cmd/cgo/internal/test: fix TestThreadLock

This test was introduced in CL 18882, but only recently enabled as of
CL 493603. It's intended to check that we don't move executing C code
between threads when it re-enters Go, but it has always contained a
flake. Go *can* preempt between the Go call to gettid and the C call
to gettid and move the goroutine to another thread because there's no
C code on the stack during the Go call to gettid. This will cause the
test to fail.

Fix this by making both gettid calls in C, with a re-entry to Go
between them.

Fixes #60265

Change-Id: I546621a541ce52b996d68b17d3bed709d2b5b1f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/496182
Auto-Submit: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/src/cmd/cgo/internal/test/cgo_thread_lock.go b/src/cmd/cgo/internal/test/cgo_thread_lock.go
index 981830b6b9..e8749384af 100644
--- a/src/cmd/cgo/internal/test/cgo_thread_lock.go
+++ b/src/cmd/cgo/internal/test/cgo_thread_lock.go
@@ -8,15 +8,19 @@ package cgotest
 
 /*
 #include <unistd.h>
+#include <stdbool.h>
 #include <sys/syscall.h>
 void Gosched(void);
-static int Ctid(void) { Gosched(); return syscall(SYS_gettid); }
+static bool Ctid(void) {
+	long tid1 = syscall(SYS_gettid);
+	Gosched();
+	return tid1 == syscall(SYS_gettid);
+}
 */
 import "C"
 
 import (
 	"runtime"
-	"syscall"
 	"testing"
 	"time"
 )
@@ -46,7 +50,7 @@ func testThreadLock(t *testing.T) {
 	defer close(stop)
 
 	for i := 0; i < 1000; i++ {
-		if C.int(syscall.Gettid()) != C.Ctid() {
+		if !C.Ctid() {
 			t.Fatalf("cgo has not locked OS thread")
 		}
 	}