Use custom implementation of read_buf in Read for &'a FileDesc

[tmiasko]

This allows to skip an unnecessary buffer initialization.

Fixes #108223.

[rustbot]

r? @joshtriplett

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[the8472]

Have you measured this properly? The linked issue only gives a single-shot benchmark that involves process spawning, that seems quite noisy (e.g. numbers vary 10x on the playground).

[tmiasko]

The problem in #108223 is that Command uses non-blocking IO and read_to_end. The read_to_end was delegating to read_buf which was initializing vec up to its capacity. The information about initialized part is not retained between calls to read_to_end, so the initialization work was repeated each time (the amount of work increases proportionally with vec spare capacity).

The test case is of course racy, but the difference is easy measurable on my system.

[the8472]

This solves a real issue and is simple enough. Doing additional optimizations as a followup is fine

@bors r+

[bors]

📌 Commit b118569 has been approved by the8472

It is now in the queue for this repository.

[bors]

⌛ Testing commit b118569 with merge 824f915...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing 824f915 to master...

[rust-timer]

Finished benchmarking commit (824f915): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

This benchmark run did not return any relevant results for this metric.