cgen: implement channel select block code generation (#6424)

vlib/sync/channel_select_3_test.v

@@ -0,0 +1,97 @@
+import time
+import sync
+
+struct St {
+	a int
+}
+
+fn f1(ch1 chan int, ch2 chan St, ch3 chan int, sem sync.Semaphore) {
+	mut a := 5
+	st := St{}
+	select {
+		a = <-ch3 {
+			a = 0
+		}
+		b := <-ch2 {
+			a = b.a
+		}
+		ch2 <- st {
+			a = 2
+		}
+		> 300 * time.millisecond {
+			a = 3
+		}
+	}
+	assert a == 3
+	sem.post()
+}
+
+fn f2(ch1 chan St, ch2 chan int, sem sync.Semaphore) {
+	mut r := 23
+	for i in 0 .. 2 {
+		select {
+			b := <- ch1 {
+				r = b.a
+			}
+			ch2 <- r {
+				r = 17
+			}
+		}
+		if i == 0 {
+			assert r == 17
+		} else {
+			assert r == 13
+		}
+	}
+	sem.post()
+} 
+
+fn test_select_blocks() {
+	ch1 := chan int{cap: 1}
+	ch2 := chan St{}
+	ch3 := chan int{}
+	sem := sync.new_semaphore()
+	mut r := false
+	t := select {
+		b := <- ch1 {
+			println(b)
+		}
+		else {
+			// no channel ready
+			r = true
+		}
+	}
+	assert r == true
+	assert t == true
+	go f2(ch2, ch3, sem)
+	n := <-ch3
+	assert n == 23
+	ch2 <- St{a: 13}
+	sem.wait()
+	stopwatch := time.new_stopwatch({})
+	go f1(ch1, ch2, ch3, sem)
+	sem.wait()
+	elapsed_ms := f64(stopwatch.elapsed()) / time.millisecond
+	assert elapsed_ms >= 295.0
+
+	ch1.close()
+	ch2.close()
+	mut h := 7
+	u := select {
+		b := <- ch2 {
+			h = 0
+		}
+		ch1 <- h {
+			h = 1
+		}
+		else {
+			h = 2
+		}
+	}
+	// no branch should have run
+	assert h == 7
+	// since all channels are closed `select` should return `false`
+	assert u == false
+}
+
+