build: support more variations on context and dockerfile + iidfile

Signed-off-by: Tibor Vass <tibor@docker.com>

vendor/github.com/moby/buildkit/session/upload/upload.go

@@ -0,0 +1,55 @@
+package upload
+
+import (
+	"context"
+	io "io"
+	"net/url"
+
+	"github.com/moby/buildkit/session"
+	"google.golang.org/grpc/metadata"
+)
+
+const (
+	keyPath = "urlpath"
+	keyHost = "urlhost"
+)
+
+func New(ctx context.Context, c session.Caller, url *url.URL) (*Upload, error) {
+	opts := map[string][]string{
+		keyPath: {url.Path},
+		keyHost: {url.Host},
+	}
+
+	client := NewUploadClient(c.Conn())
+
+	ctx = metadata.NewOutgoingContext(ctx, opts)
+
+	cc, err := client.Pull(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Upload{cc: cc}, nil
+}
+
+type Upload struct {
+	cc Upload_PullClient
+}
+
+func (u *Upload) WriteTo(w io.Writer) (int, error) {
+	n := 0
+	for {
+		var bm BytesMessage
+		if err := u.cc.RecvMsg(&bm); err != nil {
+			if err == io.EOF {
+				return n, nil
+			}
+			return n, err
+		}
+		nn, err := w.Write(bm.Data)
+		n += nn
+		if err != nil {
+			return n, err
+		}
+	}
+}