(processes, 56)

	# Don't spin up processes unless there is enough work to merit doing so.
	if not _ENABLE_MULTIPROCESSING or processes < 2:
	for arg_tuple in arg_list:
	yield target_func(*arg_tuple)
	return

	pool = multiprocessing.Pool(processes=processes)
	try:
	wrapped_func = _FuncWrapper(target_func)
	for result in pool.imap_unordered(wrapped_func, arg_list):
	if isinstance(result, _ExceptionWrapper):
	sys.stderr.write(result.formatted_trace)
	sys.exit(1)
	yield result
	finally:
	pool.close()
	pool.join() # Needed on Windows to avoid WindowsError during terminate.
	pool.terminate()


	def _ParseMojoms(mojom_files,
	input_root_paths,
	output_root_path,
	enabled_features,
	module_metadata,
	allowed_imports=None):
	"""Parses a set of mojom files and produces serialized module outputs.

	Args:
	mojom_files: A list of mojom files to process. Paths must be absolute paths
	which fall within one of the input or output root paths.
	input_root_paths: A list of absolute filesystem paths which may be used to
	resolve relative mojom file paths.
	output_root_path: An absolute filesystem path which will service as the root
	for all emitted artifacts. Artifacts produced from a given mojom file
	are based on the mojom's relative path, rebased onto this path.
	Additionally, the script expects this root to contain already-generated
	modules for any transitive dependencies not listed in mojom_files.
	enabled_features: A list of enabled feature names, controlling which AST
	nodes are filtered by [EnableIf] attributes.
	module_metadata: A list of 2-tuples representing metadata key-value pairs to
	attach to each compiled module output.

	Returns:
	None.

	Upon completion, a mojom-module file will be saved for each input mojom.
	"""
	assert input_root_paths
	assert output_root_path

	loaded_mojom_asts = {}
	loaded_modules = {}
	input_dependencies = defaultdict(set)
	mojom_files_to_parse = dict((os.path.normcase(abs_path),
	_RebaseAbsolutePath(abs_path, input_root_paths))
	for abs_path in mojom_files)
	abs_paths = dict(
	(path, abs_path) for abs_path, path in mojom_files_to_parse.items())

	logging.info('Parsing %d .mojom into ASTs', len(mojom_files_to_parse))
	map_args = ((mojom_abspath, enabled_features)
	for mojom_abspath in mojom_files_to_parse)
	for mojom_abspath, ast in _Shard(_ParseAstHelper, map_args):
	loaded_mojom_asts[mojom_abspath] = ast

	logging.info('Processing dependencies')
	for mojom_abspath, ast in sorted(loaded_mojom_asts.items()):
	invalid_imports = []
	for imp in ast.import_list:
	import_abspath = _ResolveRelativeImportPath(imp.import_filename,
	input_root_paths)
	if allowed_imports and import_abspath not in allowed_imports:
	invalid_imports.append(imp.import_filename)

	abs_paths[imp.import_filename] = import_abspath
	if import_abspath in mojom_files_to_parse:
	# This import is in the input list, so we're going to translate it
	# into a module below; however it's also a dependency of another input
	# module. We retain record of dependencies to help with input
	# processing later.
	input_dependencies[mojom_abspath].add(
	(import_abspath, imp.import_filename))
	elif import_abspath not in loaded_modules:
	# We have an import that isn't being parsed right now. It must already
	# be parsed and have a module file sitting in a corresponding output
	# location.
	module_path = _GetModuleFilename(imp.import_filename)
	module_abspath = _ResolveRelativeImportPath(module_path,
	[output_root_path])
	with open(module_abspath, 'rb') as module_file:
	loaded_modules[import_abspath] = module.Module.Load(module_file)

	if invalid_imports:
	raise ValueError(
	'\nThe file %s imports the following files not allowed by build '
	'dependencies:\n\n%s\n' % (mojom_abspath, '\n'.join(invalid_imports)))
	logging.info('Loaded %d modules from dependencies', len(loaded_modules))

	# At this point all transitive imports not listed as inputs have been loaded
	# and we have a complete dependency tree of the unprocessed inputs. Now we can
	# load all the inputs, resolving dependencies among them recursively as we go.
	logging.info('Ensuring inputs are loaded')
	num_existing_modules_loaded = len(loaded_modules)
	for mojom_abspath, mojom_path in mojom_files_to_parse.items():
	_EnsureInputLoaded(mojom_abspath, mojom_path, abs_paths, loaded_mojom_asts,
	input_dependencies, loaded_modules, module_metadata)
	assert (num_existing_modules_loaded +
	len(mojom_files_to_parse) == len(loaded_modules))

	# Now we have fully translated modules for every input and every transitive
	# dependency. We can dump the modules to disk for other tools to use.
	logging.info('Serializing %d modules', len(mojom_files_to_parse))

	# Windows does not use fork() for multiprocessing, so we'd need to pass
	# loaded_module via IPC rather than via globals. Doing so is slower than not
	# using multiprocessing.
	_SerializeHelper.loaded_modules = loaded_modules
	_SerializeHelper.output_root_path = output_root_path
	# Doesn't seem to help past 4. Perhaps IO bound here?
	processes = 4 if _MULTIPROCESSING_USES_FORK else 0
	map_args = mojom_files_to_parse.items()
	for _ in _Shard(_SerializeHelper, map_args, processes=processes):
	pass


	def Run(command_line):
	debug_logging = os.environ.get('MOJOM_PARSER_DEBUG', '0') != '0'
	logging.basicConfig(level=logging.DEBUG if debug_logging else logging.WARNING,
	format='%(levelname).1s %(relativeCreated)6d %(message)s')
	logging.info('Started (%s)', os.path.basename(sys.argv[0]))

	arg_parser = argparse.ArgumentParser(
	description="""
	Parses one or more mojom files and produces corresponding module outputs fully
	describing the definitions therein. The output is exhaustive, stable, and
	sufficient for another tool to consume and emit e.g. usable language
	bindings based on the original mojoms.""",
	epilog="""
	Note that each transitive import dependency reachable from the input mojoms must
	either also be listed as an input or must have its corresponding compiled module
	already present in the provided output root.""")

	arg_parser.add_argument(
	'--input-root',
	default=[],
	action='append',
	metavar='ROOT',
	dest='input_root_paths',
	help='Adds ROOT to the set of root paths against which relative input '
	'paths should be resolved. Provided root paths are always searched '
	'in order from longest absolute path to shortest.')
	arg_parser.add_argument(
	'--output-root',
	action='store',
	required=True,
	dest='output_root_path',
	metavar='ROOT',
	help='Use ROOT as the root path in which the parser should emit compiled '
	'modules for each processed input mojom. The path of emitted module is '
	'based on the relative input path, rebased onto this root. Note that '
	'ROOT is also searched for existing modules of any transitive imports '
	'which were not included in the set of inputs.')
	arg_parser.add_argument(
	'--mojoms',
	nargs='+',
	dest='mojom_files',
	default=[],
	metavar='MOJOM_FILE',
	help='Input mojom filename(s). Each filename must be either an absolute '
	'path which falls within one of the given input or output roots, or a '
	'relative path the parser will attempt to resolve using each of those '
	'roots in unspecified order.')
	arg_parser.add_argument(
	'--mojom-file-list',
	action='store',
	metavar='LIST_FILENAME',
	help='Input file whose contents are a list of mojoms to process. This '
	'may be provided in lieu of --mojoms to avoid hitting command line '
	'length limtations')
	arg_parser.add_argument(
	'--enable-feature',
	dest='enabled_features',
	default=[],
	action='append',
	metavar='FEATURE',
	help='Enables a named feature when parsing the given mojoms. Features '
	'are identified by arbitrary string values. Specifying this flag with a '
	'given FEATURE name will cause the parser to process any syntax elements '
	'tagged with an [EnableIf=FEATURE] attribute. If this flag is not '
	'provided for a given FEATURE, such tagged elements are discarded by the '
	'parser and will not be present in the compiled output.')
	arg_parser.add_argument(
	'--check-imports',
	dest='build_metadata_filename',
	action='store',
	metavar='METADATA_FILENAME',
	help='Instructs the parser to check imports against a set of allowed '
	'imports. Allowed imports are based on build metadata within '
	'METADATA_FILENAME. This is a JSON file with a `sources` key listing '
	'paths to the set of input mojom files being processed by this parser '
	'run, and a `deps` key listing paths to metadata files for any '
	'dependencies of these inputs. This feature can be used to implement '
	'build-time dependency checking for mojom imports, where each build '
	'metadata file corresponds to a build target in the dependency graph of '
	'a typical build system.')
	arg_parser.add_argument(
	'--add-module-metadata',
	dest='module_metadata',
	default=[],
	action='append',
	metavar='KEY=VALUE',
	help='Adds a metadata key-value pair to the output module. This can be '
	'used by build toolchains to augment parsed mojom modules with product-'
	'specific metadata for later extraction and use by custom bindings '
	'generators.')

	args, _ = arg_parser.parse_known_args(command_line)
	if args.mojom_file_list:
	with open(args.mojom_file_list) as f:
	args.mojom_files.extend(f.read().split())

	if not args.mojom_files:
	raise ValueError(
	'Must list at least one mojom file via --mojoms or --mojom-file-list')

	mojom_files = list(map(os.path.abspath, args.mojom_files))
	input_roots = list(map(os.path.abspath, args.input_root_paths))
	output_root = os.path.abspath(args.output_root_path)

	if args.build_metadata_filename:
	allowed_imports = _CollectAllowedImportsFromBuildMetadata(
	args.build_metadata_filename)
	else:
	allowed_imports = None

	module_metadata = list(
	map(lambda kvp: tuple(kvp.split('=')), args.module_metadata))
	_ParseMojoms(mojom_files, input_roots, output_root, args.enabled_features,
	module_metadata, allowed_imports)
	logging.info('Finished')
	# Exit without running GC, which can save multiple seconds due the large
	# number of object created.
	os._exit(0)


	if __name__ == '__main__':
	Run(sys.argv[1:])