tail: avoid copying each chunk buffer

[haydonryan]

Using deepseek v4 flash found another improvement. Validated tests pass and validated with codex as well.

The example below is pretty silly tbh - not really likely that you'd pipe a 400mb file, but shows the logic is sound.

LLM below here:

BytesChunkBuffer::fill and LinesChunkBuffer::fill deep-copied every 8 KiB
backing buffer with push_back(chunk.clone()), then reused the original as the
next scratch. This moves the chunk into the deque instead and takes the next
working chunk from pop_front() or a fresh allocation.

In the steady state (piped input larger than num_print), this removes one
8 KiB heap allocation and one 8 KiB memcpy per chunk read. The Box::new on
the fill path is bounded to the initial fill and replaces — not adds to — the
clone's allocation, so per-chunk allocations are unchanged or fewer. No
observable behavior change.

Allocation accounting

Steady state (large piped input — exactly what BytesChunkBuffer/LinesChunkBuffer serve, file ≫ num_print, so every iteration
is the if branch):

	push	scratch	per-chunk
baseline	clone() = 1 alloc + 8 KiB memcpy	pop_front() reuse, 0	1 alloc + 8 KiB memcpy
proposed	move = 0	pop_front() reuse, 0	0 alloc + 0 memcpy

For a 400 MB piped input that's ~49k heap allocs + ~400 MB of memcpy eliminated.

Small-file case (file < num_print, else branch every time — the only place the concern applies):

	push	scratch	per-chunk
baseline	clone() = 1 alloc + 8 KiB memcpy	*chunk = new() in place, 0	1 alloc + memcpy
proposed	move = 0	Box::new() = 1 alloc + zero-init	1 alloc + zero-init

Release multicall binary: 14,000,712 -> 14,000,456 bytes (-256 B).

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (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)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[xtqqczze]

CICD / Build (ubuntu-latest, x86_64-unknown-linux-musl, feat_os_unix_musl, use-cross) (pull_request) failure tracked by #14251.

[xtqqczze]

note: the change in this line is necessary to avoid:

error[E0382]: use of moved value: `chunk`
   --> src/uu/tail/src/chunks.rs:292:35

[haydonryan]

Thank you - checking what went wrong locally with tests, and why it missed this.

[haydonryan]

Oh wait i miss-read what you typed. My bad. Yes, the line 299 change is required.