GODRIVER-407: add test for readPreference implicit

includes reviewer's requested change

Author: tgerk

mongo/client.go

@@ -118,14 +118,20 @@ func newClient(cs connstring.ConnString, opts *ClientOptions) (*Client, error) {
 	if err != nil {
 		return nil, err
 	}
-
 	client.topology = topo
+
 	client.readConcern = readConcernFromConnString(&client.connString)
 	client.writeConcern = writeConcernFromConnString(&client.connString)
-	client.readPreference, err = readPreferenceFromConnString(&client.connString)
+
+	rp, err := readPreferenceFromConnString(&client.connString)
 	if err != nil {
 		return nil, err
 	}
+	if rp != nil {
+		client.readPreference = rp
+	} else {
+		client.readPreference = readpref.Primary()
+	}
 
 	return client, nil
 }
@@ -201,7 +207,7 @@ func readPreferenceFromConnString(cs *connstring.ConnString) (*readpref.ReadPref
 		}
 	}
 
-	return readPref.Primary();
+	return rp, nil
 }
 
 // Database returns a handle for a given database.

mongo/client_internal_test.go

@@ -331,5 +331,17 @@ func TestClient_ReadPreference(t *testing.T) {
 	d, flag := c.readPreference.MaxStaleness()
 	require.True(t, flag)
 	require.Equal(t, time.Duration(5)*time.Second, d)
+}
+
+func TestClient_ReadPreferenceAbsent(t *testing.T) {
+	t.Parallel()
 
+	cs := testutil.ConnString(t)
+	c, err := NewClient(cs.String())
+	require.NoError(t, err)
+	require.NotNil(t, c)
+	require.Equal(t, readpref.PrimaryMode, c.readPreference.Mode())
+	require.Empty(t, c.readPreference.TagSets())
+	_, flag := c.readPreference.MaxStaleness()
+	require.False(t, flag)
 }