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, 29 insertions, 0 deletions

diff --git a/src/sysdep.c b/src/sysdep.c
index 6154c1325d8..67c9bd90df7 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -130,6 +130,35 @@ static const int baud_convert[] =
     1800, 2400, 4800, 9600, 19200, 38400
   };
 
+/* If FD is not already open, arrange for it to be open with FLAGS.  */
+static void
+force_open (int fd, int flags)
+{
+  if (dup2 (fd, fd) < 0 && errno == EBADF)
+    {
+      int n = open (NULL_DEVICE, flags);
+      if (n < 0 || (fd != n && (dup2 (n, fd) < 0 || emacs_close (n) != 0)))
+        {
+          emacs_perror (NULL_DEVICE);
+          exit (EXIT_FAILURE);
+        }
+    }
+}
+
+/* Make sure stdin, stdout, and stderr are open to something, so that
+   their file descriptors are not hijacked by later system calls.  */
+void
+init_standard_fds (void)
+{
+  /* Open stdin for *writing*, and stdout and stderr for *reading*.
+     That way, any attempt to do normal I/O will result in an error,
+     just as if the files were closed, and the file descriptors will
+     not be reused by later opens.  */
+  force_open (STDIN_FILENO, O_WRONLY);
+  force_open (STDOUT_FILENO, O_RDONLY);
+  force_open (STDERR_FILENO, O_RDONLY);
+}
+
 /* Return the current working directory.  The result should be freed
    with 'free'.  Return NULL on errors.  */
 char *