unwind: refer to top-level *stderr* stream.

Exception processing should refer to the original, top-level
value of *stderr* rather than the current dynamic value, even
if unwinding has not yet taken place. Using the dynamic value
can not only cause critical output to disappear, but it leads
to a false positive identification of the "unhandled exception
in early initialization" situation, a diagnostic which is
emitted on the C stderr if the Lisp one is unavailable,
which is assumed to be due to initialization not having
yet completed.

* stream.c (top_stderr): New global variable.
(stream_init): GC-protect top_stderr, and initialize it with
the same object that goes into the *stderr* top-level binding.

* stream.h (top_stderr): Declared.

* unwind.c (uw_unwind_to_exit_point, uw_rthrow): Use
top_stderr instead of std_error macro.

1 files changed, 4 insertions, 1 deletions

diff --git a/stream.c b/stream.c
index 4f4edbe3..87eb3383 100644
--- a/stream.c
+++ b/stream.c
@@ -101,6 +101,8 @@ val socket_error_s;
 
 const wchli_t *path_sep_chars = wli("/");
 
+val top_stderr;
+
 val shell, shell_arg;
 
 void strm_base_init(struct strm_base *s)
@@ -5253,6 +5255,7 @@ void stream_init(void)
 {
   prot1(&ap_regex);
   prot1(&plp_regex);
+  prot1(&top_stderr);
 
   detect_format_string();
   detect_path_separators();
@@ -5298,7 +5301,7 @@ void stream_init(void)
   reg_var(stddebug_s = intern(lit("*stddebug*"), user_package),
           make_stdio_stream(stdout, lit("debug")));
   reg_var(stderr_s = intern(lit("*stderr*"), user_package),
-          make_stdio_stream(stderr, lit("stderr")));
+          top_stderr = make_stdio_stream(stderr, lit("stderr")));
   reg_var(stdnull_s = intern(lit("*stdnull*"), user_package),
           make_null_stream());