Add "Waiting For Goroutines To Finish" example.

Also link to it from the Channel Synchronization example

examples/channel-synchronization/channel-synchronization.go

@@ -1,6 +1,8 @@
 // We can use channels to synchronize execution
 // across goroutines. Here's an example of using a
 // blocking receive to wait for a goroutine to finish.
+// When waiting for multiple goroutines to finish,
+// you may prefer to [use a WaitGroup](waiting-for-goroutines-to-finish).
 
 package main
 

examples/channel-synchronization/channel-synchronization.hash

@@ -1,2 +1,2 @@
-fe3e2ea1a67d0f95ce4cb18f3e8aa16d416de0ce
-0DfW-1RMqi
+df90432a53832c045472981a9da23d1139fb2b5c
+f6qTtt1agmN

examples/waiting-for-goroutines-to-finish/waiting-for-goroutines-to-finish.go

@@ -0,0 +1,47 @@
+// To wait for multiple goroutines to finish, we can
+// use a sync.WaitGroup.
+
+package main
+
+import (
+    "fmt"
+    "math/rand"
+    "sync"
+    "time"
+)
+
+// This is the function we'll run in every goroutine.
+// wg is the WaitGroup it uses to notify that it's done.
+// Note that a WaitGroup must be passed to functions by
+// pointer.
+func worker(id int, wg *sync.WaitGroup) {
+    fmt.Printf("Worker %d starting\n", id)
+
+    // Sleep for a random duration between 500-700 ms
+    // to simulate work. See the [random numbers](random-numbers)
+    // example for more details on *rand*.
+    msToSleep := time.Duration(500 + rand.Intn(200))
+    time.Sleep(msToSleep * time.Millisecond)
+    fmt.Printf("Worker %d done\n", id)
+
+    // Notify the WaitGroup that we're done.
+    wg.Done()
+}
+
+func main() {
+
+    // This WaitGroup is used to wait for all the
+    // goroutines launched here to finish.
+    var wg sync.WaitGroup
+
+    // Launch several goroutines and increment the WorkGroup
+    // counter for each.
+    for i := 1; i <= 5; i++ {
+        wg.Add(1)
+        go worker(i, &wg)
+    }
+
+    // Block until the WorkGroup counter goes back to 0;
+    // all the workers notified they're done.
+    wg.Wait()
+}

examples/waiting-for-goroutines-to-finish/waiting-for-goroutines-to-finish.hash

@@ -0,0 +1,2 @@
+f068072d11ed9469174c18f5b7a6a7d9d8d3dafb
+koKzXfbq8kg

examples/waiting-for-goroutines-to-finish/waiting-for-goroutines-to-finish.sh

@@ -0,0 +1,14 @@
+$ go run waiting-for-goroutines-to-finish.go
+Worker 5 starting
+Worker 3 starting
+Worker 4 starting
+Worker 1 starting
+Worker 2 starting
+Worker 4 done
+Worker 1 done
+Worker 2 done
+Worker 5 done
+Worker 3 done
+
+# The order of workers starting up and finishing
+# is likely to be different for each invocation.