vitessdriver: OpenWithConfiguration helper now sets default values for empty fields.

Added respective unit test.

go/vt/vitessdriver/driver.go

@@ -80,11 +80,11 @@ func OpenShardForStreaming(address, keyspace, shard, tabletType string, timeout
 // It allows to pass in a Configuration struct to control all possible
 // settings of the Vitess Go SQL driver.
 func OpenWithConfiguration(c Configuration) (*sql.DB, error) {
-	jsonBytes, err := json.Marshal(c)
+	json, err := c.toJSON()
 	if err != nil {
 		return nil, err
 	}
-	return sql.Open("vitess", string(jsonBytes))
+	return sql.Open("vitess", json)
 }
 
 type drv struct {
@@ -172,11 +172,31 @@ type Configuration struct {
 }
 
 func newDefaultConfiguration() Configuration {
-	return Configuration{
-		Protocol:   "grpc",
-		TabletType: "master",
-		Streaming:  false,
+	c := Configuration{}
+	c.setDefaults()
+	return c
+}
+
+// toJSON converts Configuration to the JSON string which is required by the
+// Vitess driver. Default values for empty fields will be set.
+func (c Configuration) toJSON() (string, error) {
+	c.setDefaults()
+	jsonBytes, err := json.Marshal(c)
+	if err != nil {
+		return "", err
 	}
+	return string(jsonBytes), nil
+}
+
+// setDefaults sets the default values for empty fields.
+func (c *Configuration) setDefaults() {
+	if c.Protocol == "" {
+		c.Protocol = "grpc"
+	}
+	if c.TabletType == "" {
+		c.TabletType = "master"
+	}
+	// c.Streaming = false is enforced by Go's zero value.
 }
 
 type conn struct {

go/vt/vitessdriver/driver_test.go

@@ -218,6 +218,46 @@ func TestExec(t *testing.T) {
 	}
 }
 
+func TestConfigurationToJSON(t *testing.T) {
+	var testcases = []struct {
+		desc   string
+		config Configuration
+		json   string
+	}{
+		{
+			desc: "all fields set",
+			config: Configuration{
+				Protocol:   "some-invalid-protocol",
+				Keyspace:   "ks2",
+				Shard:      "-80",
+				TabletType: "replica",
+				Streaming:  true,
+				Timeout:    1 * time.Second,
+			},
+			json: `{"Protocol":"some-invalid-protocol","Address":"","Keyspace":"ks2","Shard":"-80","tablet_type":"replica","Streaming":true,"Timeout":1000000000}`,
+		},
+		{
+			desc: "default fields are empty",
+			config: Configuration{
+				Keyspace: "ks2",
+				Shard:    "-80",
+				Timeout:  1 * time.Second,
+			},
+			json: `{"Protocol":"grpc","Address":"","Keyspace":"ks2","Shard":"-80","tablet_type":"master","Streaming":false,"Timeout":1000000000}`,
+		},
+	}
+
+	for _, tc := range testcases {
+		json, err := tc.config.toJSON()
+		if err != nil {
+			t.Errorf("%v: JSON conversion should have succeeded but did not: %v", tc.desc, err)
+		}
+		if json != tc.json {
+			t.Errorf("%v: Configuration.JSON(): got: %v want: %v Configuration: %v", tc.desc, json, tc.json, tc.config)
+		}
+	}
+}
+
 func TestExecStreamingNotAllowed(t *testing.T) {
 	db, err := OpenForStreaming(testAddress, "rdonly", 30*time.Second)
 	if err != nil {