From 412871154a8535235c8376209377de28076a6a0b Mon Sep 17 00:00:00 2001
From: "jian.han" <jian.han@neds.com>
Date: Thu, 25 Jan 2018 11:20:15 +1000
Subject: [PATCH] added sync wait group with error handling

---
 concurrency/waitgroup/error_handling.go | 78 +++++++++++++++++++++++++
 playground/main.go                      |  8 ++-
 2 files changed, 85 insertions(+), 1 deletion(-)
 create mode 100644 concurrency/waitgroup/error_handling.go

diff --git a/concurrency/waitgroup/error_handling.go b/concurrency/waitgroup/error_handling.go
new file mode 100644
index 0000000..bf8085c
--- /dev/null
+++ b/concurrency/waitgroup/error_handling.go
@@ -0,0 +1,78 @@
+package main
+
+import "fmt"
+import "sync"
+import "time"
+
+type Error struct {
+	message string
+}
+
+func (e Error) Error() string {
+	return e.message
+}
+
+func syncWaitWithErrorHandling() {
+	var wg sync.WaitGroup
+	waitGroupLength := 8
+	errChannel := make(chan error, 1)
+
+	// Setup waitgroup to match the number of go routines we'll launch off
+	wg.Add(waitGroupLength)
+	finished := make(chan bool, 1) // this along with wg.Wait() are why the error handling works and doesn't deadlock
+
+	for i := 0; i < waitGroupLength; i++ {
+
+		go func(i int) {
+			fmt.Printf("Go routine %d executed\n", i+1)
+
+			// Sleep for the time needed for each other go routine to complete.
+			// This helps show that the program exists with the last go routine to fail.
+			// comment this line if you want to see it fail fast
+			time.Sleep(time.Duration(waitGroupLength - i))
+
+			time.Sleep(0) // only here so the time import is needed
+
+			// comment out the following 3 lines to see what happens without an error
+			// Note, the channel has a length of one so the last go routine to error
+			// will always be the last error.
+
+			if i%4 == 1 {
+				errChannel <- Error{fmt.Sprintf("Errored on routine %d", i+1)}
+			}
+
+			// Mark the wait group as Done so it does not hang
+			wg.Done()
+		}(i)
+	}
+
+	// Put the wait group in a go routine.
+	// By putting the wait group in the go routine we ensure either all pass
+	// and we close the "finished" channel or we wait forever for the wait group
+	// to finish.
+	//
+	// Waiting forever is okay because of the blocking select below.
+	go func() {
+		wg.Wait()
+		close(finished)
+	}()
+
+	// This select will block until one of the two channels returns a value.
+	// This means on the first failure in the go routines above the errChannel will release a
+	// value first. Because there is a "return" statement in the err check this function will
+	// exit when an error occurs.
+	//
+	// Due to the blocking on wg.Wait() the finished channel will not get a value unless all
+	// the go routines before were successful because not all the wg.Done() calls would have
+	// happened.
+	select {
+	case <-finished:
+	case err := <-errChannel:
+		if err != nil {
+			fmt.Println("error ", err)
+			return
+		}
+	}
+
+	fmt.Println("Successfully executed all go routines")
+}
diff --git a/playground/main.go b/playground/main.go
index 6bbadc0..596c70c 100644
--- a/playground/main.go
+++ b/playground/main.go
@@ -1,9 +1,15 @@
 package main
 
-func main() {
+import "github.com/davecgh/go-spew/spew"
 
+func main() {
+	t1 := make(DogsList, 0)
+	t2 := DogList{}
+	spew.Dump(t1, t2)
 }
 
+type DogsList []*Dog
+
 type Dog struct {
 	Name string
 }