Update to gobook@c6d7a22edd03e7738c38d5baa2174ff325d8d863

ch9/bank2/bank.go

@@ -0,0 +1,28 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 262.
+
+// Package bank provides a concurrency-safe bank with one account.
+package bank
+
+//!+
+var (
+	sema    = make(chan struct{}, 1) // a binary semaphore guarding balance
+	balance int
+)
+
+func Deposit(amount int) {
+	sema <- struct{}{} // acquire token
+	balance = balance + amount
+	<-sema // release token
+}
+
+func Balance() int {
+	sema <- struct{}{} // acquire token
+	b := balance
+	<-sema // release token
+	return b
+}
+
+//!-