runtime: support channel-based mutex in race detector

Update channel race annotations to support change in
cl/75130045: doc: allow buffered channel as semaphore without initialization
The new annotations are added only for channels with capacity 1.
Strictly saying it's possible to construct a counter-example that
will produce a false positive with capacity > 1. But it's hardly can
lead to false positives in real programs, at least I would like to see such programs first.
Any additional annotations also increase probability of false negatives,
so I would prefer to add them lazily.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, iant, khr, rsc
https://golang.org/cl/76970043

src/pkg/runtime/chan.goc

@@ -171,8 +171,11 @@ asynch:
 		goto asynch;
 	}
 
-	if(raceenabled)
+	if(raceenabled) {
+		if(c->dataqsiz == 1)
+			runtime·raceacquire(chanbuf(c, c->sendx));
 		runtime·racerelease(chanbuf(c, c->sendx));
+	}
 
 	c->elemtype->alg->copy(c->elemsize, chanbuf(c, c->sendx), ep);
 	if(++c->sendx == c->dataqsiz)
@@ -299,8 +302,11 @@ asynch:
 		goto asynch;
 	}
 
-	if(raceenabled)
+	if(raceenabled) {
 		runtime·raceacquire(chanbuf(c, c->recvx));
+		if(c->dataqsiz == 1)
+			runtime·racerelease(chanbuf(c, c->recvx));
+	}
 
 	if(ep != nil)
 		c->elemtype->alg->copy(c->elemsize, ep, chanbuf(c, c->recvx));
@@ -849,6 +855,8 @@ asyncrecv:
 		if(cas->sg.elem != nil)
 			runtime·racewriteobjectpc(cas->sg.elem, c->elemtype, cas->pc, chanrecv);
 		runtime·raceacquire(chanbuf(c, c->recvx));
+		if(c->dataqsiz == 1)
+			runtime·racerelease(chanbuf(c, c->recvx));
 	}
 	if(cas->receivedp != nil)
 		*cas->receivedp = true;
@@ -873,6 +881,8 @@ asyncrecv:
 asyncsend:
 	// can send to buffer
 	if(raceenabled) {
+		if(c->dataqsiz == 1)
+			runtime·raceacquire(chanbuf(c, c->sendx));
 		runtime·racerelease(chanbuf(c, c->sendx));
 		runtime·racereadobjectpc(cas->sg.elem, c->elemtype, cas->pc, chansend);
 	}

src/pkg/runtime/race/testdata/chan_test.go

@@ -568,12 +568,14 @@ func TestRaceChanCloseLen(t *testing.T) {
 }
 
 func TestRaceChanSameCell(t *testing.T) {
-	c := make(chan int, 1)
+	c := make(chan int, 2)
 	v := 0
 	go func() {
 		v = 1
 		c <- 42
 		<-c
+		c <- 42
+		<-c
 	}()
 	time.Sleep(1e7)
 	c <- 43
@@ -591,3 +593,64 @@ func TestRaceChanCloseSend(t *testing.T) {
 	c <- 0
 	<-compl
 }
+
+func TestNoRaceChanMutex(t *testing.T) {
+	done := make(chan struct{})
+	mtx := make(chan struct{}, 1)
+	data := 0
+	go func() {
+		mtx <- struct{}{}
+		data = 42
+		<-mtx
+		done <- struct{}{}
+	}()
+	mtx <- struct{}{}
+	data = 43
+	<-mtx
+	<-done
+}
+
+func TestNoRaceSelectMutex(t *testing.T) {
+	done := make(chan struct{})
+	mtx := make(chan struct{}, 1)
+	aux := make(chan bool)
+	data := 0
+	go func() {
+		select {
+		case mtx <- struct{}{}:
+		case <-aux:
+		}
+		data = 42
+		select {
+		case <-mtx:
+		case <-aux:
+		}
+		done <- struct{}{}
+	}()
+	select {
+	case mtx <- struct{}{}:
+	case <-aux:
+	}
+	data = 43
+	select {
+	case <-mtx:
+	case <-aux:
+	}
+	<-done
+}
+
+func TestRaceChanSem(t *testing.T) {
+	done := make(chan struct{})
+	mtx := make(chan struct{}, 2)
+	data := 0
+	go func() {
+		mtx <- struct{}{}
+		data = 42
+		<-mtx
+		done <- struct{}{}
+	}()
+	mtx <- struct{}{}
+	data = 43
+	<-mtx
+	<-done
+}