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cmp: don't reopen /dev/null on Linux #267

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5 changes: 3 additions & 2 deletions Cargo.lock
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3 changes: 3 additions & 0 deletions Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,9 @@ regex = "1.10.4"
same-file = "1.0.6"
unicode-width = "0.2.0"

[target.'cfg(any(target_os = "linux", target_os = "android"))'.dependencies]
rustix = { version = "1.1.5", features = ["fs"] }

[dev-dependencies]
assert_cmd = "2.0.14"
divan = { version = "5.0.0", package = "codspeed-divan-compat" }
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36 changes: 36 additions & 0 deletions fuzz/Cargo.lock
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24 changes: 18 additions & 6 deletions src/cmp.rs
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Expand Up @@ -11,12 +11,6 @@ use std::iter::Peekable;
use std::process::ExitCode;
use std::{cmp, fs, io};

#[cfg(unix)]
use std::os::fd::{AsRawFd, FromRawFd};

#[cfg(unix)]
use std::os::unix::fs::MetadataExt;

#[cfg(target_os = "windows")]
use std::os::windows::fs::MetadataExt;

Expand All @@ -43,8 +37,26 @@ fn usage_string(executable: &str) -> String {
format!("Usage: {executable} <from> <to>")
}

#[cfg(any(target_os = "linux", target_os = "android"))]
fn is_stdout_dev_null() -> bool {
// as_fd().try_clone_to_owned() needs additional overhead of fcntl

@sylvestre sylvestre Oct 18, 2026

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the unix version below doesn't need fcntl either (from_raw_fd + forget)
could you please do the same with MetadataExt::rdev() + FileTypeExt::is_char_device() and drop rustix?

@oech3 oech3 Oct 18, 2026

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Is needs unsafe. Is it OK? clap-rs already pulls rustix.

@oech3 oech3 Oct 18, 2026

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I think std does not have a feature to check major/minor.

use rustix::fs;
let stdout = io::stdout();
let Ok(stat) = fs::fstat(stdout) else {
return false;
};
let dev = stat.st_rdev;
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let is_char_device = fs::FileType::from_raw_mode(stat.st_mode).is_char_device();
// /dev/null's major/minor is frozen on Linux. So we don't need to open new `/dev/null` and compare
is_char_device && fs::major(dev) == 1 && fs::minor(dev) == 3
}

#[cfg(unix)]
#[cfg(not(any(target_os = "linux", target_os = "android")))]
fn is_stdout_dev_null() -> bool {
use std::os::fd::{AsRawFd, FromRawFd};
use std::os::unix::fs::MetadataExt;
let Ok(dev_null) = fs::metadata("/dev/null") else {
return false;
};
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