Hello
I agree, this goes against the main assumption people have about gensym.
I was able to reproduce the bug.
Here's a patch to libguile/symbol.c which fixes this behavior by
incrementing the gensym counter in a loop until it creates a fresh
symbol.
diff --git a/libguile/symbols.c b/libguile/symbols.c
index 71d9827..2df7433 100644
--- a/libguile/symbols.c
+++ b/libguile/symbols.c
@@ -394,19 +394,31 @@ SCM_DEFINE (scm_gensym, "gensym", 0, 1, 0,
SCM suffix, name;
int n, n_digits;
char buf[SCM_INTBUFLEN];
-
+ SCM symbol;
+
if (SCM_UNBNDP (prefix))
prefix = default_gensym_prefix;
- /* mutex in case another thread looks and incs at the exact same moment */
- scm_i_scm_pthread_mutex_lock (&scm_i_misc_mutex);
- n = gensym_counter++;
- scm_i_pthread_mutex_unlock (&scm_i_misc_mutex);
-
- n_digits = scm_iint2str (n, 10, buf);
- suffix = scm_from_latin1_stringn (buf, n_digits);
- name = scm_string_append (scm_list_2 (prefix, suffix));
- return scm_string_to_symbol (name);
+ while(1) {
+ /* mutex in case another thread looks and incs at the exact same moment */
+ scm_i_scm_pthread_mutex_lock (&scm_i_misc_mutex);
+ n = gensym_counter++;
+ scm_i_pthread_mutex_unlock (&scm_i_misc_mutex);
+
+ n_digits = scm_iint2str (n, 10, buf);
+ suffix = scm_from_latin1_stringn (buf, n_digits);
+ name = scm_string_append (scm_list_2 (prefix, suffix));
+
+ symbol = lookup_interned_symbol (name, scm_i_string_hash (name));
+ if (scm_is_true (symbol))
+ {
+ continue;
+ }
+ else
+ {
+ return scm_string_to_symbol (name);
+ }
+ }
}
#undef FUNC_NAME
