Printing classes with cycles leads to infinite recursion and bus error

[mstrout]

Summary of Problem

When I create a reference cycle between two class instances and then cast one of those instances to a string, a bus error occurs.

Better behavior would be to detect the infinite loop and possibly throw an exception or print an error?

Steps to Reproduce

Source Code:

class Node {
 var val : string;
 var other : shared Node?;
}
var n1 = new shared Node("node 1",nil);
var n2 = new shared Node("node 2",n1);
n1.other = n2;
writeln(n1:string); // THIS causes a bus error before printing anything out.

Compile command: chpl foo.chpl

Execution command: ./foo

Configuration Information

• Output of chpl --version:

  chpl version 1.28.0
  built with LLVM version 14.0.6
  Copyright 2020-2022 Hewlett Packard Enterprise Development LP
  Copyright 2004-2019 Cray Inc.
  (See LICENSE file for more details)

I am using the homebrew install of 1.28.0.

[bradcray]

Better behavior would be to detect the infinite loop

Yeah, this is a long-term known-but-ignored issue we have when printing classes (which is essentially converting them to strings in another way): We just recursively print their fields, so will recurse infinitely for anything that's not a DAG. We've discussed detecting the cycles (though that can get expensive for a large graph, and essentially equivalent to pickling or pointer swizzling, I think?) or just making the default class printer less likely to print recursively (though it's pretty cool to have it print trees automatically; DAGs are arguably a little redundant the way they're printed).

A user-level workaround is to add your own writeThis() method to the class to print it the way you want (presumably cycle-free).

[vasslitvinov]

I like Brad's proposal to detect statically where the recursion could occur, https://github.com/chapel-lang/chapel/issues/21450#issuecomment-1410868751 .

Note that recursion could potentially go through a record and/or multiple intermediaries.

Another idea is to add an argument to writeThis that's one of:

• allowed recursion depth; decremented when invoking children's writeThis
• the set of class instances seen upon the current top-most invocation of writeThis

The predefined writeThis, then, invokes the specific writeThis with the initial value of the extra argument, which is 3, say, for recursion depth or the empty set for the set.