runtime: wake select receivers for unbuffered sends

runtime/internal/runtime/z_chan.go

@@ -117,6 +117,9 @@ func ChanSend(p *Chan, v unsafe.Pointer, eltSize int) bool {
 	if n == 0 {
 		for p.getp != chanHasRecv && !p.close {
 			p.sends++
+			// A blocked unbuffered send must wake select-based receivers so they can
+			// retry ChanTryRecv after observing that a sender is now waiting.
+			notifyOps(p)
 			p.cond.Wait(&p.mutex)
 			p.sends--
 		}

test/select_test.go

@@ -0,0 +1,49 @@
+package test
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSelectRecvWakesForBlockedUnbufferedSend(t *testing.T) {
+	for i := 0; i < 50; i++ {
+		res := make(chan struct{})
+		done := make(chan struct{})
+		started := make(chan struct{})
+		received := make(chan struct{})
+		sendDone := make(chan struct{})
+
+		go func() {
+			close(started)
+			select {
+			case <-res:
+				close(received)
+			case <-done:
+			}
+		}()
+
+		<-started
+		time.Sleep(time.Millisecond)
+
+		go func() {
+			res <- struct{}{}
+			close(sendDone)
+		}()
+
+		select {
+		case <-sendDone:
+		case <-time.After(200 * time.Millisecond):
+			close(done)
+			t.Fatalf("iteration %d: unbuffered send did not wake select receiver", i)
+		}
+
+		select {
+		case <-received:
+		case <-time.After(200 * time.Millisecond):
+			close(done)
+			t.Fatalf("iteration %d: select receiver did not receive sent value", i)
+		}
+
+		close(done)
+	}
+}