add context example

go/context.go

@@ -0,0 +1,45 @@
+package main
+
+// from
+// https://www.ardanlabs.com/blog/2019/09/context-package-semantics-in-go.html
+
+import (
+	"context"
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+func main() {
+	// Set a duration.
+	duration := 150 * time.Millisecond
+
+	// Create a context that is both manually cancellable and will signal
+	// cancel at the specified duration.
+	ctx, cancel := context.WithTimeout(context.Background(), duration)
+	defer cancel()
+
+	// Create a channel to receive a signal that work is done.
+	ch := make(chan string, 1)
+
+	// Ask the goroutine to do some work for us.
+	go func() {
+
+		// Simulate work.
+		rand.Seed(time.Now().UnixNano())
+		ms := rand.Intn(300)
+		time.Sleep(time.Duration(ms) * time.Millisecond)
+
+		// Report the work is done.
+		ch <- "123"
+	}()
+
+	// Wait for the work to finish. If it takes too long, move on.
+	select {
+	case d := <-ch:
+		fmt.Println("work complete", d)
+
+	case <-ctx.Done():
+		fmt.Println("work cancelled")
+	}
+}