runtime: set raceignore to zero when starting a new goroutine

When reusing a g struct the runtime did not reset
g.raceignore. Initialize raceignore to zero when initially
setting racectx.

A goroutine can end with a non-zero raceignore if it exits
after calling runtime.RaceDisable without a matching
runtime.RaceEnable. If that goroutine's g is later reused
the race detector is in a weird state: the underlying
g.racectx is active, yet g.raceignore is non-zero, and
raceacquire/racerelease which check g.raceignore become
no-ops. This causes the race detector to report races when
there are none.

Fixes #60934

Change-Id: Ib8e412f11badbaf69a480f03740da70891f4093f
Reviewed-on: https://go-review.googlesource.com/c/go/+/505055
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/runtime/proc.go

@@ -4570,6 +4570,7 @@ func newproc1(fn *funcval, callergp *g, callerpc uintptr) *g {
 	pp.goidcache++
 	if raceenabled {
 		newg.racectx = racegostart(callerpc)
+		newg.raceignore = 0
 		if newg.labels != nil {
 			// See note in proflabel.go on labelSync's role in synchronizing
 			// with the reads in the signal handler.

src/runtime/race/testdata/mop_test.go

@@ -2093,3 +2093,40 @@ func TestNoRaceTinyAlloc(t *testing.T) {
 		<-done
 	}
 }
+
+func TestNoRaceIssue60934(t *testing.T) {
+	// Test that runtime.RaceDisable state doesn't accidentally get applied to
+	// new goroutines.
+
+	// Create several goroutines that end after calling runtime.RaceDisable.
+	var wg sync.WaitGroup
+	ready := make(chan struct{})
+	wg.Add(32)
+	for i := 0; i < 32; i++ {
+		go func() {
+			<-ready // ensure we have multiple goroutines running at the same time
+			runtime.RaceDisable()
+			wg.Done()
+		}()
+	}
+	close(ready)
+	wg.Wait()
+
+	// Make sure race detector still works. If the runtime.RaceDisable state
+	// leaks, the happens-before edges here will be ignored and a race on x will
+	// be reported.
+	var x int
+	ch := make(chan struct{}, 0)
+	wg.Add(2)
+	go func() {
+		x = 1
+		ch <- struct{}{}
+		wg.Done()
+	}()
+	go func() {
+		<-ch
+		_ = x
+		wg.Done()
+	}()
+	wg.Wait()
+}