hplot: fix race in hplot.New on gonum/plot.DefaultFont

Fixes #850.

hplot/plot.go

@@ -7,6 +7,7 @@ package hplot
 import (
 	"io"
 	"math"
+	"sync"
 
 	"gonum.org/v1/plot"
 	"gonum.org/v1/plot/vg"
@@ -19,9 +20,15 @@ type Plot struct {
 	Style Style
 }
 
+// muNewPlot protects access to gonum/plot.DefaultFont
+var muNewPlot sync.Mutex
+
 // New returns a new plot with some reasonable
 // default settings.
 func New() *Plot {
+	muNewPlot.Lock()
+	defer muNewPlot.Unlock()
+
 	style := DefaultStyle
 	defer style.reset(plot.DefaultFont)
 	plot.DefaultFont = style.Fonts.Default

hplot/plot_test.go

@@ -6,6 +6,7 @@ package hplot_test
 
 import (
 	"os"
+	"sync"
 	"testing"
 
 	"go-hep.org/x/hep/hplot"
@@ -42,3 +43,18 @@ func TestPlotWriterTo(t *testing.T) {
 		}
 	}, t, "plot_writerto.png")
 }
+
+func TestNewPlotRace(t *testing.T) {
+	const N = 100
+	var wg sync.WaitGroup
+	wg.Add(N)
+	for i := 0; i < N; i++ {
+		go func() {
+			defer wg.Done()
+
+			_ = hplot.New()
+		}()
+	}
+
+	wg.Wait()
+}