Re: `message' not outputting the newline "atomically"

[Paul Eggert <eggert@cs.ucla.edu>]

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Eli Zaretskii wrote:
> What exactly is the problem you would like to fix here?

$ strace -o /tmp/tr emacs -Q -batch -eval '(message "hello")'
hello
$ grep '^write(2,' /tmp/tr
write(2, "hello", 5)                    = 5
write(2, "\n", 1)                       = 1

There should be just one 'write' system call, not two. Because of this problem, 
if two Emacs processes output the lines "foo" and "bar" to the same file 
descriptor at nearly the same time, the output might be "foobar\n\n" or 
"barfoo\n\n" rather than the more-desirable "foo\nbar\n" or "bar\nfoo\n". This 
can cause the annoying symptoms that Lars reported in 
<https://lists.gnu.org/r/emacs-devel/2019-06/msg00710.html>.

I fixed it on GNU and similar platforms by installing the attached patch into 
Emacs master. I fixed some other minor stdio glitches while I was in the 
neighborhood.

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From 2079e40a3cffbfacc79725c8788d56d05f232222 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 20 Jun 2019 00:32:17 -0700
Subject: [PATCH] Line-buffer stderr

* src/sysdep.c (init_standard_fds) [!DOS_NT]:
Use setvbuf to ensure stderr is line-buffered.
---
 src/sysdep.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/sysdep.c b/src/sysdep.c
index bc88e70dcb..3396764d5d 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -242,6 +242,12 @@ init_standard_fds (void)
   force_open (STDIN_FILENO, O_WRONLY);
   force_open (STDOUT_FILENO, O_RDONLY);
   force_open (STDERR_FILENO, O_RDONLY);
+
+  /* Line-buffer stderr.  However, leave stderr unbuffered on
+     MS-Windows, where setvbuf treats _IOLBF like _IOFBF.  */
+#ifndef DOS_NT
+  setvbuf (stderr, NULL, _IOLBF, 0);
+#endif
 }
 
 /* Return the current working directory.  The result should be freed
-- 
2.17.1