import time import sync struct St { a int } fn getint() int { return 8 } fn f1(ch1 chan int, ch2 chan St, ch3 chan int, ch4 chan int, ch5 chan int, mut sem sync.Semaphore) { mut a := 5 select { a = <-ch3 { a = 0 } b := <-ch2 { a = b.a } ch3 <- 5 { a = 1 } ch2 <- St{ a: 37 } { a = 2 } ch4 <- (6 + 7 * 9) { a = 8 } ch5 <- getint() { a = 9 } > 300 * time.millisecond { a = 3 } } assert a == 3 sem.post() } fn f2(ch1 chan St, ch2 chan int, mut 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{} ch4 := chan int{} ch5 := chan int{} mut 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, mut sem) n := <-ch3 assert n == 23 ch2 <- St{ a: 13 } sem.wait() stopwatch := time.new_stopwatch() go f1(ch1, ch2, ch3, ch4, ch5, mut sem) sem.wait() elapsed_ms := f64(stopwatch.elapsed()) / time.millisecond // https://docs.microsoft.com/en-us/windows-hardware/drivers/kernel/high-resolution-timers // > For example, for Windows running on an x86 processor, the default interval between // > system clock ticks is typically about 15 milliseconds, and the minimum interval // > between system clock ticks is about 1 millisecond. assert elapsed_ms >= 280.0 // 300 - (15ms + 5ms just in case) ch1.close() ch2.close() mut h := 7 mut is_open := true if select { _ := <-ch2 { h = 0 } ch1 <- h { h = 1 } else { h = 2 } } { panic('channel is still open') } else { is_open = false } // no branch should have run assert h == 7 // since all channels are closed `select` should return `false` assert is_open == false }