ordered execution async wait

weed/util/limited_async_pool.go

@@ -0,0 +1,44 @@
+package util
+
+// initial version comes from https://hackernoon.com/asyncawait-in-golang-an-introductory-guide-ol1e34sg
+
+import "context"
+
+type Future interface {
+	Await() interface{}
+}
+
+type future struct {
+	await func(ctx context.Context) interface{}
+}
+
+func (f future) Await() interface{} {
+	return f.await(context.Background())
+}
+
+type LimitedAsyncExecutor struct {
+	executor *LimitedConcurrentExecutor
+}
+
+func NewLimitedAsyncExecutor(limit int) *LimitedAsyncExecutor {
+	return &LimitedAsyncExecutor{
+		executor: NewLimitedConcurrentExecutor(limit),
+	}
+}
+
+func (ae *LimitedAsyncExecutor) Execute(job func() interface{}) Future {
+	var result interface{}
+	c := make(chan struct{})
+	ae.executor.Execute(func() {
+		defer close(c)
+		result = job()
+	})
+	return future{await: func(ctx context.Context) interface{} {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-c:
+			return result
+		}
+	}}
+}