runtime: only check whether the runtime is stale once during tests

Noticed while investigating the speed of the runtime tests, as part
of debugging while Plan 9's runtime tests are timing out on GCE.

Change-Id: I95f5a3d967a0b45ec1ebf10067e193f51db84e26
Reviewed-on: https://go-review.googlesource.com/2283Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/crash_test.go

@@ -5,12 +5,14 @@
 package runtime_test
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 	"testing"
 	"text/template"
 )
@@ -78,14 +80,25 @@ func executeTest(t *testing.T, templ string, data interface{}, extra ...string)
 	return string(got)
 }
 
+var (
+	staleRuntimeOnce sync.Once // guards init of staleRuntimeErr
+	staleRuntimeErr  error
+)
+
 func checkStaleRuntime(t *testing.T) {
-	// 'go run' uses the installed copy of runtime.a, which may be out of date.
-	out, err := testEnv(exec.Command("go", "list", "-f", "{{.Stale}}", "runtime")).CombinedOutput()
-	if err != nil {
-		t.Fatalf("failed to execute 'go list': %v\n%v", err, string(out))
-	}
-	if string(out) != "false\n" {
-		t.Fatalf("Stale runtime.a. Run 'go install runtime'.")
+	staleRuntimeOnce.Do(func() {
+		// 'go run' uses the installed copy of runtime.a, which may be out of date.
+		out, err := testEnv(exec.Command("go", "list", "-f", "{{.Stale}}", "runtime")).CombinedOutput()
+		if err != nil {
+			staleRuntimeErr = fmt.Errorf("failed to execute 'go list': %v\n%v", err, string(out))
+			return
+		}
+		if string(out) != "false\n" {
+			staleRuntimeErr = fmt.Errorf("Stale runtime.a. Run 'go install runtime'.")
+		}
+	})
+	if staleRuntimeErr != nil {
+		t.Fatal(staleRuntimeErr)
 	}
 }