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
+}

vlib/sync/channel_select_5_test.v

@@ -0,0 +1,62 @@
+fn do_rec_i64(ch chan i64, sumch chan i64) {
+	mut sum := i64(0)
+	for _ in 0 .. 10000 {
+		sum += <-ch
+	}
+	sumch <- sum
+}
+
+fn do_send_int(ch chan int) {
+	for i in 0 .. 10000 {
+		ch <- i
+	}
+}
+
+fn do_send_int2(ch chan int) {
+	for i in 10000 .. 20000 {
+		ch <- i
+	}
+}
+
+fn do_send_int3(ch chan int) {
+	for i in 20000 .. 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_rec_i64(recch, chsum)
+	go do_rec_i64(recch, chsum)
+	go do_send_int(chi)
+	go do_send_int2(chi)
+	go do_send_int3(chi)
+	mut sum := i64(0)
+	mut rl := 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
+	sumrec += <- chsum
+	sumrec += <- chsum
+	// Empty receive buffer
+	for _ in 0 .. recch.cap {
+		sumrec += <- recch
+	}
+	assert sumrec == i64(30000 + recch.cap) * (30000 + recch.cap - 1) / 2
+}

vlib/sync/channels.v

@@ -304,18 +304,16 @@ fn (mut ch Channel) try_push_priv(src voidptr, no_block bool) ChanState {
 					C.memcpy(wr_ptr, src, ch.objsize)
 				}
 				C.atomic_store_u16(status_adr, u16(BufferElemStat.written))
-				old_read_avail := C.atomic_fetch_add_u32(&ch.read_avail, 1)
+				C.atomic_fetch_add_u32(&ch.read_avail, 1)
 				ch.readsem.post()
-				if old_read_avail == 0 {
-					mut null16 := u16(0)
-					for !C.atomic_compare_exchange_weak_u16(&ch.read_sub_mtx, &null16, u16(1)) {
-						null16 = u16(0)
-					}
-					if ch.read_subscriber != voidptr(0) {
-						ch.read_subscriber.sem.post()
-					}
-					C.atomic_store_u16(&ch.read_sub_mtx, u16(0))
+				mut null16 := u16(0)
+				for !C.atomic_compare_exchange_weak_u16(&ch.read_sub_mtx, &null16, u16(1)) {
+					null16 = u16(0)
 				}
+				if ch.read_subscriber != voidptr(0) {
+					ch.read_subscriber.sem.post()
+				}
+				C.atomic_store_u16(&ch.read_sub_mtx, u16(0))
 				return .success
 			} else {
 				if no_block {
@@ -421,18 +419,16 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) ChanState {
 					C.memcpy(dest, rd_ptr, ch.objsize)
 				}
 				C.atomic_store_u16(status_adr, u16(BufferElemStat.unused))
-				old_write_free := C.atomic_fetch_add_u32(&ch.write_free, 1)
+				C.atomic_fetch_add_u32(&ch.write_free, 1)
 				ch.writesem.post()
-				if old_write_free == 0 {
-					mut null16 := u16(0)
-					for !C.atomic_compare_exchange_weak_u16(&ch.write_sub_mtx, &null16, u16(1)) {
-						null16 = u16(0)
-					}
-					if ch.write_subscriber != voidptr(0) {
-						ch.write_subscriber.sem.post()
-					}
-					C.atomic_store_u16(&ch.write_sub_mtx, u16(0))
+				mut null16 := u16(0)
+				for !C.atomic_compare_exchange_weak_u16(&ch.write_sub_mtx, &null16, u16(1)) {
+					null16 = u16(0)
 				}
+				if ch.write_subscriber != voidptr(0) {
+					ch.write_subscriber.sem.post()
+				}
+				C.atomic_store_u16(&ch.write_sub_mtx, u16(0))
 				return .success
 			}
 		}