head: fix panic when -v header write fails on a long filename

[koopatroopa787]

Summary

When head -v (or when reading multiple files) prints the ==> name <== header and the filename is longer than stdout's ~1024-byte write buffer, a write error (e.g. output to /dev/full) surfaces inside print_verbatim(file).unwrap() and causes an abort rather than a graceful error message and non-zero exit.

The two adjacent writes (write!(stdout, "==> ")? and writeln!(stdout, " <==")?) already propagate errors via ?, but print_verbatim wrote directly to a separate io::stdout() handle and its return value was unwrapped — so the error was never propagated.

Fix

Replace print_verbatim(file).unwrap() with stdout.write_all_os(file.as_ref())? — writing the filename through the same locked StdoutLock as the surrounding writes. Any I/O error is now propagated consistently.

-use uucore::display::{Quotable, print_verbatim};
+use uucore::display::{OsWrite, Quotable};

-    write!(stdout, "==> ")?;
-    print_verbatim(file).unwrap();
-    writeln!(stdout, " <==")?;
+    write!(stdout, "==> ")?;
+    stdout.write_all_os(file.as_ref())?;
+    writeln!(stdout, " <==")?;

Behavior after fix

$ LONG="/dev/$(python3 -c "print('./'*512)")null"
$ head -v "$LONG" > /dev/full ; echo $?
head: No space left on device
1

Fixes #13264

[Copilot]

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[RenjiSann]

Please provide a test with this

[sylvestre]

Needs a test

[koopatroopa787]

Added a test_verbose_header_write_error test in tests/by-util/test_head.rs (Linux only, via /dev/full). It passes -v with an empty pipe input — since -v forces header output even for a single file, and the unlocked stdout handle makes any write to /dev/full fail immediately — confirming the panic is gone and the error is reported gracefully.

[RenjiSann]

There are CI fails, namely on coding style

[koopatroopa787]

Fixed — the OpenOptions chain in the test was formatted across multiple lines which violated cargo fmt. Collapsed to a single line and pushed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 339 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing koopatroopa787:fix-head-verbose-header-write-error (5e461f4) with main (ef50d8b)}

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. ↩