Re: [RFC PATCH] gpioset: only print prompt when stdout is tty

[Kent Gibson <warthog618@gmail.com>]

On Wed, May 24, 2023 at 09:53:39AM +0200, esben@geanix.com wrote:
> Kent Gibson <warthog618@gmail.com> writes:
> 
> > On Wed, May 24, 2023 at 08:30:33AM +0200, esben@geanix.com wrote:
> >> Kent Gibson <warthog618@gmail.com> writes:
> >> 
> >
> > Yeah, it isn't a whole load of fun, but it isn't intended as a full on
> > daemon.  It is an option that was added in v2 so you CAN now write a
> > shell script that can request lines and change them as necessary - without
> > releasing them.  It might not be pleasant but now it is possible.
> >
> > If that doesn't suit you then look for another solution as you are now
> > beyond the scope that gpioset was intended for.
> 
> I guess I will have to do that. Although I don't agree that I am out of
> scope. I just want to do exactly what you have described is in scope for
> gpioset. I just don't want the prompt when not using a tty, and the
> reason for the prompt being there is to make the test work, not for a
> real-world use-case.  Anyway, I can do my own thing.  No problem.
> 

Not just for testing.

In the real world the prompt is there so the controlling script can tell
a command is completed - same as a human would.

> >> > This works for me as a simple daemon script:
> >> >
> >> > #!/bin/bash
> >> >
> >> > pipe=/tmp/gpiosetd
> >> >
> >> > mkfifo $pipe
> >> >
> >> > trap "rm -f $pipe" EXIT
> >> >
> >> > # as bash will block until something is written to the pipe...
> >> > echo "" > $pipe &
> >> 
> >> I believe this is not just needed because of bash.  If you don't have a
> >> writer on the fifo, the gpioset will end up in a busy loop in readline
> >> until a writer appear, spamming a prompt out on output while eating up
> >> 100% cpu.
> >
> > I don't see that.
> >
> > What I see is that bash blocks until something writes to the fifo - not
> > even launching gpioset until that happens.
> 
> Ok.
> 
> What I am saying is if you actually do manage to run gpioset with stdin
> connected to a fifo, and the fifo not having any writers, you will end
> up eating up the cpu in a small busy loop.
> 
> Because of the problem you describe, you just haven't gotten to that
> point though.
> 

No, that is wrong. I mean I've tested it. Just now. Again. To be sure.

The only reason it would spin is if you connect it to a pipe that always
indicates it is ready to read.  And the named fifo doesn't.

> > That is typically not what you want - you want the line requested and
> > set NOW, and you can update it later through the fifo.
> > The echo is just there to get bash over the hump.
> > (btw, if there is a better way I would love to know it)
> 
> I haven't really investigated that.  I just made the process running
> gpioset hold a dummy writer open to the fifo.
> 

Yeah, I don't know what you are doing, as you haven't shared details, but
if it beahves as you say then it obviously broken.
But that is not due to gpioset.

> > With the named fifo, as used here, gpioset will start, request and set
> > the line, and then will block until something writes to the fifo.
> >
> >> > gpioset -i GPIO23=0 < $pipe > /dev/null
> >> >
> >> > Does that not work for you?
> >> 
> >> That is basically what I do.  Just output directed to a log file
> >> (actually, a pipe to a process writing to rotated log files) instead of
> >> /dev/null, and then no prompt noise in the log files.
> >
> > So redirect stdout through a filter to remove the prompt?
> 
> Yes, I could do that.  But having an extra process running, and managing
> to keep that alive...  If I need to carry a tiny out-of-tree patch to
> avoid that, I will do that.
> 

If that is easier for you.

> >> Anyway, what about adding a new CLI option. Either something like '-I'
> >> for no-prompt interactive mode, or '-n' to be used with '-i' for the
> >> same?
> >
> > I'm not keen on adding options to gpioset to massage the output for
> > different use cases - there are already better tools for that.
> 
> Ok.
> 
> That I guess leaves me with no options than working around gpioset,
> using filters and what else is needed to do what I need.
> Or out-of-tree patching.
>

You missed writing your own daemon. Or asking ChatGPT.
But you do have options.

Have a good one.

Cheers,
Kent.