bug#20087: 'gensym' is not guaranteed to return a fresh symbol

[Ludovic Courtès]

‘gensym’ returns interned symbols, but the algorithm to determine the
new symbol is simplistic and predictable.

Thus, one can arrange to produce a symbol before ‘gensym’ does, leading
‘gensym’ to return a symbol that’s not fresh (in terms of ‘eq?’), as is
the case with the second call to ‘gensym’ here:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (gensym "x")
$1 = x379
scheme@(guile-user)> 'x405
$2 = x405
scheme@(guile-user)> (gensym "x")
$3 = x405
--8<---------------cut here---------------end--------------->8---

Should we worry about it?  I think it may have hard to anticipate
security implications.

Ludo’.