Re: [PATCH] tee: Add --pipe-check to allow instantly detecting closed outputs

[Carl Edquist]

Hi Pádraig,
O2
At your convenience, please see my attached patch set.


... A couple very small implementation details I guess I'll mention here.

1. In iopoll.c, I use isapipe() from gnulib, as is done in tail.c.  I am 
not wild about the gnulib implementation of isapipe() - I would rather do 
a simple S_ISFIFO check, as the FIFO vs pipe distinction is irrelevant for 
iopoll(), and I'm not sure whether the "pipes are sockets" for Darwin 7.7 
still needs to be supported.  But I was happy to see (at least for me on 
Linux) it looks like the config.h macros end up compiling away the 
complicated parts of this function, so I guess it's not that bad  :)

2. I believe the "if (errno == EINTR) continue;" logic in iopoll() is 
unnecessary, as an interrupted poll() or select() call will result in 
repeating the loop anyway.  I've left it for clarity, but as far as I can 
tell it could be removed just the same.


On Tue, 13 Dec 2022, Arsen Arsenović wrote:

> It might also be good to give a quick test on FreeBSD, since it has some 
> popularity too.

I don't have easy access to play with this, but yeah I think I've seen the 
BSD man page for poll(2), and I imagine poll would work there.  If so we 
can tweak the IOPOLL_USES_POLL definition in iopoll.c to include an 
appropriate preprocessor check for BSD.


> If you prefer, I'll have some time in the latter part of this week too.
>
> Let's not forget to include the testcase posted previously (with -p 
> instead of -P, since it was suggested to enable polling for -p):

Hi Arsen,

Would you like to add your test case and comments in a separate commit?

Thanks!
Carl


> ( sleep 5 | (timeout 3 tee -p 2>/dev/null && echo TEST_PASSED >&8) | : ) 8>&1 | 
> grep -qx TEST_PASSED
>
> To annotate it, and let's include this info in a comment:
>
> - sleep emulates misbehaving input.
> - The timeout is our failure safety-net.
> - We ignore stderr from tee, and should have no stdout anyway.
> - If that succeeds, we print TEST_PASSED into FD 8 to grep for later.
>  (FD 8 was selected by a D20 roll, or rather, a software emulation)
> - The colon is the immediately closed output process.
> - We redirect 8 back into stdout to grep it.
>
> If tee fails, for instance because it times out, or it fails to
> recognize -P for some reason, the echo simply won't run.  The grep
> options are in POSIX (or, at least, in POSIX.1-2017).
>
> Thank you both, have a great night.
> --
> Arsen Arsenović

0001-iopoll-broken-pipe-detection.patch
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0002-tee-enhance-p-mode-using-iopoll.patch
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