On 15/06/17 02:40, Pádraig Brady wrote:
> On 14/06/17 16:03, Charlie Hagedorn wrote:
>> Hi!
>>
>> Thank you for maintaining such useful and reliable tools.
>>
>> Today I came across an unexpected warning in tail. The warning is intended
>> to handle this case:
>>
>> [:~]$ tail -f
>> tail: warning: following standard input indefinitely is ineffective
>>
>> which is both important and fun.
>>
>> Today, however, I was surprised to see it appear in this context:
>>
>> [:~]$ tail -f < /dev/ttyUSB0 > data.dat
>> tail: warning: following standard input indefinitely is ineffective
>>
>> The warning was confusing and perhaps inappropriate, as this call actually
>> *does* something, and is very effective at doing what I want; streaming the
>> port's output into data.dat.
>
> Right. The above command will go into non inotify blocking mode
> and will thus work as you expect. Note tail will not output
> anything until the first time read() returns nothing.
> The following patch should suppress the warning if we would be using this
> mode.
>
> diff --git a/src/tail.c b/src/tail.c
> index 3918373..2f9b981 100644
> --- a/src/tail.c
> +++ b/src/tail.c
> @@ -2365,12 +2365,22 @@ main (int argc, char **argv)
> if (found_hyphen && follow_mode == Follow_name)
>die (EXIT_FAILURE, 0, _("cannot follow %s by name"), quoteaf ("-"));
>
> -/* When following forever, warn if any file is '-'.
> +/* When following forever, and not using simple blocking, warn if
> + any file is '-' as the stats() used to check for input are
> ineffective.
> This is only a warning, since tail's output (before a failing seek,
> and that from any non-stdin files) might still be useful. */
> -if (forever && found_hyphen && isatty (STDIN_FILENO))
> - error (0, 0, _("warning: following standard input"
> - " indefinitely is ineffective"));
> +if (forever && found_hyphen)
> + {
> +struct stat in_stat;
> +bool blocking_stdin;
> +blocking_stdin = (pid == 0 && follow_mode == Follow_descriptor
> + && n_files == 1 && ! fstat (STDIN_FILENO, &in_stat)
> + && ! S_ISREG (in_stat.st_mode));
> +
> +if (! blocking_stdin && isatty (STDIN_FILENO))
> + error (0, 0, _("warning: following standard input"
> + " indefinitely is ineffective"));
> + }
>}
>
>> (Importantly, for reasons I don't yet understand, tail -f /dev/ttyUSB0 >
>> data.dat does not reliably tail the port; it redirects only one line of
>> output instead of a continuous stream).
>
> That looks like another case where we should be disabling inotify.
> I.E. you can see this waits forever for inotify events if you hit ctrl-d a
> few times:
>
> strace tail -f /dev/tty
>
> Ah yes that was discussed at http://bugs.gnu.org/21265
> I suppose we should improve things here with a couple of patches.
>
> 1. Disable inotify if any non regular files (except fifo/pipe)
> (the kernel should really disallow this, but best handle I think)
>
> 2. Generalise the warning in the above patch to be:
> if (n_files > 1 && any_device_or_tty)
> printf ("warning: following a stdin/device in combination with other inputs
> is ineffective")
Two proposed patches for this are attached.
cheers,
Pádraig.
>From 2a49ab8eae92859c02c0db7a14231b391287c5ee Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?=
Date: Thu, 15 Jun 2017 02:41:14 -0700
Subject: [PATCH 1/2] tail: with -f don't warn if doing a blocking read of a
tty
* src/tail.c: (main): Only issue the warning about -f being
ineffective when we're not going into simple blocking mode.
* tests/tail-2/follow-stdin.sh: Ensure the warning is output correctly.
Fixes http://bugs.gnu.org/27368
---
NEWS | 4
src/tail.c | 18 ++
tests/tail-2/follow-stdin.sh | 21 -
3 files changed, 38 insertions(+), 5 deletions(-)
diff --git a/NEWS b/NEWS
index d2672e8..22805c8 100644
--- a/NEWS
+++ b/NEWS
@@ -56,6 +56,10 @@ GNU coreutils NEWS-*- outline -*-
tail -f will now exit immediately if the output is piped
and the reader of the pipe terminates.
+ tail -f will no longer erroneously warn about being ineffective
+ when following a single tty, as the simple blocking loop used
+ is effective in this case.
+
* Noteworthy changes in release 8.27 (2017-03-08) [stable]
diff --git a/src/tail.c b/src/tail.c
index 3918373..2f9b981 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -2365,12 +2365,22 @@ main (int argc, char **argv)
if (found_hyphen && follow_mode == Follow_name)
die (EXIT_FAILURE, 0, _("cannot follow %s by name"), quoteaf ("-"));
-/* When following forever, warn if any file is '-'.
+/* When following forever, and not using simple blocking, warn if
+ any file is '-' as the stats() used to check for input are ineffective.
