protoc-gen-go/grpc: use status and code packages only if needed (#820)

In the rare event that a proto file only has a service declaration with
no declared methods, it will not depend on status and code.
Make sure these are not imported in such cases.

protoc-gen-go/grpc/grpc.go

@@ -52,10 +52,10 @@ const generatedCodeVersion = 4
 // Paths for packages used by code generated in this file,
 // relative to the import_prefix of the generator.Generator.
 const (
-	errorPkgPath   = "google.golang.org/grpc/status"
 	contextPkgPath = "context"
 	grpcPkgPath    = "google.golang.org/grpc"
 	codePkgPath    = "google.golang.org/grpc/codes"
+	statusPkgPath  = "google.golang.org/grpc/status"
 )
 
 func init() {
@@ -79,8 +79,6 @@ func (g *grpc) Name() string {
 var (
 	contextPkg string
 	grpcPkg    string
-	errorPkg   string
-	codePkg    string
 )
 
 // Init initializes the plugin.
@@ -109,8 +107,6 @@ func (g *grpc) Generate(file *generator.FileDescriptor) {
 		return
 	}
 
-	errorPkg = string(g.gen.AddImport(errorPkgPath))
-	codePkg = string(g.gen.AddImport(codePkgPath))
 	contextPkg = string(g.gen.AddImport(contextPkgPath))
 	grpcPkg = string(g.gen.AddImport(grpcPkgPath))
 
@@ -304,7 +300,9 @@ func (g *grpc) generateServerMethodConcrete(servName string, method *pb.MethodDe
 		nilArg = "nil, "
 	}
 	methName := generator.CamelCase(method.GetName())
-	g.P("return ", nilArg, errorPkg, `.Errorf(codes.Unimplemented, "method `, methName, ` not implemented")`)
+	statusPkg := string(g.gen.AddImport(statusPkgPath))
+	codePkg := string(g.gen.AddImport(codePkgPath))
+	g.P("return ", nilArg, statusPkg, `.Errorf(`, codePkg, `.Unimplemented, "method `, methName, ` not implemented")`)
 	g.P("}")
 }
 

protoc-gen-go/testdata/grpc/grpc_empty.proto

@@ -0,0 +1,38 @@
+// Go support for Protocol Buffers - Google's data interchange format
+//
+// Copyright 2019 The Go Authors.  All rights reserved.
+// https://github.com/golang/protobuf
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto3";
+
+package grpc.testing;
+
+option go_package = "github.com/golang/protobuf/protoc-gen-go/testdata/grpc;testing";
+
+service EmptyService {}