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// SPDX-License-Identifier: MIT OR Apache-2.0
//! 2026-09-26: The final `Error:` block of a child's log, quoted into the error that reports it.
//!
//! Owner: server CLI (`met benchmark`).
//! Callers: the serve lease (`bench_lease::exited_before_serving`) quotes
//! `serve-lease.log`, and certify placement (`bench_certify/remote/place.rs`)
//! quotes the gate child's log.
//! Invariants: none beyond the types.
use std::io::{Read, Seek, SeekFrom};
use std::path::Path;
/// 2026-09-26: How many bytes of a log's tail the callers read to find its final block.
pub(crate) const TAIL_BYTES: u64 = 64 * 1024;
/// 2026-09-26: The longest block, in bytes, that [`final_error_block`] returns uncut.
pub(crate) const CAUSE_CAP: usize = 4096;
/// 2026-09-26: The last `Error:` block in `text`: from the final line that starts with
/// `Error:` to the end, trimmed. A block longer than [`CAUSE_CAP`] bytes is cut
/// on a char boundary and ends with `… (+N bytes in the log)`. `None` when no
/// line starts with `Error:`.
pub(crate) fn final_error_block(text: &str) -> Option<String> {
let start = text
.rmatch_indices("Error:")
.map(|(i, _)| i)
.find(|&i| i == 0 || text.as_bytes()[i - 1] == b'\n')?;
let block = text[start..].trim();
if block.len() <= CAUSE_CAP {
return Some(block.to_string());
}
let mut cut = CAUSE_CAP;
while !block.is_char_boundary(cut) {
cut -= 1;
}
Some(format!(
"{}… (+{} bytes in the log)",
&block[..cut],
block.len() - cut
))
}
/// 2026-09-26: The last `bytes` of a file, lossily decoded. A file no longer than
/// `bytes` is read whole. Otherwise the window drops everything up to and
/// including its first newline, unless that newline is the window's last byte,
/// in which case the window is returned as read.
pub(crate) fn tail_of_file(path: &Path, bytes: u64) -> std::io::Result<String> {
let mut f = std::fs::File::open(path)?;
let len = f.metadata()?.len();
let mut text = String::new();
if len > bytes {
f.seek(SeekFrom::Start(len - bytes))?;
let mut raw = Vec::new();
f.read_to_end(&mut raw)?;
let cut = match raw.iter().position(|b| *b == b'\n') {
Some(i) if i + 1 < raw.len() => i + 1,
_ => 0,
};
text = String::from_utf8_lossy(&raw[cut..]).into_owned();
} else {
f.read_to_string(&mut text)?;
}
Ok(text)
}
/// 2026-09-26: `block` with every line indented by four spaces, so no line of it starts
/// with `Error:` and [`final_error_block`] on the embedding error finds that
/// error's own `Error:` line.
pub(crate) fn indented(block: &str) -> String {
block
.lines()
.map(|l| format!(" {l}"))
.collect::<Vec<_>>()
.join("\n")
}
#[cfg(test)]
#[path = "bench_cause_tests.rs"]
mod tests;