/** Unlinks s from the stack. */
void clean(SNode s) {
s.item = null; // forget item
s.waiter = null; // forget thread
/*
* At worst we may need to traverse entire stack to unlink
* s. If there are multiple concurrent calls to clean, we
* might not see s if another thread has already removed
* it. But we can stop when we see any node known to
* follow s. We use s.next unless it too is cancelled, in
* which case we try the node one past. We don't check any
* further because we don't want to doubly traverse just to
* find sentinel.
*/
SNode past = s.next;
if (past != null && past.isCancelled()) past = past.next;
// Absorb cancelled nodes at head
SNode p;
while ((p = head) != null && p != past && p.isCancelled()) casHead(p, p.next);
// Unsplice embedded nodes
while (p != null && p != past) {
SNode n = p.next;
if (n != null && n.isCancelled()) p.casNext(n, n.next);
else p = n;
}
}
/** Puts or takes an item. */
Object transfer(Object e, boolean timed, long nanos) {
/*
* Basic algorithm is to loop trying one of three actions:
*
* 1. If apparently empty or already containing nodes of same
* mode, try to push node on stack and wait for a match,
* returning it, or null if cancelled.
*
* 2. If apparently containing node of complementary mode,
* try to push a fulfilling node on to stack, match
* with corresponding waiting node, pop both from
* stack, and return matched item. The matching or
* unlinking might not actually be necessary because of
* other threads performing action 3:
*
* 3. If top of stack already holds another fulfilling node,
* help it out by doing its match and/or pop
* operations, and then continue. The code for helping
* is essentially the same as for fulfilling, except
* that it doesn't return the item.
*/
SNode s = null; // constructed/reused as needed
int mode = (e == null) ? REQUEST : DATA;
for (; ; ) {
SNode h = head;
if (h == null || h.mode == mode) { // empty or same-mode
if (timed && nanos <= 0) { // can't wait
if (h != null && h.isCancelled()) casHead(h, h.next); // pop cancelled node
else return null;
} else if (casHead(h, s = snode(s, e, h, mode))) {
SNode m = awaitFulfill(s, timed, nanos);
if (m == s) { // wait was cancelled
clean(s);
return null;
}
if ((h = head) != null && h.next == s) casHead(h, s.next); // help s's fulfiller
return (mode == REQUEST) ? m.item : s.item;
}
} else if (!isFulfilling(h.mode)) { // try to fulfill
if (h.isCancelled()) // already cancelled
casHead(h, h.next); // pop and retry
else if (casHead(h, s = snode(s, e, h, FULFILLING | mode))) {
for (; ; ) { // loop until matched or waiters disappear
SNode m = s.next; // m is s's match
if (m == null) { // all waiters are gone
casHead(s, null); // pop fulfill node
s = null; // use new node next time
break; // restart main loop
}
SNode mn = m.next;
if (m.tryMatch(s)) {
casHead(s, mn); // pop both s and m
return (mode == REQUEST) ? m.item : s.item;
} else // lost match
s.casNext(m, mn); // help unlink
}
}
} else { // help a fulfiller
SNode m = h.next; // m is h's match
if (m == null) // waiter is gone
casHead(h, null); // pop fulfilling node
else {
SNode mn = m.next;
if (m.tryMatch(h)) // help match
casHead(h, mn); // pop both h and m
else // lost match
h.casNext(m, mn); // help unlink
}
}
}
}