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//! Paging long output through the system pager.
//!
//! Port of `rich/pager.py` plus the pager-selection logic upstream inherits from
//! `pydoc.get_pager` (rich's `SystemPager` simply delegates to `pydoc.pager`).
//!
//! [`Console::page`](crate::console::Console::page) buffers everything printed
//! inside a closure and hands it to a [`Pager`] — the Rust analogue of upstream's
//! `with console.pager():` block, matching this crate's other capture-style
//! methods (`capture`, `export_text`, …).
use std::io::{IsTerminal, Write};
use std::process::{Command, Stdio};
/// Something that can display a block of content a screenful at a time. Mirrors
/// `rich.pager.Pager`.
pub trait Pager {
/// Show `content`, returning an error only if the content could not be
/// displayed at all.
fn show(&self, content: &str) -> std::io::Result<()>;
}
/// Pages through the pager program installed on the system. Mirrors
/// `rich.pager.SystemPager` (which defers to `pydoc.pager`).
#[derive(Debug, Default, Clone, Copy)]
pub struct SystemPager;
/// Write `content` straight to stdout — `pydoc`'s `plain_pager`, used when
/// there's no terminal to page in (piped/redirected output, `TERM=dumb`) or when
/// no pager program could be started.
fn plain(content: &str) -> std::io::Result<()> {
let stdout = std::io::stdout();
let mut handle = stdout.lock();
handle.write_all(content.as_bytes())?;
if !content.ends_with('\n') {
handle.write_all(b"\n")?;
}
handle.flush()
}
/// The pager command to run, as `(program, args)`. Port of `pydoc.get_pager`'s
/// selection order: `MANPAGER`, then `PAGER`, then a platform default.
fn pager_command() -> Option<(String, Vec<String>)> {
// An explicit pager wins. It's a command *line*, so split off any arguments
// (e.g. `PAGER="less -R"`), matching pydoc handing the string to a shell.
for variable in ["MANPAGER", "PAGER"] {
if let Ok(value) = std::env::var(variable) {
let mut parts = value.split_whitespace().map(str::to_string);
if let Some(program) = parts.next() {
return Some((program, parts.collect()));
}
}
}
if cfg!(windows) {
// Unlike pydoc's shell invocation, Command only fills in `.exe` on
// Windows. The system pager is `more.com`, so name it explicitly.
Some(("more.com".to_string(), Vec::new()))
} else {
// `less -R` keeps ANSI styling readable; pydoc tries `pager` then `less`.
Some(("less".to_string(), vec!["-R".to_string()]))
}
}
/// Whether we're attached to a terminal that can host a pager. Port of
/// `pydoc.get_pager`'s isatty + `TERM in (dumb, emacs)` guards.
fn can_page() -> bool {
if !std::io::stdin().is_terminal() || !std::io::stdout().is_terminal() {
return false;
}
!matches!(
std::env::var("TERM").unwrap_or_default().as_str(),
"dumb" | "emacs"
)
}
impl Pager for SystemPager {
fn show(&self, content: &str) -> std::io::Result<()> {
if !can_page() {
return plain(content);
}
let Some((program, args)) = pager_command() else {
return plain(content);
};
// Spawn the pager with our content on its stdin, inheriting stdout/stderr
// so it can drive the terminal. Any failure (no such program, broken
// pipe from the user quitting early) falls back to plain output.
let child = Command::new(&program)
.args(&args)
.stdin(Stdio::piped())
.spawn();
let mut child = match child {
Ok(child) => child,
Err(_) => return plain(content),
};
if let Some(mut stdin) = child.stdin.take() {
// A pager the user quits early closes the pipe; that's not an error.
let _ = stdin.write_all(content.as_bytes());
drop(stdin);
}
child.wait()?;
Ok(())
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::sync::Mutex;
/// A test pager that records what it was asked to show.
#[derive(Default)]
struct RecordingPager {
shown: Mutex<Vec<String>>,
}
impl Pager for RecordingPager {
fn show(&self, content: &str) -> std::io::Result<()> {
self.shown.lock().unwrap().push(content.to_string());
Ok(())
}
}
#[test]
fn custom_pager_receives_content() {
let pager = RecordingPager::default();
pager.show("hello").unwrap();
assert_eq!(
pager.shown.lock().unwrap().as_slice(),
&["hello".to_string()]
);
}
#[test]
fn explicit_pager_env_var_wins_and_splits_args() {
// Serialised via the env guard below; MANPAGER takes precedence over PAGER.
let _guard = EnvGuard::set(&[("MANPAGER", Some("myp --opt")), ("PAGER", Some("other"))]);
let (program, args) = pager_command().expect("a pager command");
assert_eq!(program, "myp");
assert_eq!(args, vec!["--opt".to_string()]);
}
#[test]
fn falls_back_to_a_platform_default() {
let _guard = EnvGuard::set(&[("MANPAGER", None), ("PAGER", None)]);
let (program, _) = pager_command().expect("a pager command");
assert_eq!(program, if cfg!(windows) { "more.com" } else { "less" });
}
#[test]
fn pager_env_var_wins_when_manpager_is_empty() {
let _guard = EnvGuard::set(&[("MANPAGER", Some("")), ("PAGER", Some("myp --plain"))]);
let (program, args) = pager_command().expect("a pager command");
assert_eq!(program, "myp");
assert_eq!(args, vec!["--plain".to_string()]);
}
#[cfg(windows)]
#[test]
fn windows_default_pager_displays_piped_content() {
let _guard = EnvGuard::set(&[("MANPAGER", None), ("PAGER", None)]);
let (program, args) = pager_command().expect("a pager command");
// CI has no terminal, so exercise the selected executable directly:
// SystemPager::show would correctly take the plain-output fallback.
let mut child = Command::new(program)
.args(args)
.stdin(Stdio::piped())
.stdout(Stdio::piped())
.stderr(Stdio::piped())
.spawn()
.expect("the default Windows pager must resolve and start");
let content = "rs-rich Windows pager smoke test\r\nsecond line reaches the pager\r\n";
child
.stdin
.take()
.unwrap()
.write_all(content.as_bytes())
.unwrap();
let output = child.wait_with_output().unwrap();
assert!(
output.status.success(),
"pager failed: {}",
String::from_utf8_lossy(&output.stderr)
);
assert_eq!(
String::from_utf8(output.stdout)
.unwrap()
.replace("\r\n", "\n")
.trim_end_matches('\n'),
content.replace("\r\n", "\n").trim_end_matches('\n')
);
}
/// Set/restore env vars around a test. The environment tests share a lock so
/// they can't interleave (tests run in parallel threads).
struct EnvGuard {
previous: Vec<(String, Option<String>)>,
_lock: std::sync::MutexGuard<'static, ()>,
}
static ENV_LOCK: Mutex<()> = Mutex::new(());
impl EnvGuard {
fn set(vars: &[(&str, Option<&str>)]) -> Self {
let lock = ENV_LOCK.lock().unwrap_or_else(|e| e.into_inner());
let previous = vars
.iter()
.map(|(key, _)| ((*key).to_string(), std::env::var(key).ok()))
.collect();
for (key, value) in vars {
match value {
Some(value) => std::env::set_var(key, value),
None => std::env::remove_var(key),
}
}
EnvGuard {
previous,
_lock: lock,
}
}
}
impl Drop for EnvGuard {
fn drop(&mut self) {
for (key, value) in &self.previous {
match value {
Some(value) => std::env::set_var(key, value),
None => std::env::remove_var(key),
}
}
}
}
}