Centralize storage walk emission

internal/storage/walk.go

@@ -0,0 +1,63 @@
+package storage
+
+import (
+	"context"
+	"errors"
+)
+
+type WalkEmitter struct {
+	ctx     context.Context
+	backend string
+	opts    WalkOptions
+	fn      WalkFunc
+	count   int
+}
+
+func NewWalkEmitter(ctx context.Context, backend string, opts WalkOptions, fn WalkFunc) *WalkEmitter {
+	return &WalkEmitter{
+		ctx:     ctx,
+		backend: backend,
+		opts:    opts,
+		fn:      fn,
+	}
+}
+
+func (e *WalkEmitter) Emit(entry Entry) error {
+	if err := e.ctx.Err(); err != nil {
+		return err
+	}
+	if e.opts.Limit > 0 && e.count >= e.opts.Limit {
+		return ErrStopWalk
+	}
+	e.count++
+	if err := e.fn(entry); err != nil {
+		if errors.Is(err, ErrStopWalk) {
+			return ErrStopWalk
+		}
+		return NewError(OpWalk, e.backend, entry.Path, ErrUnknown, err)
+	}
+	return nil
+}
+
+func (e *WalkEmitter) LimitReached() bool {
+	return e.opts.Limit > 0 && e.count >= e.opts.Limit
+}
+
+func FinishWalk(err error) error {
+	if errors.Is(err, ErrStopWalk) {
+		return nil
+	}
+	return err
+}
+
+func HasAny(ctx context.Context, backend Backend, prefix string) (bool, error) {
+	found := false
+	err := backend.Walk(ctx, prefix, WalkOptions{Recursive: false, Limit: 1}, func(Entry) error {
+		found = true
+		return ErrStopWalk
+	})
+	if err != nil {
+		return false, err
+	}
+	return found, nil
+}