Re: tee: not checking for EAGAIN or EWOULDBLOCK causing missing output to terminal

[Pádraig Brady]

On 25/07/2019 11:59, Kamil Dudka wrote:
> I see two major issues with the patch:
>
> 1. If the file descriptor operates in non-blocking mode and it starts
> to return EAGAIN, tee will busy-loop (consume 100% CPU) until the file
> descriptor becomes ready again.
>
> 2. If fwrite() fails with EAGAIN, you cannot repeat the call with the same
> arguments because something might have been already written before EAGAIN was
> returned from a syscall.  In that case, you would try to write the already
> written data repeatedly, which is not desired.
>
> A few months ago I proposed a patch that was free of the above issues but it
> addressed EAGAIN on tee's output only:
>
>      https://lists.gnu.org/archive/html/coreutils/2018-09/msg00010.html
>
> The bad news is that the patch was rejected because a workaround exists.

Given there are now two programs that trigger the issue,
and we've recently added portable poll() usage in the tee implementation,
I'm more inclined to add support for this now.

The attached does this using our new iopoll module.

cheers,
Pádraig

tee-nonblock.patch
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