tee: restrict to NamedReader

[oech3]

Other Read should not be accepted by copy_unbuffered since we need to show error, etc...

[codspeed-hq]

Merging this PR will degrade performance by 14.62%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 2 regressed benchmarks
✅ 317 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	true_consecutive_calls	170.3 ns	199.4 ns	-14.62%
❌	Simulation	false_consecutive_calls	170.3 ns	199.4 ns	-14.62%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing oech3:tee-nr (ee78a86) with main (55549fa)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[cakebaker]

Thanks!

[oech3]

would you review #11943 ?