sync/channels: expose `ch.closed` as `bool` pseudo attribute (#8244)

doc/docs.md

@@ -2484,6 +2484,7 @@ res := ch.try_push(a) // try to perform `ch <- a`
 println(res)
 l := ch.len // number of elements in queue
 c := ch.cap // maximum queue length
+is_closed := ch.closed // bool flag - has `ch` been closed
 println(l)
 println(c)
 mut b := Abc{}
@@ -2495,7 +2496,10 @@ The `try_push/pop()` methods will return immediately with one of the results
 `.success`, `.not_ready` or `.closed` - dependent on whether the object has been transferred or
 the reason why not.
 Usage of these methods and properties in production is not recommended -
-algorithms based on them are often subject to race conditions. Use `select` instead.
+algorithms based on them are often subject to race conditions. Especially `.len` and
+`.closed` should not be used to make decisions.
+Use `or` branches, error propagation or `select` instead (see [Syntax and Usage](#syntax-and-usage)
+and [Channel Select](#channel-select) above).
 
 ### Shared Objects
 

vlib/sync/channel_close_test.v

@@ -1,79 +1,73 @@
 import sync
 
-fn do_rec(mut ch sync.Channel, mut resch sync.Channel) {
+fn do_rec(ch chan int, resch chan i64) {
 	mut sum := i64(0)
 	for {
-		mut a := 0
-		if !ch.pop(&a) {
+		a := <-ch or {
 			break
 		}
 		sum += a
 	}
+	assert ch.closed == true
 	println(sum)
-	resch.push(&sum)
+	resch <- sum
 }
 
-fn do_send(mut ch sync.Channel) {
+fn do_send(ch chan int) {
 	for i in 0 .. 8000 {
-		ch.push(&i)
+		ch <- i
 	}
+	assert ch.closed == false
 	ch.close()
+	assert ch.closed == true
 }
 
 fn test_channel_close_buffered_multi() {
-	mut ch := sync.new_channel<int>(0)
-	mut resch := sync.new_channel<i64>(100)
-	go do_rec(mut ch, mut resch)
-	go do_rec(mut ch, mut resch)
-	go do_rec(mut ch, mut resch)
-	go do_rec(mut ch, mut resch)
-	go do_send(mut ch)
+	ch := chan int{cap: 10}
+	resch := chan i64{}
+	go do_rec(ch, resch)
+	go do_rec(ch, resch)
+	go do_rec(ch, resch)
+	go do_rec(ch, resch)
+	go do_send(ch)
 	mut sum := i64(0)
 	for _ in 0 .. 4 {
-		mut r := i64(0)
-		resch.pop(&r)
-		sum += r
+		sum += <- resch
 	}
 	assert sum == i64(8000) * (8000 - 1) / 2
 }
 
 fn test_channel_close_unbuffered_multi() {
-	mut ch := sync.new_channel<int>(0)
-	mut resch := sync.new_channel<i64>(100)
-	go do_rec(mut ch, mut resch)
-	go do_rec(mut ch, mut resch)
-	go do_rec(mut ch, mut resch)
-	go do_rec(mut ch, mut resch)
-	go do_send(mut ch)
+	ch := chan int{}
+	resch := chan i64{}
+	go do_rec(ch, resch)
+	go do_rec(ch, resch)
+	go do_rec(ch, resch)
+	go do_rec(ch, resch)
+	go do_send(ch)
 	mut sum := i64(0)
 	for _ in 0 .. 4 {
-		mut r := i64(0)
-		resch.pop(&r)
-		sum += r
+		sum += <-resch
 	}
 	assert sum == i64(8000) * (8000 - 1) / 2
 }
 
 fn test_channel_close_buffered() {
-	mut ch := sync.new_channel<int>(0)
-	mut resch := sync.new_channel<i64>(100)
-	go do_rec(mut ch, mut resch)
-	go do_send(mut ch)
+	ch := chan int{cap: 100}
+	resch := chan i64{}
+	go do_rec(ch, resch)
+	go do_send(ch)
 	mut sum := i64(0)
-	mut r := i64(0)
-	resch.pop(&r)
-	sum += r
+	sum += <-resch
 	assert sum == i64(8000) * (8000 - 1) / 2
 }
 
 fn test_channel_close_unbuffered() {
-	mut ch := sync.new_channel<int>(0)
-	mut resch := sync.new_channel<i64>(100)
-	go do_rec(mut ch, mut resch)
-	go do_send(mut ch)
+	ch := chan int{}
+	resch := chan i64{cap: 100}
+	go do_rec(ch, resch)
+	go do_send(ch)
 	mut sum := i64(0)
-	mut r := i64(0)
-	resch.pop(&r)
-	sum += r
+	sum += <-resch
 	assert sum == i64(8000) * (8000 - 1) / 2
 }

vlib/sync/channels.v

@@ -162,6 +162,11 @@ pub fn (mut ch Channel) len() int {
 	return int(C.atomic_load_u32(&ch.read_avail))
 }
 
+[inline]
+pub fn (mut ch Channel) closed() bool {
+	return C.atomic_load_u16(&ch.closed) != 0
+}
+
 [inline]
 pub fn (mut ch Channel) push(src voidptr) {
 	if ch.try_push_priv(src, false) == .closed {

vlib/v/checker/checker.v

@@ -2080,6 +2080,10 @@ pub fn (mut c Checker) selector_expr(mut selector_expr ast.SelectorExpr) table.T
 			selector_expr.typ = table.int_type
 			return table.int_type
 		}
+		if sym.kind == .chan && field_name == 'closed' {
+			selector_expr.typ = table.bool_type
+			return table.bool_type
+		}
 	}
 	mut unknown_field_msg := 'type `$sym.name` has no field or method `$field_name`'
 	mut has_field := false

vlib/v/gen/cgen.v

@@ -2933,8 +2933,8 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 		g.write('$info.size')
 		return
 	}
-	if sym.kind == .chan && node.field_name == 'len' {
-		g.write('sync__Channel_len(')
+	if sym.kind == .chan && (node.field_name == 'len' || node.field_name == 'closed') {
+		g.write('sync__Channel_${node.field_name}(')
 		g.expr(node.expr)
 		g.write(')')
 		return