bug#25498: Crash in open-file; patch attached

[Linas Vepstas <linasvepstas@gmail.com>]

The following crashes instantly; I used single-quotes by accident.

(open-file "/tmp/lg" 'w')

Stack:

Enter `,help' for help.
scheme@(guile-user)> (open-file "/tmp/lg" 'w')

Thread 1 "guile" received signal SIGSEGV, Segmentation fault.
scm_i_mode_to_open_flags (mode=mode@entry=0x555555ac5660,
    is_binary=is_binary@entry=0x7fffffffd46c,
    FUNC_NAME=FUNC_NAME@entry=0x7ffff7b89a7d "open-file")
    at ../../libguile/fports.c:168
168  switch (*md)
(gdb) bt
#0  scm_i_mode_to_open_flags (mode=mode@entry=0x555555ac5660,
    is_binary=is_binary@entry=0x7fffffffd46c,
    FUNC_NAME=FUNC_NAME@entry=0x7ffff7b89a7d "open-file")
    at ../../libguile/fports.c:168
#1  0x00007ffff7b057e9 in scm_open_file_with_encoding (
    filename=filename@entry=0x555555b7fd98, mode=mode@entry=0x555555ac5660,
    guess_encoding=0x4, encoding=0x4) at ../../libguile/fports.c:242
#2  0x00007ffff7b05b83 in scm_i_open_file (filename=0x555555b7fd98,
    mode=0x555555ac5660, keyword_args=<optimized out>)
    at ../../libguile/fports.c:380
#3  0x00007ffff7b6a221 in vm_debug_engine (thread=0x555555ac5660,
    vp=0x555555844f30, registers=0x54aad62357094bc, resume=39)
    at ../../libguile/vm-engine.c:760

A patch that seems reasonable to me:

$ git diff
diff --git a/libguile/fports.c b/libguile/fports.c
index 8fa69933d..28e666b6a 100644
--- a/libguile/fports.c
+++ b/libguile/fports.c
@@ -230,6 +230,9 @@ scm_open_file_with_encoding (SCM filename, SCM mode,
   unsigned int retries;
   char *file;

+  if (SCM_UNLIKELY (!scm_is_string (mode)))
+    scm_wrong_type_arg_msg (FUNC_NAME, 2, mode, "mode to be string");
+
   if (SCM_UNLIKELY (!(scm_is_false (encoding) || scm_is_string (encoding))))
     scm_wrong_type_arg_msg (FUNC_NAME, 0, encoding,
                             "encoding to be string or false");