X tracing: Test for start/stop the world.

go/xcommon/tracing/internal/xruntime/runtime_test.go

 package xruntime
-// TODO tests for stop/start the world.
+
+import (
+	"runtime"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestStartStopTheWorld(t *testing.T) {
+	var x, stop int32
+	ready := make(chan int)
+
+	go func() {
+		// make sure the thread running this goroutine is different from thread for main g.
+		// this way we can be sure there are 2 OS threads in action and communicating via busywait should work.
+		runtime.LockOSThread()
+		ready <- 0
+
+		for atomic.LoadInt32(&stop) == 0 {
+			atomic.AddInt32(&x, 1)
+
+			// XXX as of go19 tight loops are not preemptible (golang.org/issues/10958)
+			//     -> explicitly make sure we do not miss STW request.
+			runtime.Gosched()
+		}
+	}()
+
+
+	// wait for spawned goroutine to jump into its own thread
+	<-ready
+
+	// verify g and g2 are indeed running in parallel
+	xprev := atomic.LoadInt32(&x)
+	xnext := xprev
+	nδ := 0
+	tstart := time.Now()
+	for nδ < 100 && time.Now().Sub(tstart) < time.Second {
+		xnext = atomic.LoadInt32(&x)
+		if xnext != xprev {
+			nδ += 1
+			xprev = xnext
+		}
+	}
+
+	if nδ == 0 {
+		t.Fatal("g and g2 are not running in parallel")
+	}
+
+	// now stop the world and for 1s make sure g2 is not running in parallel with us
+	StopTheWorld("just for my reason")
+
+	xprev = atomic.LoadInt32(&x)
+	xnext = xprev
+	nδ = 0
+	tstart = time.Now()
+	for time.Now().Sub(tstart) < time.Second {
+		for i := 0; i < 100 ; i++ {
+			xnext = atomic.LoadInt32(&x)
+			if xnext != xprev {
+				nδ += 1
+				xprev = xnext
+			}
+		}
+	}
+
+	StartTheWorld()
+
+	if nδ != 0 {
+		t.Fatalf("g2 modified x at least %d times while the world was stopped", nδ)
+	}
+
+	atomic.StoreInt32(&stop, 1)
+}