split: allow writing to symlink to nonseekable

[oech3]

Fixes #13409

[codspeed-hq]

Merging this PR will degrade performance by 9.14%

⚠️ 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

❌ 4 regressed benchmarks
✅ 335 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	false_consecutive_calls	170.3 ns	199.4 ns	-14.62%
❌	Simulation	true_consecutive_calls	170.3 ns	199.4 ns	-14.62%
❌	Simulation	unexpand_large_file[10]	282.3 ms	291.9 ms	-3.3%
❌	Simulation	unexpand_many_lines[100000]	134.7 ms	139.3 ms	-3.29%

Tip

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

---

_{Comparing oech3:split-stream (bc90f0d) with main (9846eb8)}

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/rm/isatty (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.