sync: fix rare freeze on select (#7398)

vlib/sync/channel_select_4_test.v

@@ -0,0 +1,43 @@
+fn do_rec_i64(ch chan i64, sumch chan i64) {
+	mut sum := i64(0)
+	for _ in 0 .. 30000 {
+		sum += <-ch
+	}
+	sumch <- sum
+}
+
+fn do_send_int(ch chan int) {
+	for i in 0 .. 30000 {
+		ch <- i
+	}
+}
+
+fn test_select() {
+	chi := chan int{cap: 10}
+	recch := chan i64{cap: 10}
+	chsum := chan i64{}
+	go do_rec_i64(recch, chsum)
+	go do_send_int(chi)
+	mut sum := i64(0)
+	mut sl := i64(0)
+	for _ in 0 .. 60000 + recch.cap {
+		select {
+			ri := <-chi {
+				sum += ri
+			}
+			recch <- sl {
+				sl++
+			}
+		}
+	}
+	// Use Gauß' formula
+	expected_sum :=  i64(30000) * (30000 - 1) / 2
+	assert sum == expected_sum
+
+	mut sumrec := <- chsum
+	// Empty receive buffer
+	for _ in 0 .. recch.cap {
+		sumrec += <- recch
+	}
+	assert sumrec == i64(30000 + recch.cap) * (30000 + recch.cap - 1) / 2
+}