union: Implement multiple writable remotes

backend/union/policy/ff.go

@@ -0,0 +1,32 @@
+package policy
+
+import (
+	"context"
+
+	"github.com/rclone/rclone/backend/union/upstream"
+	"github.com/rclone/rclone/fs"
+)
+
+func init() {
+	registerPolicy("ff", &FF{})
+}
+
+// FF stands for first found
+// Search category: same as epff.
+// Action category: same as epff.
+// Create category: Given the order of the candiates, act on the first one found.
+type FF struct {
+	EpFF
+}
+
+// Create category policy, governing the creation of files and directories
+func (p *FF) Create(ctx context.Context, upstreams []*upstream.Fs, path string) ([]*upstream.Fs, error) {
+	if len(upstreams) == 0 {
+		return nil, fs.ErrorObjectNotFound
+	}
+	upstreams = filterNC(upstreams)
+	if len(upstreams) == 0 {
+		return upstreams, fs.ErrorPermissionDenied
+	}
+	return upstreams[:1], nil
+}