snapshot: Separate into whitebox and blackbox backup

pkg/opgroup/opgroup.go

@@ -0,0 +1,50 @@
+// Package opgroup provides OpGroup
+package opgroup
+
+import "sync"
+
+// OpGroup represents an operation group that can send messages to the waiting goroutine.
+//The zero value is not ready for use, use [NewOpGroup] instead.
+type OpGroup[M any] struct {
+	wg sync.WaitGroup
+	c  chan M
+}
+
+// NewOpGroup creates a new OpGroup.
+//
+// The internal buffer size for messages will be expectedSize.
+// If unsure about buffer size, 0 is a valid choice.
+func NewOpGroup[M any](expectedSize int) *OpGroup[M] {
+	return &OpGroup[M]{
+		c: make(chan M, expectedSize),
+	}
+}
+
+// Go schedules a new operation (implemented by worker) to run in a separate goroutine.
+// worker is passed a send-only reference to the message channel which it can uszxe to send messages to.
+func (op *OpGroup[M]) Go(worker func(c chan<- M)) {
+	op.wg.Add(1)
+	go func() {
+		defer op.wg.Done()
+		worker(op.c)
+	}()
+}
+
+// GoErr is like Go, except that once the operation is finished, it writes the returned error into dest.
+func (op *OpGroup[M]) GoErr(worker func(c chan<- M) error, dest *error) {
+	op.Go(func(c chan<- M) {
+		*dest = worker(c)
+	})
+}
+
+// Wait returns a receive-only reference to the message channel.//
+// The message channel will be closed once all operations on this group have completed.
+//
+// The Wait function may only be called once.
+func (op *OpGroup[M]) Wait() <-chan M {
+	go func() {
+		op.wg.Wait()
+		close(op.c)
+	}()
+	return op.c
+}