beam/data: fix a bug in encoding of multi-value maps

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

pkg/beam/data/data.go

@@ -91,13 +91,13 @@ func decodeString(msg string) (string, int, error) {
 		length = int(l)
 	}
 	if len(parts[1]) < length + 1 {
-		return "", 0, fmt.Errorf("message is less than %d bytes", length)
+		return "", 0, fmt.Errorf("message '%s' is %d bytes, expected at least %d", parts[1], len(parts[1]), length + 1)
 	}
 	payload := parts[1][:length + 1]
 	if payload[length] != ',' {
 		return "", 0, fmt.Errorf("message is not comma-terminated")
 	}
-	return payload[:length], len(parts[0]) + length + 1, nil
+	return payload[:length], len(parts[0]) + 1 + length + 1, nil
 }
 
 func decodeHeader(msg string) (int, int, error) {

pkg/beam/data/data_test.go

@@ -1,6 +1,7 @@
 package data
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -89,3 +90,40 @@ func TestEncodeBinaryValue(t *testing.T) {
 		t.Fatalf("'%v' != '%v'", output, expectedOutput)
 	}
 }
+
+func TestDecodeString(t *testing.T) {
+	validEncodedStrings := []struct{
+		input string
+		output string
+		skip int
+	}{
+		{"3:foo,", "foo", 6},
+		{"5:hello,", "hello", 8},
+		{"5:hello,5:world,", "hello", 8},
+	}
+	for _, sample := range validEncodedStrings {
+		output, skip, err := decodeString(sample.input)
+		if err != nil {
+			t.Fatalf("error decoding '%v': %v", sample.input, err)
+		}
+		if skip != sample.skip {
+			t.Fatalf("invalid skip: %v!=%v", skip, sample.skip)
+		}
+		if output != sample.output {
+			t.Fatalf("invalid output: %v!=%v", output, sample.output)
+		}
+	}
+}
+
+func TestDecode1Key1Value(t *testing.T) {
+	input := "000;3:foo,6:3:bar,,"
+	output, err := Decode(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v, exists := output["foo"]; !exists {
+		t.Fatalf("wrong output: %v\n", output)
+	} else if len(v) != 1 || strings.Join(v, "") != "bar" {
+		t.Fatalf("wrong output: %v\n", output)
+	}
+}