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

[Kamil Dudka]

On Tuesday, March 7, 2023 8:01:59 PM CET Pádraig Brady wrote:
> On 07/03/2023 14:39, Kamil Dudka wrote:
> > On Tuesday, March 7, 2023 2:28:05 AM CET Pádraig Brady wrote:
> >> 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.
> > 
> > The proposed patch works for me.  Thank you for working on it!
> 
> Thanks for the review.
> Updated patch attached with a test.
> 
> cheers,
> Pádraig

Great idea with the test!  I confirm it works in my environment.

Kamil