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

[esben@geanix.com]

Kent Gibson <warthog618@gmail.com> writes:

> 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.

And still you say that I should probably redirect it to /dev/null ...

>> >> > 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.

So when you call read(2) on a named pipe opened in blocking mode, but
without any writers, you don't simply get an immediate return with 0,
indicating EOF?  That is what I saw when I tested it previously.
If not, I probably just messed up back then.  Sorry about that.

>> > 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.

Yes, you always have options, and I will go with one of them. I will
probably not go with ChatGPT though ;)

/Esben