add missing merge.go

cmd/x-check-mailserver/merge.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"sync"
+)
+
+// T is a type which is handled by merge function.
+// Type alias is used to allow easy type change.
+type T = error
+
+func merge(cs ...<-chan T) <-chan T {
+	out := make(chan T)
+	var wg sync.WaitGroup
+	wg.Add(len(cs))
+	for _, c := range cs {
+		go func(c <-chan T) {
+			for v := range c {
+				out <- v
+			}
+			wg.Done()
+		}(c)
+	}
+	go func() {
+		wg.Wait()
+		close(out)
+	}()
+	return out
+}