add signal 'snapshot', rename existing signal 'wakeup' to 'replication'

daemon/job/doreplication/doreplication.go

@@ -0,0 +1,35 @@
+package doreplication
+
+import (
+	"context"
+	"errors"
+)
+
+type contextKey int
+
+const contextKeyReplication contextKey = iota
+
+func Wait(ctx context.Context) <-chan struct{} {
+	wc, ok := ctx.Value(contextKeyReplication).(chan struct{})
+	if !ok {
+		wc = make(chan struct{})
+	}
+	return wc
+}
+
+type Func func() error
+
+var AlreadyReplicating = errors.New("already replicating")
+
+func Context(ctx context.Context) (context.Context, Func) {
+	wc := make(chan struct{})
+	wuf := func() error {
+		select {
+		case wc <- struct{}{}:
+			return nil
+		default:
+			return AlreadyReplicating
+		}
+	}
+	return context.WithValue(ctx, contextKeyReplication, wc), wuf
+}