utils: ipc: Update mojo

Update mojo from the Chromium repository. The commit from which this was
taken is:

9c138d992bfc1fb8f4f7bcf58d00bf19c219e4e2 "Updating trunk VERSION from
4523.0 to 4524.0"

The update-mojo.sh script was used for this update.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=34
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

9 files changed, 376 insertions, 100 deletions

diff --git a/utils/ipc/mojo/public/tools/mojom/README.md b/utils/ipc/mojo/public/tools/mojom/README.md
index 6a4ff78a..e5d17ab0 100644
--- a/utils/ipc/mojo/public/tools/mojom/README.md
+++ b/utils/ipc/mojo/public/tools/mojom/README.md
@@ -3,7 +3,7 @@
 The Mojom format is an interface definition language (IDL) for describing
 interprocess communication (IPC) messages and data types for use with the
 low-level cross-platform
-[Mojo IPC library](https://chromium.googlesource.com/chromium/src/+/master/mojo/public/c/system/README.md).
+[Mojo IPC library](https://chromium.googlesource.com/chromium/src/+/main/mojo/public/c/system/README.md).
 
 This directory consists of a `mojom` Python module, its tests, and supporting
 command-line tools. The Python module implements the parser used by the
diff --git a/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py b/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py
index 7e746112..08bd672f 100755
--- a/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py
+++ b/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py
@@ -131,7 +131,8 @@ def _ValidateDelta(root, delta):
           'renamed, please add a [RenamedFrom] attribute to the new type. This '
           'can be deleted by a subsequent change.' % qualified_name)
 
-    if not new_types[new_name].IsBackwardCompatible(kind):
+    checker = module.BackwardCompatibilityChecker()
+    if not checker.IsBackwardCompatible(new_types[new_name], kind):
       raise Exception('Stable type %s appears to have changed in a way which '
                       'breaks backward-compatibility. Please fix!\n\nIf you '
                       'believe this assessment to be incorrect, please file a '
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn b/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn
index 7416ef19..51facc0c 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn
@@ -8,7 +8,6 @@ group("mojom") {
     "error.py",
     "fileutil.py",
     "generate/__init__.py",
-    "generate/constant_resolver.py",
     "generate/generator.py",
     "generate/module.py",
     "generate/pack.py",
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py
index de62260a..4a1c73fc 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py
@@ -136,9 +136,14 @@ class Stylizer(object):
 
 def WriteFile(contents, full_path):
   # If |contents| is same with the file content, we skip updating.
+  if not isinstance(contents, bytes):
+    data = contents.encode('utf8')
+  else:
+    data = contents
+
   if os.path.isfile(full_path):
     with open(full_path, 'rb') as destination_file:
-      if destination_file.read() == contents:
+      if destination_file.read() == data:
         return
 
   # Make sure the containing directory exists.
@@ -146,11 +151,8 @@ def WriteFile(contents, full_path):
   fileutil.EnsureDirectoryExists(full_dir)
 
   # Dump the data to disk.
-  with open(full_path, "wb") as f:
-    if not isinstance(contents, bytes):
-      f.write(contents.encode('utf-8'))
-    else:
-      f.write(contents)
+  with open(full_path, 'wb') as f:
+    f.write(data)
 
 
 def AddComputedData(module):
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py
index 8547ff64..9bdb28e0 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py
@@ -12,7 +12,33 @@
 # method = interface.AddMethod('Tat', 0)
 # method.AddParameter('baz', 0, mojom.INT32)
 
-import pickle
+import sys
+if sys.version_info.major == 2:
+  import cPickle as pickle
+else:
+  import pickle
+from uuid import UUID
+
+
+class BackwardCompatibilityChecker(object):
+  """Used for memoization while recursively checking two type definitions for
+  backward-compatibility."""
+
+  def __init__(self):
+    self._cache = {}
+
+  def IsBackwardCompatible(self, new_kind, old_kind):
+    key = (new_kind, old_kind)
+    result = self._cache.get(key)
+    if result is None:
+      # Assume they're compatible at first to effectively ignore recursive
+      # checks between these types, e.g. if both kinds are a struct or union
+      # that references itself in a field.
+      self._cache[key] = True
+      result = new_kind.IsBackwardCompatible(old_kind, self)
+      self._cache[key] = result
+    return result
+
 
 # We use our own version of __repr__ when displaying the AST, as the
 # AST currently doesn't capture which nodes are reference (e.g. to
@@ -114,6 +140,10 @@ class Kind(object):
     # during a subsequent run of the parser.
     return hash((self.spec, self.parent_kind))
 
+  # pylint: disable=unused-argument
+  def IsBackwardCompatible(self, rhs, checker):
+    return self == rhs
+
 
 class ReferenceKind(Kind):
   """ReferenceKind represents pointer and handle types.
@@ -195,6 +225,10 @@ class ReferenceKind(Kind):
   def __hash__(self):
     return hash((super(ReferenceKind, self).__hash__(), self.is_nullable))
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return (super(ReferenceKind, self).IsBackwardCompatible(rhs, checker)
+            and self.is_nullable == rhs.is_nullable)
+
 
 # Initialize the set of primitive types. These can be accessed by clients.
 BOOL = Kind('b')
@@ -253,9 +287,13 @@ PRIMITIVES = (
 )
 
 ATTRIBUTE_MIN_VERSION = 'MinVersion'
+ATTRIBUTE_DEFAULT = 'Default'
 ATTRIBUTE_EXTENSIBLE = 'Extensible'
+ATTRIBUTE_NO_INTERRUPT = 'NoInterrupt'
 ATTRIBUTE_STABLE = 'Stable'
 ATTRIBUTE_SYNC = 'Sync'
+ATTRIBUTE_UNLIMITED_SIZE = 'UnlimitedSize'
+ATTRIBUTE_UUID = 'Uuid'
 
 
 class NamedValue(object):
@@ -274,6 +312,9 @@ class NamedValue(object):
             and (self.parent_kind, self.mojom_name) == (rhs.parent_kind,
                                                         rhs.mojom_name))
 
+  def __hash__(self):
+    return hash((self.parent_kind, self.mojom_name))
+
 
 class BuiltinValue(object):
   def __init__(self, value):
@@ -368,21 +409,19 @@ class Field(object):
 
 
 class StructField(Field):
-  pass
+  def __hash__(self):
+    return super(Field, self).__hash__()
 
 
 class UnionField(Field):
   pass
 
 
-def _IsFieldBackwardCompatible(new_field, old_field):
+def _IsFieldBackwardCompatible(new_field, old_field, checker):
   if (new_field.min_version or 0) != (old_field.min_version or 0):
     return False
 
-  if isinstance(new_field.kind, (Enum, Struct, Union)):
-    return new_field.kind.IsBackwardCompatible(old_field.kind)
-
-  return new_field.kind == old_field.kind
+  return checker.IsBackwardCompatible(new_field.kind, old_field.kind)
 
 
 class Struct(ReferenceKind):
@@ -457,7 +496,7 @@ class Struct(ReferenceKind):
     for constant in self.constants:
       constant.Stylize(stylizer)
 
-  def IsBackwardCompatible(self, older_struct):
+  def IsBackwardCompatible(self, older_struct, checker):
     """This struct is backward-compatible with older_struct if and only if all
     of the following conditions hold:
       - Any newly added field is tagged with a [MinVersion] attribute specifying
@@ -496,7 +535,7 @@ class Struct(ReferenceKind):
       old_field = old_fields[ordinal]
       if (old_field.min_version or 0) > max_old_min_version:
         max_old_min_version = old_field.min_version
-      if not _IsFieldBackwardCompatible(new_field, old_field):
+      if not _IsFieldBackwardCompatible(new_field, old_field, checker):
         # Type or min-version mismatch between old and new versions of the same
         # ordinal field.
         return False
@@ -590,7 +629,7 @@ class Union(ReferenceKind):
     for field in self.fields:
       field.Stylize(stylizer)
 
-  def IsBackwardCompatible(self, older_union):
+  def IsBackwardCompatible(self, older_union, checker):
     """This union is backward-compatible with older_union if and only if all
     of the following conditions hold:
       - Any newly added field is tagged with a [MinVersion] attribute specifying
@@ -623,7 +662,7 @@ class Union(ReferenceKind):
       if not new_field:
         # A field was removed, which is not OK.
         return False
-      if not _IsFieldBackwardCompatible(new_field, old_field):
+      if not _IsFieldBackwardCompatible(new_field, old_field, checker):
         # An field changed its type or MinVersion, which is not OK.
         return False
       old_min_version = old_field.min_version or 0
@@ -703,6 +742,10 @@ class Array(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return (isinstance(rhs, Array) and self.length == rhs.length
+            and checker.IsBackwardCompatible(self.kind, rhs.kind))
+
 
 class Map(ReferenceKind):
   """A map.
@@ -747,6 +790,11 @@ class Map(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return (isinstance(rhs, Map)
+            and checker.IsBackwardCompatible(self.key_kind, rhs.key_kind)
+            and checker.IsBackwardCompatible(self.value_kind, rhs.value_kind))
+
 
 class PendingRemote(ReferenceKind):
   ReferenceKind.AddSharedProperty('kind')
@@ -768,6 +816,10 @@ class PendingRemote(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return (isinstance(rhs, PendingRemote)
+            and checker.IsBackwardCompatible(self.kind, rhs.kind))
+
 
 class PendingReceiver(ReferenceKind):
   ReferenceKind.AddSharedProperty('kind')
@@ -789,6 +841,10 @@ class PendingReceiver(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return isinstance(rhs, PendingReceiver) and checker.IsBackwardCompatible(
+        self.kind, rhs.kind)
+
 
 class PendingAssociatedRemote(ReferenceKind):
   ReferenceKind.AddSharedProperty('kind')
@@ -810,6 +866,11 @@ class PendingAssociatedRemote(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return isinstance(rhs,
+                      PendingAssociatedRemote) and checker.IsBackwardCompatible(
+                          self.kind, rhs.kind)
+
 
 class PendingAssociatedReceiver(ReferenceKind):
   ReferenceKind.AddSharedProperty('kind')
@@ -831,6 +892,11 @@ class PendingAssociatedReceiver(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return isinstance(
+        rhs, PendingAssociatedReceiver) and checker.IsBackwardCompatible(
+            self.kind, rhs.kind)
+
 
 class InterfaceRequest(ReferenceKind):
   ReferenceKind.AddSharedProperty('kind')
@@ -851,6 +917,10 @@ class InterfaceRequest(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return isinstance(rhs, InterfaceRequest) and checker.IsBackwardCompatible(
+        self.kind, rhs.kind)
+
 
 class AssociatedInterfaceRequest(ReferenceKind):
   ReferenceKind.AddSharedProperty('kind')
@@ -873,6 +943,11 @@ class AssociatedInterfaceRequest(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return isinstance(
+        rhs, AssociatedInterfaceRequest) and checker.IsBackwardCompatible(
+            self.kind, rhs.kind)
+
 
 class Parameter(object):
   def __init__(self,
@@ -976,6 +1051,16 @@ class Method(object):
     return self.attributes.get(ATTRIBUTE_SYNC) \
         if self.attributes else None
 
+  @property
+  def allow_interrupt(self):
+    return not self.attributes.get(ATTRIBUTE_NO_INTERRUPT) \
+        if self.attributes else True
+
+  @property
+  def unlimited_message_size(self):
+    return self.attributes.get(ATTRIBUTE_UNLIMITED_SIZE) \
+        if self.attributes else False
+
   def __eq__(self, rhs):
     return (isinstance(rhs, Method) and
             (self.mojom_name, self.ordinal, self.parameters,
@@ -1029,7 +1114,7 @@ class Interface(ReferenceKind):
     for constant in self.constants:
       constant.Stylize(stylizer)
 
-  def IsBackwardCompatible(self, older_interface):
+  def IsBackwardCompatible(self, older_interface, checker):
     """This interface is backward-compatible with older_interface if and only
     if all of the following conditions hold:
       - All defined methods in older_interface (when identified by ordinal) have
@@ -1067,8 +1152,8 @@ class Interface(ReferenceKind):
         # A method was removed, which is not OK.
         return False
 
-      if not new_method.param_struct.IsBackwardCompatible(
-          old_method.param_struct):
+      if not checker.IsBackwardCompatible(new_method.param_struct,
+                                          old_method.param_struct):
         # The parameter list is not backward-compatible, which is not OK.
         return False
 
@@ -1081,8 +1166,8 @@ class Interface(ReferenceKind):
         if new_method.response_param_struct is None:
           # A reply was removed from a message, which is not OK.
           return False
-        if not new_method.response_param_struct.IsBackwardCompatible(
-            old_method.response_param_struct):
+        if not checker.IsBackwardCompatible(new_method.response_param_struct,
+                                            old_method.response_param_struct):
           # The new message's reply is not backward-compatible with the old
           # message's reply, which is not OK.
           return False
@@ -1120,6 +1205,20 @@ class Interface(ReferenceKind):
                  self.attributes) == (rhs.mojom_name, rhs.methods, rhs.enums,
                                       rhs.constants, rhs.attributes))
 
+  @property
+  def uuid(self):
+    uuid_str = self.attributes.get(ATTRIBUTE_UUID) if self.attributes else None
+    if uuid_str is None:
+      return None
+
+    try:
+      u = UUID(uuid_str)
+    except:
+      raise ValueError('Invalid format for Uuid attribute on interface {}. '
+                       'Expected standard RFC 4122 string representation of '
+                       'a UUID.'.format(self.mojom_name))
+    return (int(u.hex[:16], 16), int(u.hex[16:], 16))
+
   def __hash__(self):
     return id(self)
 
@@ -1144,6 +1243,11 @@ class AssociatedInterface(ReferenceKind):
   def __hash__(self):
     return id(self)
 
+  def IsBackwardCompatible(self, rhs, checker):
+    return isinstance(rhs,
+                      AssociatedInterface) and checker.IsBackwardCompatible(
+                          self.kind, rhs.kind)
+
 
 class EnumField(object):
   def __init__(self,
@@ -1161,6 +1265,11 @@ class EnumField(object):
     self.name = stylizer.StylizeEnumField(self.mojom_name)
 
   @property
+  def default(self):
+    return self.attributes.get(ATTRIBUTE_DEFAULT, False) \
+        if self.attributes else False
+
+  @property
   def min_version(self):
     return self.attributes.get(ATTRIBUTE_MIN_VERSION) \
         if self.attributes else None
@@ -1186,6 +1295,7 @@ class Enum(Kind):
     self.attributes = attributes
     self.min_value = None
     self.max_value = None
+    self.default_field = None
 
   def Repr(self, as_ref=True):
     if as_ref:
@@ -1216,7 +1326,8 @@ class Enum(Kind):
       prefix = self.module.GetNamespacePrefix()
     return '%s%s' % (prefix, self.mojom_name)
 
-  def IsBackwardCompatible(self, older_enum):
+  # pylint: disable=unused-argument
+  def IsBackwardCompatible(self, older_enum, checker):
     """This enum is backward-compatible with older_enum if and only if one of
     the following conditions holds:
         - Neither enum is [Extensible] and both have the exact same set of valid
@@ -1250,9 +1361,10 @@ class Enum(Kind):
   def __eq__(self, rhs):
     return (isinstance(rhs, Enum) and
             (self.mojom_name, self.native_only, self.fields, self.attributes,
-             self.min_value,
-             self.max_value) == (rhs.mojom_name, rhs.native_only, rhs.fields,
-                                 rhs.attributes, rhs.min_value, rhs.max_value))
+             self.min_value, self.max_value,
+             self.default_field) == (rhs.mojom_name, rhs.native_only,
+                                     rhs.fields, rhs.attributes, rhs.min_value,
+                                     rhs.max_value, rhs.default_field))
 
   def __hash__(self):
     return id(self)
@@ -1272,6 +1384,7 @@ class Module(object):
     self.attributes = attributes
     self.imports = []
     self.imported_kinds = {}
+    self.metadata = {}
 
   def __repr__(self):
     # Gives us a decent __repr__ for modules.
@@ -1285,6 +1398,9 @@ class Module(object):
                                   rhs.imports, rhs.constants, rhs.enums,
                                   rhs.structs, rhs.unions, rhs.interfaces))
 
+  def __hash__(self):
+    return id(self)
+
   def Repr(self, as_ref=True):
     if as_ref:
       return '<%s path=%r mojom_namespace=%r>' % (
@@ -1555,6 +1671,13 @@ def HasSyncMethods(interface):
   return False
 
 
+def HasUninterruptableMethods(interface):
+  for method in interface.methods:
+    if not method.allow_interrupt:
+      return True
+  return False
+
+
 def ContainsHandlesOrInterfaces(kind):
   """Check if the kind contains any handles.
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py
index 7a300560..0da90058 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py
@@ -75,9 +75,8 @@ def PrecompileTemplates(generator_modules, output_dir):
                 os.path.dirname(module.__file__), generator.GetTemplatePrefix())
         ]))
     jinja_env.filters.update(generator.GetFilters())
-    jinja_env.compile_templates(
-        os.path.join(output_dir, "%s.zip" % generator.GetTemplatePrefix()),
-        extensions=["tmpl"],
-        zip="stored",
-        py_compile=True,
-        ignore_errors=False)
+    jinja_env.compile_templates(os.path.join(
+        output_dir, "%s.zip" % generator.GetTemplatePrefix()),
+                                extensions=["tmpl"],
+                                zip="stored",
+                                ignore_errors=False)
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py
index d6df3ca6..7580b780 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py
@@ -472,6 +472,9 @@ def _Method(module, parsed_method, interface):
                     "attribute. If no response parameters are needed, you "
                     "could use an empty response parameter list, i.e., "
                     "\"=> ()\".")
+  # And only methods with the [Sync] attribute can specify [NoInterrupt].
+  if not method.allow_interrupt and not method.sync:
+    raise Exception("Only [Sync] methods can be marked [NoInterrupt].")
 
   return method
 
@@ -592,6 +595,16 @@ def _Enum(module, parsed_enum, parent_kind):
         map(lambda field: _EnumField(module, enum, field),
             parsed_enum.enum_value_list))
     _ResolveNumericEnumValues(enum)
+    # TODO(https://crbug.com/731893): Require a default value to be
+    # specified.
+    for field in enum.fields:
+      if field.default:
+        if not enum.extensible:
+          raise Exception('Non-extensible enums may not specify a default')
+        if enum.default_field is not None:
+          raise Exception(
+              'Only one enumerator value may be specified as the default')
+        enum.default_field = field
 
   module.kinds[enum.spec] = enum
 
@@ -650,7 +663,9 @@ def _CollectReferencedKinds(module, all_defined_kinds):
     if mojom.IsMapKind(kind):
       return (extract_referenced_user_kinds(kind.key_kind) +
               extract_referenced_user_kinds(kind.value_kind))
-    if mojom.IsInterfaceRequestKind(kind) or mojom.IsAssociatedKind(kind):
+    if (mojom.IsInterfaceRequestKind(kind) or mojom.IsAssociatedKind(kind)
+        or mojom.IsPendingRemoteKind(kind)
+        or mojom.IsPendingReceiverKind(kind)):
       return [kind.kind]
     if mojom.IsStructKind(kind):
       return [kind]
@@ -678,12 +693,9 @@ def _CollectReferencedKinds(module, all_defined_kinds):
     for method in interface.methods:
       for param in itertools.chain(method.parameters or [],
                                    method.response_parameters or []):
-        if (mojom.IsStructKind(param.kind) or mojom.IsUnionKind(param.kind)
-            or mojom.IsEnumKind(param.kind)
-            or mojom.IsAnyInterfaceKind(param.kind)):
-          for referenced_kind in extract_referenced_user_kinds(param.kind):
-            sanitized_kind = sanitize_kind(referenced_kind)
-            referenced_user_kinds[sanitized_kind.spec] = sanitized_kind
+        for referenced_kind in extract_referenced_user_kinds(param.kind):
+          sanitized_kind = sanitize_kind(referenced_kind)
+          referenced_user_kinds[sanitized_kind.spec] = sanitized_kind
 
   return referenced_user_kinds
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom_parser.py b/utils/ipc/mojo/public/tools/mojom/mojom_parser.py
index 12adbfb9..eb90c825 100755
--- a/utils/ipc/mojo/public/tools/mojom/mojom_parser.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom_parser.py
@@ -14,6 +14,8 @@ import argparse
 import codecs
 import errno
 import json
+import logging
+import multiprocessing
 import os
 import os.path
 import sys
@@ -25,6 +27,19 @@ 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.
 
@@ -98,7 +113,7 @@ def _GetModuleFilename(mojom_filename):
 
 
 def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
-                       dependencies, loaded_modules):
+                       dependencies, loaded_modules, module_metadata):
   """Recursively ensures that a module and its dependencies are loaded.
 
   Args:
@@ -111,10 +126,8 @@ def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
         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.
-    import_set: The working set of mojom imports processed so far in this
-        call stack. Used to detect circular dependencies.
-    import_stack: An ordered list of imports processed so far in this call
-        stack. Used to report circular dependencies.
+    module_metadata: Metadata to be attached to every module loaded by this
+        helper.
 
   Returns:
     None
@@ -129,7 +142,7 @@ def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
   for dep_abspath, dep_path in dependencies[mojom_abspath]:
     if dep_abspath not in loaded_modules:
       _EnsureInputLoaded(dep_abspath, dep_path, abs_paths, asts, dependencies,
-                         loaded_modules)
+                         loaded_modules, module_metadata)
 
   imports = {}
   for imp in asts[mojom_abspath].import_list:
@@ -137,6 +150,7 @@ def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
     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):
@@ -157,10 +171,67 @@ def _CollectAllowedImportsFromBuildMetadata(build_metadata_filename):
   return allowed_imports
 
 
+# multiprocessing helper.
+def _ParseAstHelper(args):
+  mojom_abspath, enabled_features = args
+  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(args):
+  mojom_abspath, mojom_path = args
+  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)
+
+
+def _Shard(target_func, args, processes=None):
+  args = list(args)
+  if processes is None:
+    processes = multiprocessing.cpu_count()
+  # Seems optimal to have each process perform at least 2 tasks.
+  processes = min(processes, len(args) // 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 result in map(target_func, args):
+      yield result
+    return
+
+  pool = multiprocessing.Pool(processes=processes)
+  try:
+    for result in pool.imap_unordered(target_func, args):
+      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.
 
@@ -176,6 +247,8 @@ def _ParseMojoms(mojom_files,
         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.
@@ -193,72 +266,79 @@ def _ParseMojoms(mojom_files,
                               for abs_path in mojom_files)
   abs_paths = dict(
       (path, abs_path) for abs_path, path in mojom_files_to_parse.items())
-  for mojom_abspath, _ in mojom_files_to_parse.items():
-    with codecs.open(mojom_abspath, encoding='utf-8') as f:
-      ast = parser.Parse(''.join(f.readlines()), mojom_abspath)
-      conditional_features.RemoveDisabledDefinitions(ast, enabled_features)
-      loaded_mojom_asts[mojom_abspath] = ast
-      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))
-        else:
-          # 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('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 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)
+                       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.
-  for mojom_abspath, mojom_path in mojom_files_to_parse.items():
-    module_path = os.path.join(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:
-      loaded_modules[mojom_abspath].Dump(f)
+  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
@@ -333,6 +413,16 @@ already present in the provided output root.""")
       '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:
@@ -353,8 +443,14 @@ already present in the provided output root.""")
   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,
-               allowed_imports)
+               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__':
diff --git a/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py b/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py
index a0ee150e..65db4dc9 100644
--- a/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+from mojom.generate import module
 from mojom_parser_test_case import MojomParserTestCase
 
 
@@ -20,9 +21,11 @@ class VersionCompatibilityTest(MojomParserTestCase):
     self.assertEqual(set(old.keys()), set(new.keys()),
                      'Old and new test mojoms should use the same type names.')
 
+    checker = module.BackwardCompatibilityChecker()
     compatibility_map = {}
     for name in old.keys():
-      compatibility_map[name] = new[name].IsBackwardCompatible(old[name])
+      compatibility_map[name] = checker.IsBackwardCompatible(
+          new[name], old[name])
     return compatibility_map
 
   def assertBackwardCompatible(self, old_mojom, new_mojom):
@@ -234,6 +237,47 @@ class VersionCompatibilityTest(MojomParserTestCase):
     self.assertNotBackwardCompatible('union U { string a; };',
                                      'union U { string? a; };')
 
+  def testFieldNestedTypeChanged(self):
+    """Changing the definition of a nested type within a field (such as an array
+    element or interface endpoint type) should only break backward-compatibility
+    if the changes to that type are not backward-compatible."""
+    self.assertBackwardCompatible(
+        """\
+        struct S { string a; };
+        struct T { array<S> ss; };
+        """, """\
+        struct S {
+          string a;
+          [MinVersion=1] string? b;
+        };
+        struct T { array<S> ss; };
+        """)
+    self.assertBackwardCompatible(
+        """\
+        interface F { Do(); };
+        struct S { pending_receiver<F> r; };
+        """, """\
+        interface F {
+          Do();
+          [MinVersion=1] Say();
+        };
+        struct S { pending_receiver<F> r; };
+        """)
+
+  def testRecursiveTypeChange(self):
+    """Recursive types do not break the compatibility checker."""
+    self.assertBackwardCompatible(
+        """\
+        struct S {
+          string a;
+          array<S> others;
+        };""", """\
+        struct S {
+          string a;
+          array<S> others;
+          [MinVersion=1] string? b;
+        };""")
+
   def testUnionFieldBecomingNonOptional(self):
     """Changing a field from optional to non-optional breaks
     backward-compatibility."""