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PuTTY semi-bug pterm-timer

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pterm currently runs a GTK timer function every 20ms (fifty times a
second). This timer function takes care of calling term_update() to
handle actual window updates, calling term_blink() to handle
blinking cursors etc, and checking whether the child process has
terminated.

It would be nice if pterm could avoid doing this unless it was
absolutely necessary. So, alternative ways of doing the same things:

 - to detect stuff in the child process, I'd have to get
   asynchronous notification of SIGCHLD. I suppose this means having
   an intra-process pipe in the same way Unix plink does it, so that
   pty.c's SIGCHLD handler writes a byte to the pipe and the GTK
   main function calls a function in response.

 - the reason term_update() is called in the timer function is so
   that it won't happen more than 50 times per second, which means
   that really fast-scrolling text doesn't slow down pterm the way
   it does xterm. So it would be a step backwards to move to just
   calling term_update() after every term_out(). I think a sensible
   way to fix this might be:

    * Introduce a wrapper function in pterm.c, whose job is to
      arrange a call to term_update either now or later.

    * The wrapper function will first check how long it's been since
      the last term_update. If it's been more than 20ms, it will
      call term_update. If not, it should _schedule_ a GTK timer
      function which will call term_update at some point 20ms or
      less in the future. That timer function will then disable
      itself the first time it runs.

    * Then we call the wrapper function after every term_out().

 - Regular calls to the terminal code are actually necessary if
   blinking text or blinking cursor are enabled, or if a visual bell
   is currently active. So I could introduce a spare function in
   terminal.c which allows the front end to ask the terminal `Do you
   have any pending events for which I should call you back in
   future?' The wrapper function described above could call this
   after term_update, and schedule the timer function _anyway_ if it
   says yes.

Patch (unreviewed): Pine.LNX.4.53mb.0304291625260.25393@ask.diku.dk