[imp-hackers] problems with *DEFAULT-SPECIAL-BINDINGS*

[Tobias C. Rittweiler]

Most implementations that provide multi-threading also provide a
variable which contains an association lists for variable bindings to be
established in new threads.

I wanted to introduce the same to SBCL.

However, Gábor Melis made me aware of the following issue:


One use of *DEFAULT-SPECIAL-BINDINGS* is for libraries to establish
thread-local variable bindings to new threads by pushing onto
*DEFAULT-SPECIAL-BINDINGS*.

Howsoever, that will only affect new threads, not already running
threads.

That's a big problem for the following scenario:

Let's say your Lisp development environment is multi-threaded and allows
you to run multiple listeners at the same time. Each listener is run in
its own thread.

Now, you opened a few listeners, then loaded in this library which
pushes onto *DEFAULT-SPECIAL-BINDINGS*. Now all running listeners, when
the user uses them to call into that library, will operate on the _same_
global binding. 

Hilarity ensues.


Do you have ideas on how to provide a mechanism that handles this
scenario reliably?

  -T.