Port redirect-debugging-output to non-GNU/Linux

Problem reported by Kylie McClain for musl in:
http://lists.gnu.org/archive/html/emacs-devel/2016-03/msg01592.html
* etc/DEBUG, etc/NEWS: Mention this.
* src/callproc.c (child_setup) [!MSDOS]:
* src/dispnew.c (init_display):
* src/emacs.c (main, Fdaemon_initialized):
* src/minibuf.c (read_minibuf_noninteractive):
* src/regex.c (xmalloc, xrealloc):
Prefer symbolic names like STDERR_FILENO to magic numbers like 2,
to make file-descriptor manipulation easier to follow.
* src/emacs.c (relocate_fd) [!WINDOWSNT]: Remove; no longer needed
now that we make sure stdin, stdout and stderr are open.  All uses
removed.
(main): Make sure standard FDs are OK.  Prefer symbolic names like
EXIT_FAILURE to magic numbers like 1.  Use bool for boolean.
* src/lisp.h (init_standard_fds): New decl.
* src/print.c (WITH_REDIRECT_DEBUGGING_OUTPUT) [GNU_LINUX]:
Remove; no longer needed.
(Fredirect_debugging_output): Define on all platforms, not just
GNU/Linux.  Redirect file descriptor, not stream, so that the code
works even if stderr is not an lvalue.  Report an error if the
file arg is neither a string nor nil.
(syms_of_print): Always define redirect-debugging-output.
* src/sysdep.c (force_open, init_standard_fds): New functions.

1 files changed, 24 insertions, 27 deletions

diff --git a/src/print.c b/src/print.c
index 2b53d7580b1..db2918ff478 100644
--- a/src/print.c
+++ b/src/print.c
@@ -775,15 +775,6 @@ debug_output_compilation_hack (bool x)
   print_output_debug_flag = x;
 }
 
-#if defined (GNU_LINUX)
-
-/* This functionality is not vitally important in general, so we rely on
-   non-portable ability to use stderr as lvalue.  */
-
-#define WITH_REDIRECT_DEBUGGING_OUTPUT 1
-
-static FILE *initial_stderr_stream = NULL;
-
 DEFUN ("redirect-debugging-output", Fredirect_debugging_output, Sredirect_debugging_output,
        1, 2,
        "FDebug output file: \nP",
@@ -793,30 +784,38 @@ Optional arg APPEND non-nil (interactively, with prefix arg) means
 append to existing target file.  */)
   (Lisp_Object file, Lisp_Object append)
 {
-  if (initial_stderr_stream != NULL)
-    {
-      block_input ();
-      fclose (stderr);
-      unblock_input ();
-    }
-  stderr = initial_stderr_stream;
-  initial_stderr_stream = NULL;
+  /* If equal to STDERR_FILENO, stderr has not been duplicated and is OK as-is.
+     Otherwise, this is a close-on-exec duplicate of the original stderr. */
+  static int stderr_dup = STDERR_FILENO;
+  int fd = stderr_dup;
 
-  if (STRINGP (file))
+  if (! NILP (file))
     {
       file = Fexpand_file_name (file, Qnil);
-      initial_stderr_stream = stderr;
-      stderr = emacs_fopen (SSDATA (file), NILP (append) ? "w" : "a");
-      if (stderr == NULL)
+
+      if (stderr_dup == STDERR_FILENO)
         {
-          stderr = initial_stderr_stream;
-          initial_stderr_stream = NULL;
-          report_file_error ("Cannot open debugging output stream", file);
+          int n = fcntl (STDERR_FILENO, F_DUPFD_CLOEXEC, STDERR_FILENO + 1);
+          if (n < 0)
+            report_file_error ("dup", file);
+          stderr_dup = n;
         }
+
+      fd = emacs_open (SSDATA (ENCODE_FILE (file)),
+                       (O_WRONLY | O_CREAT
+                        | (! NILP (append) ? O_APPEND : O_TRUNC)),
+                       0666);
+      if (fd < 0)
+        report_file_error ("Cannot open debugging output stream", file);
     }
+
+  fflush (stderr);
+  if (dup2 (fd, STDERR_FILENO) < 0)
+    report_file_error ("dup2", file);
+  if (fd != stderr_dup)
+    emacs_close (fd);
   return Qnil;
 }
-#endif /* GNU_LINUX */
 
 
 /* This is the interface for debugging printing.  */
@@ -2305,9 +2304,7 @@ priorities.  */);
   defsubr (&Sprint);
   defsubr (&Sterpri);
   defsubr (&Swrite_char);
-#ifdef WITH_REDIRECT_DEBUGGING_OUTPUT
   defsubr (&Sredirect_debugging_output);
-#endif
 
   DEFSYM (Qprint_escape_newlines, "print-escape-newlines");
   DEFSYM (Qprint_escape_multibyte, "print-escape-multibyte");