Plugin support for protobuf compiler (protoc) targets.

third_party/protobuf/proto_library.gni

 # 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.
 
 # Compile a protocol buffer.
 #
 # Protobuf parameters:
 #
 #   proto_out_dir (optional)
 #       Specifies the path suffix that output files are generated under. This
 #       path will be appended to the root_gen_dir.
 #
 #       Targets that depend on the proto target will be able to include the
 #       resulting proto headers with an include like:
 #         #include "dir/for/my_proto_lib/foo.pb.h"
 #       If undefined, this defaults to matching the input directory for each
 #       .proto file (you should almost always use the default mode).
 #
 #   cc_generator_options (optional)
 #       List of extra flags passed to the protocol compiler.  If you need to
 #       add an EXPORT macro to a protobuf's C++ header, set the
 #       'cc_generator_options' variable with the value:
 #       'dllexport_decl=FOO_EXPORT:' (note trailing colon).
 #
 #       It is likely you also need to #include a file for the above EXPORT
 #       macro to work. See cc_include.
 #
+#   cc_generator_plugin (optional)
+#       Plugin executable name. Expected .pb.h and .pb.cc as output.

[xyzzyz, 2016/06/20 20:17:46]

If you drop the requirement that the plugin outputs need to be .pb.h and .pb.cc,
I think the code will be both more general and simpler at the same time.

It will be more general, because e.g. gRPC plugin generates .grpc.pb.h and
.grpc.pb.cc files in addition to .pb.h/.pb.cc files generated by regular protoc,
and so the current code will not be usable with gRPC plugin. You will probably
need to add some parameter here to let the GN/GYP know about the desired
extension (and so about desired output artifacts). Also, it looks like your
plugin generates the file with the same as regular "--cpp_out". Is it necessary
in your case? If so, you could also add parameters that would determine if
regular "--cpp_out" is passed.

It will be simpler, because you will not need to make any changes to
protoc_wrapper.py at all -- you will just add an additional "--plugin_out" flag
alongside currently existing "--cpp_out" and "--python_out", conditional on a
plugin being used.

[kraynov, 2016/06/20 20:19:47]

Acknowledged.

+#
 #   cc_include (optional)
 #       String listing an extra include that should be passed.
 #       Example: cc_include = "foo/bar.h"
 #
 #   deps (optional)
 #       Additional dependencies.
 #
 # Parameters for compiling the generated code:
 #
 #   defines (optional)
@@ skipping 55 matching lines @@
     ]
 
     args = []
     if (defined(invoker.cc_include)) {
       args += [
         "--include",
         invoker.cc_include,
       ]
     }
 
+    if (defined(invoker.cc_generator_plugin)) {
+      cc_generator_plugin = invoker.cc_generator_plugin
+    } else {
+      cc_generator_plugin = ""
+    }
     args += [
       "--protobuf",
       "$rel_out_dir/{{source_name_part}}.pb.h",
       "--proto-in-dir",
       "{{source_dir}}",
       "--proto-in-file",
       "{{source_file_part}}",
+      "--cc-generator-plugin=$cc_generator_plugin",
 
       # TODO(brettw) support system protobuf compiler.
       "--use-system-protobuf=0",
     ]
 
     protoc_label = "//third_party/protobuf:protoc($host_toolchain)"
     args += [
       "--",
 
       # Prepend with "./" so this will never pick up the system one (normally
@@ skipping 38 matching lines @@
                            ])
 
     sources = get_target_outputs(":$action_name")
 
     if (defined(invoker.extra_configs)) {
       configs += invoker.extra_configs
     }
 
     public_configs = [ "//third_party/protobuf:using_proto" ]
 
-    public_deps = [
-      # The generated headers reference headers within protobuf_lite, so
-      # dependencies must be able to find those headers too.
-      "//third_party/protobuf:protobuf_lite",
-    ]
+    # If using built-in cc generator the resulting headers reference headers
+    # within protobuf_lite, hence dependencies require those headers too.
+    # In case of generator plugin such issues should be resolved by invoker.
+    if (!defined(invoker.cc_generator_plugin)) {
+      public_deps = [
+        "//third_party/protobuf:protobuf_lite",
+      ]
+    }
     deps = [
       ":$action_name",
     ]
 
     # This will link any libraries in the deps (the use of invoker.deps in the
     # action won't link it).
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
   }
 }