misc/cgo/testcshared: force descriptor 30 to be closed when execing test

Fixes #26730

Change-Id: I3396598282c814e75c0c4ef16f692dbe83d2935e
Reviewed-on: https://go-review.googlesource.com/127395
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/testcshared/cshared_test.go

@@ -201,6 +201,16 @@ func run(t *testing.T, env []string, args ...string) string {
 	t.Helper()
 	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Env = env
+
+	if GOOS != "windows" {
+		// TestUnexportedSymbols relies on file descriptor 30
+		// being closed when the program starts, so enforce
+		// that in all cases. (The first three descriptors are
+		// stdin/stdout/stderr, so we just need to make sure
+		// that cmd.ExtraFiles[27] exists and is nil.)
+		cmd.ExtraFiles = make([]*os.File, 28)
+	}
+
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("command failed: %v\n%v\n%s\n", args, err, out)