crypto/x509: properly gate test on macos version

Fixes the gating of TestIssue51759 by shelling out to sw_vers to check
what version of macOS we are on.

Fixes #64677

Change-Id: I5eef4fa39e5449e7b2aa73864625c3abf002aef8
Reviewed-on: https://go-review.googlesource.com/c/go/+/549195
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: rolandshoemaker

src/crypto/x509/verify_test.go

@@ -16,9 +16,11 @@ import (
 	"fmt"
 	"internal/testenv"
 	"math/big"
+	"os/exec"
 	"reflect"
 	"runtime"
 	"sort"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -1867,17 +1869,40 @@ func TestSystemRootsErrorUnwrap(t *testing.T) {
 	}
 }
 
+func macosMajorVersion(t *testing.T) (int, error) {
+	cmd := testenv.Command(t, "sw_vers", "-productVersion")
+	out, err := cmd.Output()
+	if err != nil {
+		if ee, ok := err.(*exec.ExitError); ok && len(ee.Stderr) > 0 {
+			return 0, fmt.Errorf("%v: %v\n%s", cmd, err, ee.Stderr)
+		}
+		return 0, fmt.Errorf("%v: %v", cmd, err)
+	}
+	before, _, ok := strings.Cut(string(out), ".")
+	major, err := strconv.Atoi(before)
+	if !ok || err != nil {
+		return 0, fmt.Errorf("%v: unexpected output: %q", cmd, out)
+	}
+
+	return major, nil
+}
+
 func TestIssue51759(t *testing.T) {
 	if runtime.GOOS != "darwin" {
 		t.Skip("only affects darwin")
 	}
-	builder := testenv.Builder()
-	if builder == "" {
-		t.Skip("only run this test on the builders, as we have no reasonable way to gate tests on macOS versions elsewhere")
-	}
-	if builder == "darwin-amd64-10_14" || builder == "darwin-amd64-10_15" {
+
+	testenv.MustHaveExecPath(t, "sw_vers")
+	if vers, err := macosMajorVersion(t); err != nil {
+		if builder := testenv.Builder(); builder != "" {
+			t.Fatalf("unable to determine macOS version: %s", err)
+		} else {
+			t.Skip("unable to determine macOS version")
+		}
+	} else if vers < 11 {
 		t.Skip("behavior only enforced in macOS 11 and after")
 	}
+
 	// badCertData contains a cert that we parse as valid
 	// but that macOS SecCertificateCreateWithData rejects.
 	const badCertData = "0\x82\x01U0\x82\x01\a\xa0\x03\x02\x01\x02\x02\x01\x020\x05\x06\x03+ep0R1P0N\x06\x03U\x04\x03\x13Gderpkey8dc58100b2493614ee1692831a461f3f4dd3f9b3b088e244f887f81b4906ac260\x1e\x17\r220112235755Z\x17\r220313235755Z0R1P0N\x06\x03U\x04\x03\x13Gderpkey8dc58100b2493614ee1692831a461f3f4dd3f9b3b088e244f887f81b4906ac260*0\x05\x06\x03+ep\x03!\x00bA\xd8e\xadW\xcb\xefZ\x89\xb5\"\x1eR\x9d\xba\x0e:\x1042Q@\u007f\xbd\xfb{ks\x04\xd1£\x020\x000\x05\x06\x03+ep\x03A\x00[\xa7\x06y\x86(\x94\x97\x9eLwA\x00\x01x\xaa\xbc\xbd Ê]\n(΅!ف0\xf5\x9a%I\x19<\xffo\xf1\xeaaf@\xb1\xa7\xaf\xfd\xe9R\xc7\x0f\x8d&\xd5\xfc\x0f;Ϙ\x82\x84a\xbc\r"