rpc: implement full bi-directional communication (#18471)

New APIs added:

    client.RegisterName(namespace, service) // makes service available to server
    client.Notify(ctx, method, args...)     // sends a notification
    ClientFromContext(ctx)                  // to get a client in handler method

This is essentially a rewrite of the server-side code. JSON-RPC
processing code is now the same on both server and client side. Many
minor issues were fixed in the process and there is a new test suite for
JSON-RPC spec compliance (and non-compliance in some cases).

List of behavior changes:

- Method handlers are now called with a per-request context instead of a
  per-connection context. The context is canceled right after the method
  returns.
- Subscription error channels are always closed when the connection
  ends. There is no need to also wait on the Notifier's Closed channel
  to detect whether the subscription has ended.
- Client now omits "params" instead of sending "params": null when there
  are no arguments to a call. The previous behavior was not compliant
  with the spec. The server still accepts "params": null.
- Floating point numbers are allowed as "id". The spec doesn't allow
  them, but we handle request "id" as json.RawMessage and guarantee that
  the same number will be sent back.
- Logging is improved significantly. There is now a message at DEBUG
  level for each RPC call served.

rpc/testservice_test.go

@@ -0,0 +1,180 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package rpc
+
+import (
+	"context"
+	"encoding/binary"
+	"errors"
+	"sync"
+	"time"
+)
+
+func newTestServer() *Server {
+	server := NewServer()
+	server.idgen = sequentialIDGenerator()
+	if err := server.RegisterName("test", new(testService)); err != nil {
+		panic(err)
+	}
+	if err := server.RegisterName("nftest", new(notificationTestService)); err != nil {
+		panic(err)
+	}
+	return server
+}
+
+func sequentialIDGenerator() func() ID {
+	var (
+		mu      sync.Mutex
+		counter uint64
+	)
+	return func() ID {
+		mu.Lock()
+		defer mu.Unlock()
+		counter++
+		id := make([]byte, 8)
+		binary.BigEndian.PutUint64(id, counter)
+		return encodeID(id)
+	}
+}
+
+type testService struct{}
+
+type Args struct {
+	S string
+}
+
+type Result struct {
+	String string
+	Int    int
+	Args   *Args
+}
+
+func (s *testService) NoArgsRets() {}
+
+func (s *testService) Echo(str string, i int, args *Args) Result {
+	return Result{str, i, args}
+}
+
+func (s *testService) EchoWithCtx(ctx context.Context, str string, i int, args *Args) Result {
+	return Result{str, i, args}
+}
+
+func (s *testService) Sleep(ctx context.Context, duration time.Duration) {
+	time.Sleep(duration)
+}
+
+func (s *testService) Rets() (string, error) {
+	return "", nil
+}
+
+func (s *testService) InvalidRets1() (error, string) {
+	return nil, ""
+}
+
+func (s *testService) InvalidRets2() (string, string) {
+	return "", ""
+}
+
+func (s *testService) InvalidRets3() (string, string, error) {
+	return "", "", nil
+}
+
+func (s *testService) CallMeBack(ctx context.Context, method string, args []interface{}) (interface{}, error) {
+	c, ok := ClientFromContext(ctx)
+	if !ok {
+		return nil, errors.New("no client")
+	}
+	var result interface{}
+	err := c.Call(&result, method, args...)
+	return result, err
+}
+
+func (s *testService) CallMeBackLater(ctx context.Context, method string, args []interface{}) error {
+	c, ok := ClientFromContext(ctx)
+	if !ok {
+		return errors.New("no client")
+	}
+	go func() {
+		<-ctx.Done()
+		var result interface{}
+		c.Call(&result, method, args...)
+	}()
+	return nil
+}
+
+func (s *testService) Subscription(ctx context.Context) (*Subscription, error) {
+	return nil, nil
+}
+
+type notificationTestService struct {
+	unsubscribed            chan string
+	gotHangSubscriptionReq  chan struct{}
+	unblockHangSubscription chan struct{}
+}
+
+func (s *notificationTestService) Echo(i int) int {
+	return i
+}
+
+func (s *notificationTestService) Unsubscribe(subid string) {
+	if s.unsubscribed != nil {
+		s.unsubscribed <- subid
+	}
+}
+
+func (s *notificationTestService) SomeSubscription(ctx context.Context, n, val int) (*Subscription, error) {
+	notifier, supported := NotifierFromContext(ctx)
+	if !supported {
+		return nil, ErrNotificationsUnsupported
+	}
+
+	// By explicitly creating an subscription we make sure that the subscription id is send
+	// back to the client before the first subscription.Notify is called. Otherwise the
+	// events might be send before the response for the *_subscribe method.
+	subscription := notifier.CreateSubscription()
+	go func() {
+		for i := 0; i < n; i++ {
+			if err := notifier.Notify(subscription.ID, val+i); err != nil {
+				return
+			}
+		}
+		select {
+		case <-notifier.Closed():
+		case <-subscription.Err():
+		}
+		if s.unsubscribed != nil {
+			s.unsubscribed <- string(subscription.ID)
+		}
+	}()
+	return subscription, nil
+}
+
+// HangSubscription blocks on s.unblockHangSubscription before sending anything.
+func (s *notificationTestService) HangSubscription(ctx context.Context, val int) (*Subscription, error) {
+	notifier, supported := NotifierFromContext(ctx)
+	if !supported {
+		return nil, ErrNotificationsUnsupported
+	}
+	s.gotHangSubscriptionReq <- struct{}{}
+	<-s.unblockHangSubscription
+	subscription := notifier.CreateSubscription()
+
+	go func() {
+		notifier.Notify(subscription.ID, val)
+	}()
+	return subscription, nil
+}