bug#45339: Readline filename completion

Hi. Filename completion just doesn't work at all. Using
filename-completion-function as a completer for readline always throws
an error.

Minimal example:
  (use-modules (ice-9 readline))

  (with-readline-completion-function
   filename-completion-function
   readline)

Hitting TAB when prompted results in:
  File: Backtrace:
  In ice-9/boot-9.scm:
    1736:10  8 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  In unknown file:
             7 (apply-smob/0 #<thunk 7ffa51f5e500>)
  In ice-9/boot-9.scm:
      718:2  6 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
  In ice-9/eval.scm:
      619:8  5 (_ #(#(#<directory (guile-user) 7ffa51f63c80>)))
  In ice-9/boot-9.scm:
     2806:4  4 (save-module-excursion _)
    4351:12  3 (_)
  In ice-9/readline.scm:
      213:4  2 (with-readline-completion-function _ #<procedure 7ffa4f684328 at /data/lampilelo/Misc-Scripts/install-wizard.scm:27:1 ()>)
  In unknown file:
             1 (%readline "File: " #<input: file /dev/pts/1> #<undefined> #<undefined>)
             0 (filename-completion-function "" #t)

  ERROR: In procedure filename-completion-function:
  In procedure scm_from_stringn: NULL string pointer

My version of Guile is 3.0.4 but I expect it to fail on every version.
It results in an error because the scm_filename_completion_function()
calls readline's rl_filename_completion_function() and doesn't check if
it returned NULL instead of a string.
Readline's info manual states:
"The generator function returns '(char *)NULL' to inform
'rl_completion_matches()' that there are no more possibilities left."
So it's impossible for the current implementation to work, since
scm_filename_completion_function() throws an error every time NULL is
returned.

I'm attaching a patch.

Jakub

From 1f8fce206dc657a04694f8b546df5d1fe1ca1856 Mon Sep 17 00:00:00 2001
From: Jakub Wojciech <[email protected]>
Date: Sun, 20 Dec 2020 10:56:51 +0100
Subject: [PATCH] Fixed filename-completion-function for readline completion

* guile-readline/readline.c (scm_filename_completion_function):
  A completion function should return #f when there's no more
  candidates.  Since the result of readline's
  rl_filename_completion_function was never checked it was impossible
  for it to work as intended and instead of #f it threw an error from
  trying to convert NULL to an scm string.
---
 guile-readline/readline.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/guile-readline/readline.c b/guile-readline/readline.c
index 5c4a32689..7bb2394ba 100644
--- a/guile-readline/readline.c
+++ b/guile-readline/readline.c
@@ -386,8 +386,11 @@ SCM_DEFINE (scm_filename_completion_function, "filename-completion-function", 2,
 #else
   s = filename_completion_function (c_text, scm_is_true (continuep));
 #endif
-  ans = scm_take_locale_string (s);
   free (c_text);
+  if (!s) {
+    return SCM_BOOL_F;
+  }
+  ans = scm_take_locale_string (s);
   return ans;
 }
 #undef FUNC_NAME
--
2.29.2

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