font: add test

font.go

@@ -20,9 +20,12 @@ type defaultFont struct {
 	once sync.Once
 }
 
+var _testingHook = func() {}
+
 func (df *defaultFont) Font() (*truetype.Font, error) {
 	df.once.Do(func() {
 		df.font, df.err = truetype.Parse(roboto.Roboto)
+		_testingHook()
 	})
 	return df.font, df.err
 }

font_test.go

@@ -0,0 +1,42 @@
+package chart
+
+import (
+	"testing"
+
+	"github.com/golang/freetype/truetype"
+)
+
+func TestDefaultFont(t *testing.T) {
+	inits := make(chan struct{}, 2)
+	old := _testingHook
+	_testingHook = func() { inits <- struct{}{} }
+	defer func() { _testingHook = old }()
+
+	type testData struct {
+		font *truetype.Font
+		err  error
+	}
+	calls := make(chan testData, 2)
+	for i := 0; i < cap(calls); i++ {
+		go func() {
+			font, err := GetDefaultFont()
+			calls <- testData{font, err}
+		}()
+	}
+
+	var c []testData
+	for i := 0; i < cap(calls); i++ {
+		c = append(c, <-calls)
+	}
+
+	if c[0].err != nil || c[1].err != nil {
+		t.Error("GetDefaultFont got unexpected error: ", c)
+	}
+	if c[0].font != c[1].font {
+		t.Error("GetDefaultFont got different fonts: ", c)
+	}
+
+	if len(inits) > 1 { // Safe to check since we already have two results.
+		t.Error("GetDefaultFont initialized more than once")
+	}
+}