Added detached context example

Author: Dmitry Sinyavskiy

context/detached.go

@@ -0,0 +1,43 @@
+package context
+
+import (
+	"context"
+	"time"
+)
+
+type detached struct {
+	ctx context.Context
+}
+
+func (detached) Deadline() (time.Time, bool) {
+	return time.Time{}, false
+}
+
+func (detached) Done() <-chan struct{} {
+	return nil
+}
+
+func (detached) Err() error {
+	return nil
+}
+
+func (d detached) Value(key interface{}) interface{} {
+	return d.ctx.Value(key)
+}
+
+// Detach creates a detached context (without cancel and deadline) from a parent ctx.
+// Example:
+//	import myapp/context
+//	func home(w http.ResponseWriter, r *http.Request) {
+//		log.Println("home")
+//		// Create a Detached context
+//		detachedCtx := context.Detach(r.Context())
+//		detachedDeadline, ok := detachedCtx.Deadline()
+//		fmt.Println("DETACHED ---")
+//		fmt.Println("Deadline():", detachedDeadline, ok) // 0001-01-01 00:00:00 +0000 UTC false
+//		fmt.Println("Err():", r.Context().Err()) // <empty>
+// 		backgroundTask.Run(detachedCtx) // prevents task stop when r.Context has cancelled
+//	}
+func Detach(ctx context.Context) context.Context {
+	return detached{ctx: ctx}
+}