cli: Add pager to `rad diff`

Invokes a pager when the output doesn't fit on the screen.

Cargo.lock

@@ -1824,6 +1824,7 @@ dependencies = [
  "serde",
  "serde_json",
  "serde_yaml",
+ "shlex",
  "tempfile",
  "thiserror",
  "timeago",

radicle-cli/Cargo.toml

@@ -26,6 +26,7 @@ radicle-surf = { version = "0.17.0" }
 serde = { version = "1.0" }
 serde_json = { version = "1" }
 serde_yaml = { version = "0.8" }
+shlex = { version = "1.1.0" }
 tempfile = { version = "3.3.0" }
 thiserror = { version = "1" }
 timeago = { version = "0.3", default-features = false }

radicle-cli/src/commands/diff.rs

@@ -8,10 +8,10 @@ use radicle_surf as surf;
 
 use crate::git::pretty_diff::ToPretty as _;
 use crate::git::Rev;
+use crate::pager;
 use crate::terminal as term;
 use crate::terminal::args::{Args, Error, Help};
 use crate::terminal::highlight::Highlighter;
-use crate::terminal::{Constraint, Element as _};
 
 pub const HELP: Help = Help {
     name: "diff",
@@ -145,7 +145,7 @@ pub fn run(options: Options, _ctx: impl term::Context) -> anyhow::Result<()> {
     let mut hi = Highlighter::default();
     let pretty = diff.pretty(&mut hi, &(), &repo);
 
-    pretty.write(Constraint::from_env().unwrap_or_default());
+    pager::run(pretty)?;
 
     Ok(())
 }

radicle-cli/src/lib.rs

@@ -3,5 +3,6 @@
 #![allow(clippy::too_many_arguments)]
 pub mod commands;
 pub mod git;
+pub mod pager;
 pub mod project;
 pub mod terminal;

radicle-cli/src/pager.rs

@@ -0,0 +1,51 @@
+use std::io;
+use std::process::{Command, Stdio};
+
+use radicle_term::element;
+use radicle_term::{Constraint, Element};
+
+use crate::terminal;
+
+/// Output the given element through a pager, if necessary.
+/// If it fits within the screen, don't run it through a pager.
+pub fn run(elem: impl Element) -> io::Result<()> {
+    let Some(constraint) = Constraint::from_env() else {
+        return elem.write(Constraint::UNBOUNDED);
+    };
+    let Some(rows) = terminal::rows() else {
+        return elem.write(Constraint::UNBOUNDED);
+    };
+    if elem.size(Constraint::UNBOUNDED).rows <= rows {
+        return elem.write(Constraint::UNBOUNDED);
+    }
+    let Some(pager) = radicle::profile::env::pager() else {
+        return elem.write(Constraint::UNBOUNDED);
+    };
+    let Some(parts) = shlex::split(&pager) else {
+        return elem.write(Constraint::UNBOUNDED);
+    };
+    let Some((program, args)) = parts.split_first() else {
+        return elem.write(Constraint::UNBOUNDED);
+    };
+
+    let mut child = Command::new(program)
+        .stdin(Stdio::piped())
+        .stdout(Stdio::inherit())
+        .stderr(Stdio::inherit())
+        .args(args)
+        .spawn()?;
+
+    let writer = child.stdin.as_mut().unwrap();
+    let result = element::write_to(&elem, writer, constraint);
+
+    child.wait()?;
+
+    match result {
+        // This error is expected when the pager is exited.
+        Err(e) if e.kind() == io::ErrorKind::BrokenPipe => {}
+        Err(e) => return Err(e),
+        Ok(_) => {}
+    }
+
+    Ok(())
+}

radicle-term/src/element.rs

@@ -43,19 +43,12 @@ impl Constraint {
         }
     }
 
-    /// A constraint that can only be satisfied by a single size.
+    /// A constraint that can only be satisfied by a single column size.
-    pub fn tight(size: Size) -> Self {
+    /// The rows are unconstrained.
+    pub fn tight(cols: usize) -> Self {
         Self {
-            min: size,
+            min: Size::new(cols, 1),
-            max: size,
+            max: Size::new(cols, usize::MAX),
-        }
-    }
-
-    /// Return a new constraint that forces objects to the maximum size.
-    pub fn maximize(self) -> Self {
-        Self {
-            min: self.max,
-            max: self.max,
         }
     }
 
@@ -96,11 +89,12 @@ pub trait Element: fmt::Debug {
         }
     }
 
-    /// Write to a writer.
+    /// Write using the given constraints to `stdout`.
-    fn write(&self, constraints: Constraint) {
+    fn write(&self, constraints: Constraint) -> io::Result<()>
-        for line in self.render(constraints) {
+    where
-            println!("{}", line.to_string().trim_end());
+        Self: Sized,
-        }
+    {
+        self::write_to(self, &mut io::stdout(), constraints)
     }
 
     #[must_use]
@@ -143,6 +137,18 @@ impl<T: Element> Element for &T {
     }
 }
 
+/// Write using the given constraints, to a writer.
+pub fn write_to(
+    elem: &impl Element,
+    writer: &mut impl io::Write,
+    constraints: Constraint,
+) -> io::Result<()> {
+    for line in elem.render(constraints) {
+        writeln!(writer, "{}", line.to_string().trim_end())?;
+    }
+    Ok(())
+}
+
 /// A line of text that has styling and can be displayed.
 #[derive(Clone, Default, Debug)]
 pub struct Line {

radicle-term/src/io.rs

@@ -92,6 +92,10 @@ pub fn columns() -> Option<usize> {
     termion::terminal_size().map(|(cols, _)| cols as usize).ok()
 }
 
+pub fn rows() -> Option<usize> {
+    termion::terminal_size().map(|(_, rows)| rows as usize).ok()
+}
+
 pub fn viewport() -> Option<Size> {
     termion::terminal_size()
         .map(|(cols, rows)| Size::new(cols as usize, rows as usize))

radicle-term/src/vstack.rs

@@ -32,6 +32,13 @@ impl<'a> Row<'a> {
             Self::Dividier => c.min.cols,
         }
     }
+
+    fn height(&self, c: Constraint) -> usize {
+        match self {
+            Self::Element(e) => e.rows(c),
+            Self::Dividier => 1,
+        }
+    }
 }
 
 /// Vertical stack of [`Element`] objects that implements [`Element`].
@@ -109,7 +116,7 @@ impl<'a> VStack<'a> {
     fn outer(&self, c: Constraint) -> Size {
         let padding = self.opts.padding * 2;
         let mut cols = self.rows.iter().map(|r| r.width(c)).max().unwrap_or(0) + padding;
-        let mut rows = self.rows.len();
+        let mut rows = self.rows.iter().map(|r| r.height(c)).sum();
 
         // Account for outer borders.
         if self.opts.border.is_some() {
@@ -129,10 +136,7 @@ impl<'a> Element for VStack<'a> {
         let mut lines = Vec::new();
         let padding = self.opts.padding;
         let inner = self.inner(parent);
-        let child = Constraint::tight(Size {
+        let child = Constraint::tight(inner.cols - padding * 2);
-            cols: inner.cols - padding * 2,
-            rows: usize::MAX,
-        });
 
         if let Some(color) = self.opts.border {
             lines.push(

radicle/src/profile.rs

@@ -40,9 +40,22 @@ pub mod env {
     /// RNG seed. Must be convertible to a `u64`.
     pub const RAD_RNG_SEED: &str = "RAD_RNG_SEED";
 
+    /// Get the configured pager program from the environment.
+    pub fn pager() -> Option<String> {
+        if let Ok(cfg) = git2::Config::open_default() {
+            if let Ok(pager) = cfg.get_string("core.pager") {
+                return Some(pager);
+            }
+        }
+        if let Ok(pager) = var("PAGER") {
+            return Some(pager);
+        }
+        None
+    }
+
     /// Get the radicle passphrase from the environment.
     pub fn passphrase() -> Option<super::Passphrase> {
-        let Ok(passphrase) = std::env::var(RAD_PASSPHRASE) else {
+        let Ok(passphrase) = var(RAD_PASSPHRASE) else {
             return None;
         };
         Some(super::Passphrase::from(passphrase))
@@ -50,7 +63,7 @@ pub mod env {
 
     /// Get a random number generator from the environment.
     pub fn rng() -> fastrand::Rng {
-        if let Ok(seed) = std::env::var(RAD_RNG_SEED) {
+        if let Ok(seed) = var(RAD_RNG_SEED) {
             return fastrand::Rng::with_seed(
                 seed.parse()
                     .expect("env::rng: invalid seed specified in `RAD_RNG_SEED`"),