defvar: bugfix: bad state caused by exception.

The following situation can occur:

  (defvar v expr) ;; expr throws!

What happens is that the internal hash table of global
variables ends up with an entry that has a nil value,
instead of the expected (v . <value>) cons cell.
The hash table entry is created when the table is probed
for the existence of v, but then is never populated
with a binding because expr throws.

The consequence then is that (boundp v) returns nil, yet
subsequent (defvar v ...) expressions think that v exists, and
refuse to define it. (defparm v ...) also fails; it relies on
(defvar ...) which fails to define the variable, and then
tries to assign to it, which throws.

* eval.c (rt_defvarl): If the hash table exists, but has a nil
value, treat that as an undefined variable.  Thus, define the
variable not only if the hash cell is newly made, but also if
it already exists, with a null cdr.

1 files changed, 1 insertions, 1 deletions

diff --git a/eval.c b/eval.c
index 9cfeba03..2cf3e36e 100644
--- a/eval.c
+++ b/eval.c
@@ -1895,7 +1895,7 @@ static val rt_defvarl(val sym)
   val new_p;
   val cell = gethash_c(self, top_vb, sym, mkcloc(new_p));
 
-  if (new_p) {
+  if (new_p || !cdr(cell)) {
     uw_purge_deferred_warning(cons(var_s, sym));
     uw_purge_deferred_warning(cons(sym_s, sym));
     remhash(top_smb, sym);