tail: avoid unnecessary pipe2 syscall for empty files

[dragutreis]

Fixes #12464

bounded_tail calls print_target_section unconditionally, which creates
a broker pipe via splice_unbounded_broker even when the file is empty or
exhausted, causing an unnecessary pipe2 syscall.

Added is_file_exhausted to check before calling print_target_section:

• Regular files: compare stream position against file length
• Char/block devices: probe with a single-byte read; if data exists, write
  it directly to stdout since char devices don't support seeking back

Before:
strace -c -e pipe,pipe2 tail -c +1 /dev/null → 1 pipe2 call
After:
strace -c -e pipe,pipe2 tail -c +1 /dev/null → 0 pipe2 calls

[oech3]

Does this require additional stat or other syscall? I'm not sure which syscall is cheap, but I thins this is not needed considering usage for 0-sized file is very rare.

[dragutreis]

Compared full syscall counts with strace -c:

Empty file (/dev/null):

• Without fix: 129 syscalls (includes pipe2 + splice)
• With fix: 124 syscalls

No new syscalls added. statx goes down by 1.

Non-empty file:

• Without fix: 124 syscalls
• With fix: 124 syscalls

No overhead for the common case.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.

[oech3]

it directly to stdout since char devices don't support seeking back

Are /dev/sd* and /dev/nvme* missing splice driver yet? I saw cat does not call splice yet. (Though I want to cover the case kernel started supporting someday).

[dragutreis]

it directly to stdout since char devices don't support seeking back

Are /dev/sd* and /dev/nvme* missing splice driver yet? I saw cat does not call splice yet. (Though I want to cover the case kernel started supporting someday).

tested on /dev/nvme0n1, splice is called successfully, so block devices do support it

[sylvestre]

please remove this allow

[dragutreis]

alright

[oech3]

Would you add test for char/block devises with 2 byte input?

[codspeed-hq]

Merging this PR will create unknown performance changes

⚠️ No benchmarks were detected in both the base of the PR and the PR.
Please ensure that your benchmarks are correctly instrumented with CodSpeed.

Check out the benchmarks creation guide

---

_{Comparing dragutreis:fix/tail-is-file-exhausted (9794752) with main (c8af53a)}

[oech3]

It would be better to include read/write path for small input to splice modules for all utilities instead of patching tail only. Then we only pay 0 byte read at here. I'll do that soon.