tail: do not abort on a -c +N offset past the end of the file

[AlejandroCoronadoN]

Closes #13887.

Problem

$ yes | head -c 8192 > big
$ tail -c +18446744073709551615 big
thread 'main' panicked ... called `Result::unwrap()` on an `Err` value ...
$ echo $?
134

bounded_tail does file.seek(SeekFrom::Start(*count - 1)).unwrap() for a
positive byte offset. When the offset is past the end of the file and not
seekable (here it exceeds i64::MAX), the seek returns an error and the unwrap
aborts. GNU tail prints nothing for a start offset past end of file and exits
0.

Fix

Mirror the existing negative-bytes arm: if the forward seek fails, fall back to
SeekFrom::End(0) so the following copy reads nothing, instead of unwrapping.

Verification

$ tail -c +18446744073709551615 big ; echo $?   # was 134 (abort), now 0, no output
0

Compared against GNU tail over offsets +18446744073709551615,
+17592186040322, +8193, +100: exit codes and byte counts match. Added a
regression test; the full test_tail suite (122 tests) passes and cargo fmt
/ cargo clippy are clean.

[codspeed-hq]

Merging this PR will improve performance by 3.21%

⚠️ 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
✅ 352 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	ls_recursive_balanced_tree[(6, 4, 15)]	120.7 ms	117 ms	+3.21%

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 AlejandroCoronadoN:fix-tail-c-plus-seek-panic (3f5f713) with main (d8bee62)}

Footnotes

1. 50 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. ↩

[sylvestre]

please fix the lint:

error: this `if` can be collapsed into the outer `match`
     --> src/uu/tail/src/tail.rs:487:13
      |
  487 | /             if file.seek(SeekFrom::Start(*count - 1)).is_err() {
  488 | |                 file.seek(SeekFrom::End(0))?;
  489 | |             }
      | |_____________^
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#collapsible_match
      = note: `-D clippy::collapsible-match` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(clippy::collapsible_match)]`
  help: collapse nested if block
      |
  480 ~         FilterMode::Bytes(Signum::Positive(count)) if count > &1
  481 |             // GNU `tail` seems to index bytes and lines starting at 1, not
  ...
  486 |             // than aborting on the failed seek.
  487 ~             && file.seek(SeekFrom::Start(*count - 1)).is_err() => {
  488 |                 file.seek(SeekFrom::End(0))?;
  489 ~             }
      |
  

[Copilot]

Pull request overview

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Prevents tail -c +N from panicking when the requested start offset is past EOF and the underlying seek fails (notably for offsets exceeding i64::MAX), matching GNU tail behavior.

Changes:

• Replace an unwrap() on a forward seek with a graceful fallback to seeking to EOF when the seek fails.
• Add a regression test covering very large positive byte offsets past end-of-file.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/uu/tail/src/tail.rs	Avoids aborting on failed forward seek by falling back to EOF for -c +N.
tests/by-util/test_tail.rs	Adds a regression test ensuring large -c +N offsets do not crash and produce no output.

---

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cat/splice is no longer failing!
Congrats! The gnu test tests/cut/mb-non-utf8 is no longer failing!
Congrats! The gnu test tests/ls/stat-free-symlinks is no longer failing!
Congrats! The gnu test tests/mv/dir2dir is no longer failing!
Congrats! The gnu test tests/mv/mv-exchange is no longer failing!
Congrats! The gnu test tests/nl/multibyte is no longer failing!
Congrats! The gnu test tests/od/od-j is no longer failing!
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/dd/no-allocate is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[AlejandroCoronadoN]

Thanks for the review. Fixed the lint by replacing the if/is_err with a .or_else fallback on the seek; clippy and fmt are clean now.

[sylvestre]

Please make the comments shorter

[AlejandroCoronadoN]

Shortened the comment. Thanks!