bug#27368: Minor concern: Confusing tail warning

[Pádraig Brady]

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.

tail-pipe.diff
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