app: support merging configs

config.go

@@ -20,6 +20,33 @@ type config struct {
 	Commands map[string]string
 }
 
+// Merge the configurations. Anything in the "new" configuration takes
+// precedence over the "old" configuration.
+func mergeConfig(a, b *config) *config {
+	configs := []*config{a, b}
+	result := newConfig()
+
+	for _, config := range configs {
+		for k, v := range config.Builders {
+			result.Builders[k] = v
+		}
+
+		for k, v := range config.Commands {
+			result.Commands[k] = v
+		}
+	}
+
+	return result
+}
+
+// Creates and initializes a new config struct.
+func newConfig() *config {
+	result := new(config)
+	result.Builders = make(map[string]string)
+	result.Commands = make(map[string]string)
+	return result
+}
+
 // Parses a configuration file and returns a proper configuration
 // struct.
 func parseConfig(data string) (result *config, err error) {

config_test.go

@@ -5,6 +5,31 @@ import (
 	"testing"
 )
 
+func TestConfig_MergeConfig(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	aString := `
+	[commands]
+	a = "1"
+	b = "1"
+	`
+
+	bString := `
+	[commands]
+	a = "1"
+	b = "2"
+	c = "3"
+	`
+
+	a, _ := parseConfig(aString)
+	b, _ := parseConfig(bString)
+	result := mergeConfig(a, b)
+
+	assert.Equal(result.Commands["a"], "1", "a should be 1")
+	assert.Equal(result.Commands["b"], "2", "a should be 2")
+	assert.Equal(result.Commands["c"], "3", "a should be 3")
+}
+
 func TestConfig_ParseConfig_Bad(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)