tee: handle short writes to stdout correctly

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.

@mattsu2020

taken by @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.

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?

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.

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

I think it is impossible to do same strace test for splice(2) since strace is a tool to modify returned value, not a len passed to splice(2).

I think it is impossible to do same strace test for splice(2) since strace is a tool to modify returned value, not a len passed to splice(2).

How about changing the test to something like the following?

strace -qqq -o /dev/null --trace-fds=1 -e trace=write,splice \
  -e fault=splice \
  -e inject=write:retval=1:when=1..5 "$@"

#14981 (comment) tests write code path only.

OK. So splice(2) and write(2) are very different.

retval=1 for write(2) means write(2) returns 1 without actually writing. ~~So next read(2) consumes buffer. Then we lost "a".~~ So we lost "a" by previous read(2). It is expected and used at GnuTests.
retval=1 for splice(2) means splice(2) returns 1 without actually sending. So pipe buffer is NOT changed. It is "correct" behaviour. Then we see "a" after all retval'ed call finished since it is still in pipe buffer.

I fixed the previous comment about read.

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tee: handle short writes to stdout correctly #14981

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tee: handle short writes to stdout correctly #14981

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xtqqczze commented on Sep 30, 2026

oech3 commented on Sep 30, 2026

mattsu2020 commented on Sep 30, 2026

oech3 commented on Sep 30, 2026

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oech3 commented on Oct 7, 2026

mattsu2020 commented on Oct 7, 2026

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oech3 commented on Oct 7, 2026

oech3 commented on Oct 7, 2026

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oech3 commented on Oct 8, 2026

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