Deterministic example for atomics.

Fixes #265

examples/atomic-counters/atomic-counters.go

@@ -7,9 +7,11 @@
 
 package main
 
-import "fmt"
-import "time"
-import "sync/atomic"
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+)
 
 func main() {
 
@@ -17,33 +19,28 @@ func main() {
 	// (always-positive) counter.
 	var ops uint64
 
-	// To simulate concurrent updates, we'll start 50
-	// goroutines that each increment the counter about
-	// once a millisecond.
+	// A WaitGroup will help us wait for all goroutines
+	// to finish their work.
+	var wg sync.WaitGroup
+
+	// We'll start 50 goroutines that each increment the
+	// counter exactly 1000 times.
 	for i := 0; i < 50; i++ {
+		wg.Add(1)
+
 		go func() {
-			for {
+			for c := 0; c < 1000; c++ {
 				// To atomically increment the counter we
 				// use `AddUint64`, giving it the memory
 				// address of our `ops` counter with the
 				// `&` syntax.
 				atomic.AddUint64(&ops, 1)
-
-				// Wait a bit between increments.
-				time.Sleep(time.Millisecond)
 			}
+			wg.Done()
 		}()
 	}
 
-	// Wait a second to allow some ops to accumulate.
-	time.Sleep(time.Second)
-
-	// In order to safely use the counter while it's still
-	// being updated by other goroutines, we extract a
-	// copy of the current value into `opsFinal` via
-	// `LoadUint64`. As above we need to give this
-	// function the memory address `&ops` from which to
-	// fetch the value.
-	opsFinal := atomic.LoadUint64(&ops)
-	fmt.Println("ops:", opsFinal)
+	// Wait until all the goroutines are done.
+	wg.Wait()
+	fmt.Println("ops:", ops)
 }

examples/atomic-counters/atomic-counters.hash

@@ -1,2 +1,2 @@
-a4190094ea0405b5f2733101beb97939a1d43aee
-KDr9EMMPMgi
+103c9b7d036e3a5c14dc481755b78b10dc9f894e
+GRkVf6J1--B

examples/atomic-counters/atomic-counters.sh

@@ -1,7 +1,11 @@
-# Running the program shows that we executed about
-# 40,000 operations.
+# We expect to get exactly 50,000 operations. Had we
+# used the non-atomic `ops++` to increment the counter,
+# we'd likely get a different number, changing between
+# runs, because the goroutines would interfere with
+# each other. Moreover, we'd get data race failures
+# when running with the `-race` flag.
 $ go run atomic-counters.go
-ops: 41419
+ops: 50000
 
 # Next we'll look at mutexes, another tool for managing
 # state.