R devel - May 2024 - [External] R hang/bug with circular references and promises

On Sat, 11 May 2024, Travers Ching wrote:
> The following code snippet causes R to hang. This example might be a
> bit contrived as I was experimenting and trying to understand
> promises, but uses only base R.
>
> It looks like it is looking for "not_a_variable" recursively but
since
> it doesn't exist it goes on indefinitely.
>
> x0 <- new.env()
> x1 <- new.env(parent = x0)
> parent.env(x0) <- x1
> delayedAssign("v", not_a_variable, eval.env=x1)
> delayedAssign("w", v, assign.env=x1, eval.env=x0)
> x1$w
This has nothing to do with promises. You created a cycle in the
environment chain. A simpler variant:

e <- new.env()
parent.env(e) <- e
get("x", e)

This will hang and is not interruptable -- loops searching up
environment chains are too speed-critical to check for interrupts.  It
is, however, pretty easy to check whether the parent change would
create a cycle and throw an error if it would. Need to think a bit
about exactly where the check should go.

Best,

luke
>
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>
-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu/

On Sat, May 11, 2024 at 9:34?AM luke-tierney--- via R-devel
<r-devel at r-project.org> wrote:
>
> On Sat, 11 May 2024, Travers Ching wrote:
>
> > The following code snippet causes R to hang. This example might be a
> > bit contrived as I was experimenting and trying to understand
> > promises, but uses only base R.
>
> This has nothing to do with promises. You created a cycle in the
> environment chain. A simpler variant:
>
> e <- new.env()
> parent.env(e) <- e
> get("x", e)
>
> This will hang and is not interruptable -- loops searching up
> environment chains are too speed-critical to check for interrupts.  It
> is, however, pretty easy to check whether the parent change would
> create a cycle and throw an error if it would. Need to think a bit
> about exactly where the check should go.
FWIW, the help for parent.env already explicitly warns against using
parent.env <-:

The replacement function ?parent.env<-? is extremely dangerous as
     it can be used to destructively change environments in ways that
     violate assumptions made by the internal C code.  It may be
     removed in the near future.

Peter