socketpair: directly export *net.UnixConn (and add test for that behavior)

util/socketpair/socketpair_test.go

@@ -0,0 +1,18 @@
+package socketpair
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// This is test is mostly to verify that the assumption about
+// net.FileConn returning *net.UnixConn for AF_UNIX FDs works.
+func TestSocketPairWorks(t *testing.T) {
+	assert.NotPanics(t, func() {
+		a, b, err := SocketPair()
+		assert.NoError(t, err)
+		a.Close()
+		b.Close()
+	})
+}