scons: Add support for GRPC protobuf files.

These files are used to generate stubs for calling across GRPC
protocols, an RPC mechanism which is based around the protocol buffer
system.

The support for these files is heavily based on and calls into the
existing protobuf file support, but with the extra step which generates
the additional .grpc.pb.cc and .grpc.pb.h files.

Change-Id: I89022928c08aa9f7ed024b7380ddcc54ca75b55e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37277
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -41,6 +41,7 @@ from __future__ import print_function
 
 import array
 import bisect
+import distutils.spawn
 import functools
 import imp
 import os
@@ -435,6 +436,45 @@ class ProtoBuf(SourceFile):
                 append={'CXXFLAGS': '-Wno-array-bounds'})
 
 
+
+env['PROTOC_GRPC'] = distutils.spawn.find_executable('grpc_cpp_plugin')
+if env['PROTOC_GRPC']:
+    env.Append(LIBS=['grpc++'])
+
+def protoc_grpc_emitter(target, source, env):
+    root, ext = os.path.splitext(source[0].get_abspath())
+    return [root + '.grpc.pb.cc', root + '.grpc.pb.h'], source
+
+env.Append(BUILDERS={'GrpcProtoBufCC' : Builder(
+            action=MakeAction('${PROTOC} --grpc_out ${BUILDDIR} '
+                              '--plugin=protoc-gen-grpc=${PROTOC_GRPC} '
+                              '--proto_path ${BUILDDIR} '
+                              '${SOURCE.get_abspath()}',
+                              Transform("PROTOC")),
+            emitter=protoc_grpc_emitter
+        )})
+
+class GrpcProtoBuf(SourceFile):
+    '''Add a GRPC protocol buffer to the build'''
+
+    def __init__(self, source, tags=None, add_tags=None):
+        '''Specify the source file, and any tags'''
+
+        super(GrpcProtoBuf, self).__init__(source, tags, add_tags)
+
+        if not env['PROTOC_GRPC']:
+            error('No grpc_cpp_plugin found')
+
+        self.cc_file, self.hh_file = env.GrpcProtoBufCC(source=source)
+
+        # We still need to build the normal protobuf code too.
+        self.protobuf = ProtoBuf(source, tags=self.tags)
+
+        # Add the C++ source file.
+        Source(self.cc_file, tags=self.tags,
+                append={'CXXFLAGS': '-Wno-array-bounds'})
+
+
 exectuable_classes = []
 class ExecutableMeta(type):
     '''Meta class for Executables.'''
@@ -573,6 +613,7 @@ Export('Source')
 Export('PySource')
 Export('SimObject')
 Export('ProtoBuf')
+Export('GrpcProtoBuf')
 Export('Executable')
 Export('UnitTest')
 Export('GTest')