The Problem
===========

Certain applications embedding Guile (Scwm, Guppi) have found it
necessary to include hacked versions of scm_call_with_dynamic_root.

They want to run user callbacks, but don't want the callback to be
able to longjmp (via exceptions or continuations) randomly in and out,
since the C code hasn't been written to dynamically wind/unwind local
state.  This is likely to be a common problem for users of Guile as an
extension language.

libguile/root.c:scm_call_with_dynamic_root seems to almost do this,
but it has the apparently undesirable behaviour of unwinding the
dynamic state when the protected procedure is called.  In addition
the implementation looks a bit heavy for use in every callback.

scm_call_with_dynamic_root was implemented to support threading, so
the needs of libguile itself should be considered.  Other
considerations are how any new interface interacts with error handling
and reporting; whether a new interface is convenient to use from C;
whether a new interface should also be available to Scheme code.

Discussion
==========

Proposal
========