head/tail: parse zero byte counts with size suffix as 0 bytes

[MeGaurav4]

head/tail: parse zero byte counts with size suffix as 0 bytes, not 1KiB

head --bytes=0K, tail -c 0K and similar inputs (00K, +0K, 0M) print the whole input instead of nothing. GNU coreutils prints nothing for all of these.

Closes #13703

Root cause

parse_signed_num_max strips leading zeros before the size suffix is interpreted, so 0K becomes the bare suffix K. A bare suffix means "1 of that unit" in parse_size (empty numeric part parses as 1), so 0K parsed as 1KiB instead of 0 bytes.

Fix

When the trimmed string contains no digits (only a suffix remains after stripping leading zeros), parse the suffix once to validate it, then return 0: a zero count times any unit is zero bytes. Invalid bare suffixes still error (e.g. head -c 0B errors, matching GNU, because B is not valid for head's parser).

Affects only head -c/--bytes and tail -c/--bytes (the only users of this parser).

Verification

• cargo test -p uucore --lib: 94 passed (includes 5 new unit tests in parse_signed_num)
• cargo test --features "head tail" test_zero_bytes_with_suffix: 2 new integration tests passed
• Full suites, no regressions: test_head:: 63 passed, test_tail:: 154 passed (12 ignored)
• Behavior cross-checked against GNU coreutils: head -c 0K, head -c 00K, head -c +0K, tail -c 0K, tail -c 00K print nothing; tail -c +0K prints everything; head -c 0B errors

[codspeed-hq]

Merging this PR will improve performance by 3.03%

⚠️ 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

⚡ 1 improved benchmark
✅ 344 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	cksum_crc32b	40.9 ms	39.7 ms	+3.03%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing MeGaurav4:fix/head-zero-bytes-suffix (c7bde4b) with main (0c8a3c7)}

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/tail/retry (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)

[RenjiSann]

This change makes tests::test_gnu_compatibility in head.rs fail