server/discovery: add discovery service

server/discovery/discovery.go

Index: server/discovery/discovery.go
diff --git a/server/discovery/discovery.go b/server/discovery/discovery.go
new file mode 100644
index 0000000000000000000000000000000000000000..060b76d8ef47ea8fbc4799c80175605d21cadc6e
--- /dev/null
+++ b/server/discovery/discovery.go
@@ -0,0 +1,94 @@
+// Copyright 2016 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.
+
+// Package discovery implements RPC service introspection.
+package discovery
+
+import (
+	"crypto/sha1"
+	"fmt"
+
+	"github.com/golang/protobuf/proto"
+	"golang.org/x/net/context"
+
+	"github.com/luci/luci-go/common/proto/google/descriptor"
+	"github.com/luci/luci-go/server/prpc"
+)
+
+// New creates a discovery server for all the given services.
+// The service descriptions must be registered using
+// RegisterDescriptorSetCompressed which is called by init() function
+// generated by github.com/luci/luci-go/tools/cmd/cproto.
+func New(serviceNames ...string) (DiscoveryServer, error) {
+	desc, err := combineDescriptors(serviceNames)
+	if err != nil {
+		return nil, err
+	}
+	descBytes, err := proto.Marshal(desc)
+	if err != nil {
+		return nil, err
+	}
+
+	cpy := make([]string, len(serviceNames))
+	copy(cpy, serviceNames)
+	return &service{cpy, descBytes}, nil
+}
+
+// Enable registers a discovery service on the server.
+// It makes all currently registered services and the discovery service
+// discoverable.
+func Enable(server *prpc.Server) {
+	serviceNames := append(server.ServiceNames(), "discovery.Discovery")
+	service, err := New(serviceNames...)
+	if err != nil {
+		panic(err)
+	}
+	RegisterDiscoveryServer(server, service)
+}
+
+type service struct {
+	serviceNames       []string
+	fileDescriptionSet []byte
+}
+
+func (s *service) Describe(c context.Context, _ *Void) (*DescribeResponse, error) {
+	return &DescribeResponse{
+		FileDescriptionSet: s.fileDescriptionSet,
+		Services:           s.serviceNames,
+	}, nil
+}
+
+// combineDescriptors creates one FileDescriptorSet that covers all services
+// and their dependencies.
+func combineDescriptors(serviceNames []string) (*descriptor.FileDescriptorSet, error) {
+	result := &descriptor.FileDescriptorSet{}
+	// seenFiles is a set of descriptor files keyed by SHA1 of their contents..
+	seenFiles := map[[sha1.Size]byte]bool{}
+
+	for _, s := range serviceNames {
+		desc, err := GetDescriptorSet(s)
+		if err != nil {
+			return nil, fmt.Errorf("service %s: %s", s, err)
+		}
+		if desc == nil {
+			return nil, fmt.Errorf(
+				"descriptor for service %q is not found. "+
+					"Did you compile it with github.com/luci/luci-go/tools/cmd/cproto?",
+				s)
+		}
+		for _, f := range desc.GetFile() {
+			binary, err := proto.Marshal(f)
+			if err != nil {
+				return nil, fmt.Errorf("could not marshal description of %s", f.GetName())
+			}
+			hash := sha1.Sum(binary)
+			if !seenFiles[hash] {
+				result.File = append(result.File, f)
+				seenFiles[hash] = true
+			}
+
+		}
+	}
+	return result, nil
+}