event: add new Subscription type and related utilities

This commit introduces a new Subscription type, which is synonymous with
ethereum.Subscription. It also adds a couple of utilities that make
working with Subscriptions easier. The mot complex utility is Feed, a
synchronisation device that implements broadcast subscriptions. Feed is
slightly faster than TypeMux and will replace uses of TypeMux across the
go-ethereum codebase in the future.

event/example_feed_test.go

@@ -0,0 +1,73 @@
+// Copyright 2017 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 event_test
+
+import (
+	"fmt"
+
+	"github.com/ethereum/go-ethereum/event"
+)
+
+func ExampleFeed_acknowledgedEvents() {
+	// This example shows how the return value of Send can be used for request/reply
+	// interaction between event consumers and producers.
+	var feed event.Feed
+	type ackedEvent struct {
+		i   int
+		ack chan<- struct{}
+	}
+
+	// Consumers wait for events on the feed and acknowledge processing.
+	done := make(chan struct{})
+	defer close(done)
+	for i := 0; i < 3; i++ {
+		ch := make(chan ackedEvent, 100)
+		sub := feed.Subscribe(ch)
+		go func() {
+			defer sub.Unsubscribe()
+			for {
+				select {
+				case ev := <-ch:
+					fmt.Println(ev.i) // "process" the event
+					ev.ack <- struct{}{}
+				case <-done:
+					return
+				}
+			}
+		}()
+	}
+
+	// The producer sends values of type ackedEvent with increasing values of i.
+	// It waits for all consumers to acknowledge before sending the next event.
+	for i := 0; i < 3; i++ {
+		acksignal := make(chan struct{})
+		n := feed.Send(ackedEvent{i, acksignal})
+		for ack := 0; ack < n; ack++ {
+			<-acksignal
+		}
+	}
+	// Output:
+	// 0
+	// 0
+	// 0
+	// 1
+	// 1
+	// 1
+	// 2
+	// 2
+	// 2
+}