private Token xfer0(SelectActionImpl<Message> e) {
final boolean haveData = e.isData();
Node s = null; // the node to append, if needed
retry:
for (; ; ) { // restart on append race
if (!e.lease()) return null;
if (isClosed() || (isSendClosed() && e.isData())) {
e.setItem(null);
e.won();
return null;
}
Object item = tryMatch(e, e.message(), haveData);
if (item == LOST) return null;
if (item != NO_MATCH) {
e.setItem(item == CHANNEL_CLOSED ? null : (Message) item);
e.won();
return null;
}
e.returnLease();
if (s == null) {
s = new Node(e);
requestUnpark(s, e.selector().getWaiter());
}
Node pred = tryAppend(s, haveData);
if (pred == null) continue retry; // lost race vs opposite mode
return new Token(s, pred);
}
}
void won() {
if (sa != null) {
Object x = item;
sa.setItem(x == CHANNEL_CLOSED ? null : x);
sa.won();
}
}
private Object tryMatch(SelectActionImpl sa, Message e, boolean haveData) {
boolean closed = isSendClosed(); // must be read before trying to match so as not to miss puts
for (Node h = head, p = h; p != null; ) { // find & match first node
boolean isData = p.isData;
Object item = p.item;
if (item != p && (item != null) == isData) { // unmatched
if (isData == haveData) // can't match
break;
// avoid deadlock by orderdering lease acquisition:
// if p requires a lease and is of lower hashCode than sa, we return sa's lease, acquire
// p's, and then reacquire sa's.
SelectActionImpl sa2 = p.sa;
boolean leasedp;
if (sa != null && sa2 != null && sa2.selector().id < sa.selector().id) {
sa.returnLease();
leasedp = sa2.lease();
if (!sa.lease()) {
if (leasedp) sa2.returnLease();
return LOST;
}
} else leasedp = p.lease();
if (leasedp) {
if (p.casItem(item, e)) { // match
p.won();
for (Node q = p; q != h; ) {
Node n = q.next; // update by 2 unless singleton
if (head == h && casHead(h, n == null ? q : n)) {
h.forgetNext();
break;
} // advance and retry
if ((h = head) == null || (q = h.next) == null || !q.isMatched())
break; // unless slack < 2
}
Strand.unpark(p.waiter, this);
return item;
} else p.returnLease();
}
}
Node n = p.next;
p = (p != n) ? n : (h = head); // Use head if p offlist
}
if (closed) {
assert !haveData;
setReceiveClosed();
return CHANNEL_CLOSED;
}
return NO_MATCH;
}
void returnLease() {
if (sa != null) sa.returnLease();
}
boolean lease() {
if (sa == null) return true;
return sa.lease();
}
/**
* Constructs a new node. Uses relaxed write because item can only be seen after publication via
* casNext.
*/
Node(SelectActionImpl sa) {
UNSAFE.putObject(this, itemOffset, sa.message()); // relaxed write
UNSAFE.putObject(this, saOffset, sa); // relaxed write
this.isData = sa.isData();
}
