Add a golang solution for the Queue problem from section 3.8

LittleBookOfSemaphores/chapter3/queue/README.md

@@ -0,0 +1,11 @@
+# 3.8 Queue
+
+For example, imagine that threads represent ballroom dancers and that two
+kinds of dancers, leaders and followers, wait in two queues before entering the
+dance floor. When a leader arrives, it checks to see if there is a follower waiting.
+If so, they can both proceed. Otherwise it waits.
+
+Similarly, when a follower arrives, it checks for a leader and either proceeds
+or waits, accordingly.
+
+Puzzle: write code for leaders and followers that enforces these constraints.

LittleBookOfSemaphores/chapter3/queue/go/main.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+type leader struct{ id int }
+type follower struct{ id int }
+
+func main() {
+	lq := make(chan *leader)
+	fq := make(chan *follower)
+
+	dance := func(l *leader, f *follower) {
+		fmt.Printf("Leader (%d) is dancing with Follower (%d)\n", l.id, f.id)
+	}
+
+	triggerLeader := func(id int) {
+		leaderArrival(id, lq, fq, dance)
+	}
+	triggerFollower := func(id int) {
+		followerArrival(id, lq, fq, dance)
+	}
+
+	go trigger("Leader", triggerLeader)
+	go trigger("Follower", triggerFollower)
+
+	time.Sleep(time.Duration(15) * time.Second)
+}
+
+func trigger(name string, event func(int)) {
+	for i := 0; ; i++ {
+		sleep := rand.Intn(999) + 1
+		time.Sleep(time.Duration(sleep) * time.Millisecond)
+		fmt.Printf("Triggering (%s)\n", name)
+		go event(i)
+	}
+}
+
+func leaderArrival(id int, lq chan *leader, fq chan *follower, dance func(*leader, *follower)) {
+	l := &leader{id: id}
+	select {
+	case f := <-fq:
+		dance(l, f)
+	default:
+		lq <- l
+	}
+}
+
+func followerArrival(id int, lq chan *leader, fq chan *follower, dance func(*leader, *follower)) {
+	f := &follower{id: id}
+	select {
+	case l := <-lq:
+		dance(l, f)
+	default:
+		fq <- f
+	}
+}