[zengfanfan]

When stdout is a terminal, matching lines currently remain buffered while grep waits for more input. Enable the existing line-flushing path for terminal stdout so interactive output becomes visible before EOF.

Redirected output keeps its default buffering, and explicit --line-buffered continues to flush redirected output. A single terminal check is shared with automatic color selection; color behavior and dependencies are unchanged.

Following review, the added buffering test module has been removed to keep this change small.

Validation on Ubuntu 24.04.4 / WSL2:

• Existing suite: 13 unit tests and 102 integration tests passed (cargo test --locked -j 2 -- --test-threads=2).
• Strict Clippy, formatting, and diff whitespace checks passed.
• External terminal and pipe checks confirmed output before EOF where line buffering applies, expected color behavior, and no early output during a bounded observation of default pipe buffering.

Windows/macOS have not been rerun locally for this revision. Hosted CI results are tracked below. AI assistance was used for the implementation, validation, and review.

Fixes #145.

[codspeed]

Merging this PR will not alter performance

⚠️ 3 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• fixed_string
• search_pattern
• literal_no_match

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing zengfanfan:codex/145-terminal-line-buffering (af1697f) with main (01fb6c3)}

Footnotes

1. 17 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. ↩

[lhecker]

I'm not sure if the comment adds much nuance beyond what the code expresses. Line-buffering is the default for all terminal IO.

[lhecker]

is_terminal is a comparatively very expensive call. Consider calling it once and using the variable both here and for --color flag handling.

[lhecker]

I think this unit test is very overkill. If we cannot express this in a meaningfully compact way, I'd favor no tests at all, as the backing logic for --line-buffered is and will always remain a primitive if condition (= easy to review and verify).

[zengfanfan]

Thanks for the review! Updated in af1697f: removed the redundant comment, reused a single stdout terminal check for line buffering and automatic color, and removed the added buffering test module as suggested.

The existing test suite (13 unit tests and 102 integration tests), formatting, and Clippy pass on Linux. I also checked terminal and pipe output behavior locally.

[sylvestre]

Thanks for your PR