sync: support concurrent selects on same channel(s) (#7415)

vlib/sync/channel_select_6_test.v

@@ -0,0 +1,75 @@
+// This test case runs concurrent 3 instances of `do_select` that
+// communicate with 6 other threads doing send and receive operations.
+// There are buffered and unbuffered channels - handled by one or two
+// concurrend threads on the other side
+
+fn do_select(ch1 chan int, ch2 chan int, chf1 chan f64, chf2 chan f64, sumch1 chan i64, sumch2 chan i64) {
+	mut sum1 := i64(0)
+	mut sum2 := i64(0)
+	f1 := 17.
+	f2 := 7.	
+	for _ in 0 .. 20000 + chf1.cap / 3 {
+		select {
+			chf1 <- f1 {}
+			i := <-ch1 {
+				sum1 += i
+			}
+			j := <-ch2 {
+				sum2 += j
+			}
+			chf2 <- f2 {}
+		}
+	}
+	sumch1 <- sum1
+	sumch2 <- sum2
+}
+
+fn do_send_int(ch chan int, factor int) {
+	for i in 0 .. 10000 {
+		ch <- (i * factor)
+	}
+}
+
+fn do_rec_f64(ch chan f64, sumch chan f64) {
+	mut sum:= 0.
+	for _ in 0 .. 10000 {
+		sum += <-ch
+	}
+	sumch <- sum
+}
+
+fn test_select() {
+	ch1 := chan int{cap: 3}
+	ch2 := chan int{}
+	// buffer length of chf1 mus be mutiple of 3 (# select threads)
+	chf1 := chan f64{cap: 30}
+	chf2 := chan f64{}
+	chsum1 := chan i64{}
+	chsum2 := chan i64{}
+	chsumf1 := chan f64{}
+	chsumf2 := chan f64{}
+	go do_send_int(ch1, 3)
+	go do_select(ch1, ch2, chf1, chf2, chsum1, chsum2)
+	go do_rec_f64(chf1, chsumf1)
+	go do_rec_f64(chf2, chsumf2)
+	go do_rec_f64(chf2, chsumf2)
+	go do_select(ch1, ch2, chf1, chf2, chsum1, chsum2)
+	go do_send_int(ch2, 7)
+	go do_send_int(ch2, 17)
+	go do_select(ch1, ch2, chf1, chf2, chsum1, chsum2)
+
+	sum1 := <-chsum1 + <-chsum1 + <-chsum1
+	sum2 := <-chsum2 + <-chsum2 + <-chsum2
+	mut sumf1 := <-chsumf1
+	// empty channel buffer
+	for _ in 0 .. chf1.cap {
+		sumf1 += <-chf1
+	}
+	sumf2 := <-chsumf2 + <-chsumf2
+	// Use Gauß' formula
+	expected_sum :=  i64(10000) * (10000 - 1) / 2
+	assert sum1 == 3 * expected_sum
+	assert sum2 == (7 + 17) * expected_sum
+	assert sumf1 == 17. * f64(10000 + chf1.cap)
+	assert sumf2 == 7. * 20000
+}