controller: print result outside of controller

This will allow result printing to work with the remote controller
(though this currently causes a panic, to be fixed in a follow-up).

Signed-off-by: Justin Chadwell <me@jedevc.com>

util/buildflags/printfunc.go

@@ -0,0 +1,37 @@
+package buildflags
+
+import (
+	"encoding/csv"
+	"strings"
+
+	controllerapi "github.com/docker/buildx/controller/pb"
+	"github.com/pkg/errors"
+)
+
+func ParsePrintFunc(str string) (*controllerapi.PrintFunc, error) {
+	if str == "" {
+		return nil, nil
+	}
+	csvReader := csv.NewReader(strings.NewReader(str))
+	fields, err := csvReader.Read()
+	if err != nil {
+		return nil, err
+	}
+	f := &controllerapi.PrintFunc{}
+	for _, field := range fields {
+		parts := strings.SplitN(field, "=", 2)
+		if len(parts) == 2 {
+			if parts[0] == "format" {
+				f.Format = parts[1]
+			} else {
+				return nil, errors.Errorf("invalid print field: %s", field)
+			}
+		} else {
+			if f.Name != "" {
+				return nil, errors.Errorf("invalid print value: %s", str)
+			}
+			f.Name = field
+		}
+	}
+	return f, nil
+}