bug#10404: [PATCH] Power: sleep longer than two seconds at a time

[Glenn Morris <rgm@gnu.org>]

This sounds like a good idea.
Does anyone have any comments?

Daniel Colascione wrote:

> Emacs uses an atimer to poll for input every so often so it can detect
> C-g presses while lisp code is running; atimer arranges for this polling
> to be done by having a SIGALRM delivered every so often --- by default,
> every two seconds.  But while lisp code is not running and emacs is
> blocked in select() [or a platform-specific analogue], we want to
> stop this polling to save power: we don't need it because the select
> will return as soon as there's input.
>
> Emacs has code that's meant to turn off atimers while we wait for
> input --- wait_reading_process_output calls stop_polling and
> turn_on_atimers (0).  But time ago, we started calling redisplay
> code inside the select loop, and this select code turns atimers
> back on.  The effect is that we don't sleep longer than two second
> at a time.
>
> This patch turns off SIGALRM delivery around select, making sure that
> we stay asleep.  With this patch (and blink-cursor-mode off), Emacs
> will process input, then sleep for 30 seconds, and if no input
> arrives in that time, will sleep for several hours.  The patch
> does not adversely any functionality.
> ---
>  src/process.c |   16 ++++++++++++++++
>  1 files changed, 16 insertions(+), 0 deletions(-)
>
> diff --git a/src/process.c b/src/process.c
> index 5c8eef7..f81a5c4 100644
> --- a/src/process.c
> +++ b/src/process.c
> @@ -4304,6 +4304,10 @@ wait_reading_process_output (int time_limit, int microsecs, int read_kbd,
>    int got_some_input = 0;
>    int count = SPECPDL_INDEX ();
>  
> +#ifdef SIGALRM
> +  SIGMASKTYPE mask;
> +#endif /* SIGALRM */
> +
>    FD_ZERO (&Available);
>    FD_ZERO (&Writeok);
>  
> @@ -4606,6 +4610,14 @@ wait_reading_process_output (int time_limit, int microsecs, int read_kbd,
>  	    }
>  #endif
>  
> +#ifdef SIGALRM
> +          /* We don't want SIGALRM going off while we're blocked in
> +             select.  If there any any pending timers, timeout has
> +             been set appropriately already and we'll wake up
> +             automatically.  */
> +          mask = sigblock (sigmask (SIGALRM));
> +#endif /* SIGALRM */
> +
>  #if defined (USE_GTK) || defined (HAVE_GCONF) || defined (HAVE_GSETTINGS)
>            nfds = xg_select
>  #elif defined (HAVE_NS)
> @@ -4618,6 +4630,10 @@ wait_reading_process_output (int time_limit, int microsecs, int read_kbd,
>               (check_write ? &Writeok : (SELECT_TYPE *)0),
>               (SELECT_TYPE *)0, &timeout);
>  
> +#ifdef SIGALRM
> +          sigsetmask (mask);
> +#endif /* SIGALRM */
> +
>  #ifdef HAVE_GNUTLS
>            /* GnuTLS buffers data internally.  In lowat mode it leaves
>               some data in the TCP buffers so that select works, but