[Bug 344163] Re: Default config uses bad check_mail setting.

[Miciah Dashiel Butler Masters]

If no spool file is set, the check_mail feature also causes multitail to
wake up every 10 milliseconds.

This bug is easily verified using strace(1) or powertop(1):

1. unset the MAIL environmental variable (e.g. using `env -u MAIL`);

2. run multitail with the default setting of 'check_mail:5' in
/etc/multitail.conf; and

3. use strace or powertop to see that multitail wakes up every 10ms
because of timeout on select(2).

4. Then add 'check_mail:0' to ~/.multitailrc;

5. run multitail again; and

6. use strace or powertop to see that multitail calls select(2) without
a timeout.

The concerned routines are wait_for_keypress in mt.c and
do_check_for_mail in misc.c.  wait_for_keypress contains the select(2)
loop for multitail.  It also calls do_check_for_mail in every iteration
of the select(2) loop.  wait_for_keypress checks msf_last_check in
computing the time until the mail next needs to be checked, and it uses
this time as an upper bound for the timeout for the select(2) call.
do_check_for_mail is supposed to reset msf_last_check to the current
time whenever it checks mail.  The problem is that if mail_spool_file is
NULL, do_check_for_mail returns immediately.  Thus msf_last_check is
never updated, and wait_for_keypress always gives select(2) a very short
timeout.

Should I open a separate bug report regarding the excessive wakeups?  I
have a patch for this issue.

-- 
Default config uses bad check_mail setting.
https://bugs.launchpad.net/bugs/344163
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