Break the circular dependency in the new mojom bindings generation pipeline.

mojo/public/python/BUILD.gn

 # Copyright 2014 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.
 
 import("../mojo_sdk.gni")
 import("//third_party/cython/rules.gni")
 import("rules.gni")
 
 group("python") {
   deps = [
@@ skipping 71 matching lines @@
 python_package("packaged_application") {
   sources = [
     "mojo_application/__init__.py",
     "mojo_application/application_delegate.py",
     "mojo_application/application_impl.py",
     "mojo_application/application_runner.py",
     "mojo_application/service_provider_impl.py",
   ]
 }
 
 action("import_interface_bindings") {

[rudominer, 2015/11/06 23:07:25]

I know you did not really touch this target but if you understand what it does
could you add a brief comment please?

   visibility = [ ":bindings" ]
   script = rebase_path("mojo/public/tools/gn/unzip.py", ".", mojo_root)
   timestamp = "$target_gen_dir/${target_name}.outputstamp"
   mojom_deps = [ "../interfaces/bindings:bindings_python" ]
   inputs = []
   foreach(d, mojom_deps) {
     dep_name = get_label_info(d, "name")
     dep_target_out_dir = get_label_info(d, "target_out_dir")
     inputs += [ "$dep_target_out_dir/$dep_name.pyzip" ]
   }
   outputs = [
     timestamp,
   ]
   rebase_inputs = rebase_path(inputs, root_build_dir)
   rabase_output = rebase_path("$root_out_dir/python", root_build_dir)
   rebase_timestamp = rebase_path(timestamp, root_build_dir)
   args = [
     "--inputs=${rebase_inputs}",
     "--output=${rabase_output}",
     "--timestamp=${rebase_timestamp}",
   ]
 
   deps = mojom_deps
 }
 
-copy("bindings") {
+group("bindings") {

[rudominer, 2015/11/06 23:07:25]

This target is depended on by the "python" target. Previously that meant the
Python target was depending on the thing that is now called
"serialization_bindings". Do you know why that dependency was there before and
why it is not needed now?

[azani, 2015/11/06 23:39:00]

Sorry, this should depend on serialization_bindings.

+  deps = [

[rudominer, 2015/11/06 23:07:25]

should this target depend on :mojo_system_impl? Or should there be a comment
explaining any consumer of this target must depend on :mojo_system_impl?

[azani, 2015/11/06 23:39:00]

I don't know. I think that in practice, yes, you will always need
mojo_system_impl, but I don't understand what is going on enough to make that
change.

+    ":import_interface_bindings",
+  ]
+}
+
+# This target lists dependencies for mojom serialization only.

[rudominer, 2015/11/06 23:07:25]

Could you add one more sentence fleshing this out a little. Do you mean that
this target allows one to get Mojo serialization in Python without pulling in
the full Mojo application stack?

[azani, 2015/11/06 23:39:00]

Done.

+copy("serialization_bindings") {
   sources = [
     "mojo_bindings/__init__.py",
     "mojo_bindings/descriptor.py",
     "mojo_bindings/interface_reflection.py",
     "mojo_bindings/messaging.py",
     "mojo_bindings/promise.py",
     "mojo_bindings/reflection.py",
     "mojo_bindings/serialization.py",
   ]
   outputs = [
     "$root_out_dir/python/mojo_bindings/{{source_file_part}}",
   ]
   deps = [
-    ":import_interface_bindings",
     ":mojo_system",
   ]
 }
 
 python_package("packaged_bindings") {
   sources = [
     "mojo_bindings/__init__.py",
     "mojo_bindings/descriptor.py",
     "mojo_bindings/interface_reflection.py",
     "mojo_bindings/messaging.py",
     "mojo_bindings/promise.py",
     "mojo_bindings/reflection.py",
     "mojo_bindings/serialization.py",
   ]
 
   deps = [
     "../interfaces/bindings:bindings_python",
   ]
 }