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// program inspired by "Proving Copyless Message Passing" (Villard, Lozes and Calcagno, APLAS 2009)
// msg tag indicates what the type of the message
// channel is freed by Getter when it completes
// ack works, but an assume is needed and negative credits are sent over channels!
// Conjecture: it is ok to send debit for yourself over yourself.
// Why: Suppose a channel that allows self-debt is involved in a deadlock. The either that channel is empty, which means there's no difference between the situation with or with self-debt. Or the channel is non-empty. This means that we can make progress by receiving the message stored in the channel! Does this make any sense?
channel C(msg: int, n: Node) where
(msg == 0 || msg == 1 || msg == 2) &&
(msg == 0 ==> credit(this, -1)) && // ack
(msg == 1 ==> n != null && acc(n.next) && acc(n.mu) && credit(this, -1)) && // cell
(msg == 2 ==> acc(this.mu, 50)); // done
class Node {
var next: Node;
function length(): int
requires this.list;
{
unfolding this.list in 1 + (next == null ? 0 : next.length())
}
predicate list {
acc(next) && acc(mu) && (next != null ==> next.list)
}
}
class Program {
method Putter(e: C, x0: Node)
requires e!= null && acc(e.mu, 50) && e.mu == waitlevel && (x0 != null ==> x0.list) && (x0 != null ==> credit(e, - 1));
{
var x: Node := x0;
var t: Node;
while(x != null)
invariant (x != null ==> x.list) && acc(e.mu, 50) && credit(e, - 1);
{
unfold x.list;
t := x.next;
send e(1, x);
x := t;
var ack;
assume waitlevel << e.mu; // Chalice should be able to figure this out itself
receive ack, t := e;
if(ack != 2) { assume false; /* abort */ }
}
send e(2, null);
}
method Getter(f: C)
requires f!= null && credit(f, 1) && acc(f.mu, 50) && waitlevel << f.mu;
{
var x: Node := null;
var msg := 1;
while(msg != 0)
invariant msg == 0 || msg == 1;
invariant acc(f.mu, 50) && waitlevel << f.mu && (credit(f, 1));
{
assert msg == 1
receive msg, x := f;
if(msg == 1) {
free x;
}
send f(0, null);
if(msg == 2) { assume false; /* abort */ }
}
receive msg, x := f;
if (msg != 2) { assume false; /* abort */ }
free f; // close the channel
}
method Main(x: Node)
requires x != null;
requires x.list;
{
var e := new C;
fork Putter(e, x);
fork Getter(e);
}
}
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