sync/channels: provide `.cap` and `.len()` (#6104)

doc/upcoming.md

@@ -250,6 +250,15 @@ if ch.pop(&m) {
 }
 ```
 
+There are also methods `try_push()` and `try_pop()` which return immediatelly with the return value `.not_ready` if the transaction
+cannot be performed without waiting. The return value is of type `sync.TransactionState` which can also be
+`.success` or `.closed`.
+
+To monitor a channel there is a method `len()` which returns the number of elements currently in the queue and the attribute
+`cap` for the queue length. Please be aware that in general `channel.len() > 0` does not guarantee that the next
+`pop()` will succeed without waiting, since other threads may already have "stolen" elements from the queue. Use `try_pop()` to
+accomplish this kind of task.
+
 The select call is somewhat tricky. The `channel_select()` function needs three arrays that
 contain the channels, the directions (pop/push) and the object references and
 a timeout of type `time.Duration` (`time.infinite` or `-1` to wait unlimited) as parameters. It returns the

vlib/sync/channel_fill_test.v

@@ -0,0 +1,22 @@
+import sync
+
+const (
+	queue_len = 1000
+	queue_fill = 763
+)
+
+fn do_send(mut ch sync.Channel, fin sync.Semaphore) {
+	for i in 0 .. queue_fill {
+		ch.push(&i)
+	}
+	fin.post()
+}
+
+fn test_channel_len_cap() {
+	mut ch := sync.new_channel<int>(queue_len)
+	sem := sync.new_semaphore()
+	go do_send(mut ch, sem)
+	sem.wait()
+	assert ch.cap == queue_len
+	assert ch.len() == queue_fill
+}

vlib/sync/channels.v

@@ -90,7 +90,6 @@ struct Channel {
 	ringbuf            byteptr // queue for buffered channels
 	statusbuf          byteptr // flags to synchronize write/read in ringbuf
 	objsize            u32
-	queue_length       u32 // in #objects
 mut: // atomic
 	write_adr          C.atomic_uintptr_t // if != NULL the next obj can be written here without wait
 	read_adr           C.atomic_uintptr_t // if != NULL an obj can be read from here without wait
@@ -106,6 +105,8 @@ mut: // atomic
 	write_sub_mtx      u16
 	read_sub_mtx       u16
 	closed             u16
+pub:
+	cap                u32 // queue length in #objects
 }
 
 pub fn new_channel<T>(n u32) &Channel {
@@ -116,7 +117,7 @@ pub fn new_channel<T>(n u32) &Channel {
 		writesem_im: new_semaphore()
 		readsem_im: new_semaphore()
 		objsize: st
-		queue_length: n
+		cap: n
 		write_free: n
 		read_avail: 0
 		ringbuf: if n > 0 { malloc(int(n * st)) } else { byteptr(0) }
@@ -152,6 +153,11 @@ pub fn (mut ch Channel) close() {
 	C.atomic_store_u16(&ch.write_sub_mtx, u16(0))
 }
 
+[inline]
+pub fn (mut ch Channel) len() int {
+	return int(C.atomic_load_u32(&ch.read_avail))
+}
+
 [inline]
 pub fn (mut ch Channel) push(src voidptr) {
 	if ch.try_push_priv(src, false) == .closed {
@@ -188,7 +194,7 @@ fn (mut ch Channel) try_push_priv(src voidptr, no_block bool) TransactionState {
 				return .success
 			}
 		}
-		if no_block && ch.queue_length == 0 {
+		if no_block && ch.cap == 0 {
 			return .not_ready
 		}
 		// get token to read
@@ -201,7 +207,7 @@ fn (mut ch Channel) try_push_priv(src voidptr, no_block bool) TransactionState {
 		if !got_sem {
 			ch.writesem.wait()
 		}
-		if ch.queue_length == 0 {
+		if ch.cap == 0 {
 			// try to advertise current object as readable
 			mut read_in_progress := false
 			C.atomic_store_ptr(&ch.read_adr, src)
@@ -275,8 +281,8 @@ fn (mut ch Channel) try_push_priv(src voidptr, no_block bool) TransactionState {
 				mut wr_idx := C.atomic_load_u32(&ch.buf_elem_write_idx)
 				for {
 					mut new_wr_idx := wr_idx + 1
-					for new_wr_idx >= ch.queue_length {
+					for new_wr_idx >= ch.cap {
-						new_wr_idx -= ch.queue_length
+						new_wr_idx -= ch.cap
 					}
 					if C.atomic_compare_exchange_strong_u32(&ch.buf_elem_write_idx, &wr_idx, new_wr_idx) {
 						break
@@ -335,7 +341,7 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) TransactionState {
 	mut write_in_progress := false
 	for {
 		mut got_sem := false
-		if ch.queue_length == 0 {
+		if ch.cap == 0 {
 			// unbuffered channel - first see if a `push()` has adversized
 			mut rdadr := C.atomic_load_ptr(&ch.read_adr)
 			for rdadr != C.NULL {
@@ -367,7 +373,7 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) TransactionState {
 			}
 			ch.readsem.wait()
 		}
-		if ch.queue_length > 0 {
+		if ch.cap > 0 {
 			// try to get buffer token
 			mut obj_in_queue := false
 			mut rd_avail := C.atomic_load_u32(&ch.read_avail)
@@ -381,8 +387,8 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) TransactionState {
 				mut rd_idx := C.atomic_load_u32(&ch.buf_elem_read_idx)
 				for {
 					mut new_rd_idx := rd_idx + 1
-					for new_rd_idx >= ch.queue_length {
+					for new_rd_idx >= ch.cap {
-						new_rd_idx -= ch.queue_length
+						new_rd_idx -= ch.cap
 					}
 					if C.atomic_compare_exchange_weak_u32(&ch.buf_elem_read_idx, &rd_idx, new_rd_idx) {
 						break
@@ -419,7 +425,7 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) TransactionState {
 		}
 		// try to advertise `dest` as writable
 		C.atomic_store_ptr(&ch.write_adr, dest)
-		if ch.queue_length == 0 {
+		if ch.cap == 0 {
 			mut rdadr := C.atomic_load_ptr(&ch.read_adr)
 			if rdadr != C.NULL {
 				mut dest2 := dest
@@ -431,7 +437,7 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) TransactionState {
 				}
 			}
 		}
-		if ch.queue_length == 0 && !write_in_progress {
+		if ch.cap == 0 && !write_in_progress {
 			mut null16 := u16(0)
 			for !C.atomic_compare_exchange_weak_u16(&ch.write_sub_mtx, &null16, u16(1)) {
 				null16 = u16(0)