Programs exit instead of blocking when cyclic dependency exists.

[TheGrandmother]

Hello! I have found an odd bug.

This is only tested for the new-ponyrt branch.

Some encore programs terminate prematurely. The program doesn't crash it just exits successfully.

For example the following program

class Main
    def main() : void {
        let

            boo = new Boo
            far = new Far

        in{
            print "main  start";
            far = new Far(boo);
            print "main  middle";
            get far.bar();
            print "main  end";


        }
    }

class Boo

    def foo(far : Far) : int{
        print "foo start";
        let
            a = get far.getSilly()
        in{
            print "foo end";
            a
        }

    }

class  Far

    boo : Boo
    silly : int

    def init(boo : Boo) : void {
        print "Far init start"; 
        this.boo = boo;
        this.silly = embed int rand(); end;
        print "Far init end";
    }

    def bar() : void{
        print "bar start";
        let

           lol = this.boo.foo(this) 
        in{
           print "bar midle";
           print get lol;
           print "bar end";
        }

    }

    def getSilly() : int{
        print "get silly start";
        this.silly;
    }

Produces the following output when run in gdb:

[New Thread 0x7ffff5ff6700 (LWP 11947)]
main  start
main  middle
Far init start
Far init end
bar start
bar midle
foo start
[New Thread 0x7ffff57f5700 (LWP 11948)]
[New Thread 0x7ffff37f4700 (LWP 11949)]
[Thread 0x7ffff37f4700 (LWP 11949) exited]
[Thread 0x7ffff57f5700 (LWP 11948) exited]
[Thread 0x7ffff5ff6700 (LWP 11947) exited]
[Inferior 1 (process 11943) exited normally]

So the program shuts down prematurely. I know that the program contains a deadlock and its behavior is thus a bit iffy.

---

I have done some forensic and pinpointed the error too: int ret = swapcontext(&ctx, &this_context->ctx); at line 190 in encore.c.

According to the the internet and the man pages. swapcontext/setcontext can cause the program to prematurely exit successfully.

Also, according to the man pages:

SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specifications of makecontext() and swapcontext(), citing portability issues, and recommending that applications be rewritten to use POSIX threads instead.

I have only tested this on Linux so it might be a portability issue.

[albertnetymk]

Using the head of new-ponyrt, this is my output on linux:

main  10
main  12
Far init start
Far init end
bar start
bar midle
foo start

[TheGrandmother]

@albertnetymk my bad i changed the printouts after I copy pasted the code. But you run osx right

[albertnetymk]

@TheGrandmother My laptop is mac air, but I run the above example using linux via ssh.

[TheGrandmother]

@albertnetymk Oki. Can you test it on osx?

[kikofernandez]

in Mac:

(encore)➜  threadring git:(brussels) ✗ encorec -run test.enc
main  10
main  12
Far init start
Far init end
bar start
bar midle
foo start

[TheGrandmother]

@kikofernandez Oki then it's not just a Linux thing.

[albertnetymk]

@TheGrandmother There seems one circular dependency: main -> far -> boo -> far. Hope I understand the program logic right.

[TheGrandmother]

@albertnetymk Yes there is a cyclic dependency. I found the bug when I was experimenting with deadlocks.

[albertnetymk]

Currently, encore doesn't handle cyclic dependency, so you need to enforce it yourself for now.

[albertnetymk]

@TheGrandmother It's probably good to change the title of this issue, for we know the reason why the program exits prematurely. Maybe sth like this: 'Programs exit instead of blocking when cyclic dependency exists.'?

[TobiasWrigstad]

What is the status of this?

[albertnetymk]

It's captured in my last comment. To my knowledge, it's not decided what's the behavior encountering cyclic dependency, so the program just exits.

[TobiasWrigstad]

I believe the decision was to deadlock, but maybe you can take 5 minutes on Friday to talk about this and we can resolve it?