cmd/vet: reenable cgo test

The reason the 386 trybot was happy but 'GOARCH=386 go test cmd/vet'
was not is that CgoEnabled defaults to false in a cross build;
I have no idea why. Now we ask the go command for the effective
value so that the test works in both cases.

Also, remove stale comment.

Fixes #28829

Change-Id: I1210af34da6986f47924059de5c1f08b2824ace9
Reviewed-on: https://go-review.googlesource.com/c/149958
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/vet/vet_test.go

@@ -70,11 +70,6 @@ func vetCmd(t *testing.T, args ...string) *exec.Cmd {
 	return cmd
 }
 
-// TestVet is equivalent to running this:
-// 	go build -o ./testvet
-// 	errorCheck the output of ./testvet -printfuncs='Warn:1,Warnf:1' testdata/*.go testdata/*.s
-// 	rm ./testvet
-//
 func TestVet(t *testing.T) {
 	t.Parallel()
 	Build(t)
@@ -106,9 +101,8 @@ func TestVet(t *testing.T) {
 		t.Run(pkg, func(t *testing.T) {
 			t.Parallel()
 
-			// Skip for now, pending investigation.
-			if pkg == "cgo" {
-				t.Skip("cgo test disabled -- github.com/golang/go/issues/28829")
+			// Skip cgo test on platforms without cgo.
+			if pkg == "cgo" && !cgoEnabled(t) {
 				return
 			}
 
@@ -137,6 +131,17 @@ func TestVet(t *testing.T) {
 	}
 }
 
+func cgoEnabled(t *testing.T) bool {
+	// Don't trust build.Default.CgoEnabled as it is false for
+	// cross-builds unless CGO_ENABLED is explicitly specified.
+	// That's fine for the builders, but causes commands like
+	// 'GOARCH=386 go test .' to fail.
+	// Instead, we ask the go command.
+	cmd := exec.Command(testenv.GoToolPath(t), "list", "-f", "{{context.CgoEnabled}}")
+	out, _ := cmd.CombinedOutput()
+	return string(out) == "true\n"
+}
+
 func errchk(c *exec.Cmd, files []string, t *testing.T) {
 	output, err := c.CombinedOutput()
 	if _, ok := err.(*exec.ExitError); !ok {