Add support for --Import_path (grpc-ecosystem#507)

Fixes: grpc-ecosystem#443

Author: achew22

protoc-gen-grpc-gateway/descriptor/registry.go

@@ -25,6 +25,9 @@ type Registry struct {
 	// prefix is a prefix to be inserted to golang package paths generated from proto package names.
 	prefix string
 
+	// importPath is used as the package if no input files declare go_package. If it contains slashes, everything up to the rightmost slash is ignored.
+	importPath string
+
 	// pkgMap is a user-specified mapping from file path to proto package.
 	pkgMap map[string]string
 
@@ -212,6 +215,13 @@ func (r *Registry) SetPrefix(prefix string) {
 	r.prefix = prefix
 }
 
+// SetImportPath registers the importPath which is used as the package if no
+// input files declare go_package. If it contains slashes, everything up to the
+// rightmost slash is ignored.
+func (r *Registry) SetImportPath(importPath string) {
+	r.importPath = importPath
+}
+
 // ReserveGoPackageAlias reserves the unique alias of go package.
 // If succeeded, the alias will be never used for other packages in generated go files.
 // If failed, the alias is already taken by another package, so you need to use another
@@ -237,6 +247,9 @@ func (r *Registry) goPackagePath(f *descriptor.FileDescriptorProto) string {
 	}
 
 	gopkg := f.Options.GetGoPackage()
+	if len(gopkg) == 0 {
+		gopkg = r.importPath
+	}
 	idx := strings.LastIndex(gopkg, "/")
 	if idx >= 0 {
 		if sc := strings.LastIndex(gopkg, ";"); sc > 0 {

protoc-gen-grpc-gateway/main.go

@@ -24,6 +24,7 @@ import (
 
 var (
 	importPrefix      = flag.String("import_prefix", "", "prefix to be added to go package paths for imported proto files")
+	importPath        = flag.String("import_path", "", "used as the package if no input files declare go_package. If it contains slashes, everything up to the rightmost slash is ignored.")
 	useRequestContext = flag.Bool("request_context", true, "determine whether to use http.Request's context or not")
 	allowDeleteBody   = flag.Bool("allow_delete_body", false, "unless set, HTTP DELETE methods may not have a body")
 )
@@ -78,6 +79,7 @@ func main() {
 	g := gengateway.New(reg, *useRequestContext)
 
 	reg.SetPrefix(*importPrefix)
+	reg.SetImportPath(*importPath)
 	reg.SetAllowDeleteBody(*allowDeleteBody)
 	if err := reg.Load(req); err != nil {
 		emitError(err)