tee: handle short writes to stdout correctly

[mattsu2020]

tee does not correctly handle short writes to stdout when stdout is a regular file on Linux.

The GNU tee/short-write.sh test expects short writes to be retried until the remaining data is written, but the current implementation uses the splice path for regular-file stdout, bypassing the injected write(2) short writes.

We should ensure that short writes to regular-file stdout are handled correctly.

Related to #14705.

[xtqqczze]

taken by @mattsu2020

[oech3]

Difference of used syscall is not a problem if the used syscall is an optional dependency.
We can patch GnuTests in this case to support our syscall.

[mattsu2020]

Difference of used syscall is not a problem if the used syscall is an optional dependency. We can patch GnuTests in this case to support our syscall.

Should we update the GNU test to use splice instead?

[oech3]

I think so. But it is bit tricky to the test meaningful for splice(2) path. I'd like to patch to fail splice path at fallback to write path at a moment.

It is done by adding -e fault=splice.

[oech3]

The test says the behaviour of tee was changed at GNU > 9.11 . So we need an actual fix too.

[oech3]

We could use named pipe with 1 byte remain to cause short-write. But it would be too complex.