CLI: a closed stdout ends vectorcraft-cli quietly

`vectorcraft-cli commands | head -1` panicked and exited with status 101:

  thread 'main' panicked at library/std/src/io/stdio.rs:1168:9:
  failed printing to stdout: Broken pipe (os error 32)

println! and print! panic when stdout is closed. Output now goes through
outln! and out!, which end the program with status 0 on a broken pipe (the
reader has what it wanted) and report any other write error on stderr with
status 1.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/vectorcraft-cli/src/main.rs

@@ -14,11 +14,41 @@
 use std::io::Write;
 use std::process::ExitCode;
 
+/// `println!` / `print!` that end the program quietly when stdout is closed
+/// (`vectorcraft-cli commands | head`) instead of panicking with "failed printing to stdout:
+/// Broken pipe (os error 32)".
+macro_rules! outln {
+    ($($arg:tt)*) => {{
+        use std::io::Write as _;
+        if let Err(e) = writeln!(std::io::stdout(), $($arg)*) {
+            $crate::stdout_failed(e);
+        }
+    }};
+}
+macro_rules! out {
+    ($($arg:tt)*) => {{
+        use std::io::Write as _;
+        if let Err(e) = write!(std::io::stdout(), $($arg)*) {
+            $crate::stdout_failed(e);
+        }
+    }};
+}
+
 mod perf;
 
 use serde_json::{Value, json};
 use vectorcraft_mcp::{Backend, DEFAULT_ADDR, Headless, Remote, Server};
 
+/// stdout went away. A reader that stopped early (a closed pipe) ends the program quietly, as
+/// ripgrep does; any other write error is reported.
+fn stdout_failed(e: std::io::Error) -> ! {
+    if e.kind() == std::io::ErrorKind::BrokenPipe {
+        std::process::exit(0);
+    }
+    eprintln!("vectorcraft-cli: can't write to stdout: {e}");
+    std::process::exit(1);
+}
+
 const USAGE: &str = "\
 vectorcraft-cli — VectorCraft automation
 
@@ -68,11 +98,11 @@ fn main() -> ExitCode {
         Some("bench") => bench(&args[1..]),
         Some("perf") => perf::run(&args[1..]),
         Some("-h" | "--help" | "help") | None => {
-            print!("{}", usage());
+            out!("{}", usage());
             Ok(())
         }
         Some("-V" | "--version") => {
-            println!("vectorcraft-cli {}", env!("CARGO_PKG_VERSION"));
+            outln!("vectorcraft-cli {}", env!("CARGO_PKG_VERSION"));
             Ok(())
         }
         Some(other) => Err(format!("unknown subcommand `{other}`\n\n{}", usage())),
@@ -120,7 +150,7 @@ fn mcp(args: &[String]) -> Result<(), String> {
 fn commands() -> Result<(), String> {
     let mut h = Headless::new();
     let v = h.call("engine.commands", json!({}))?;
-    println!("{}", serde_json::to_string_pretty(&v).unwrap_or_default());
+    outln!("{}", serde_json::to_string_pretty(&v).unwrap_or_default());
     Ok(())
 }
 
@@ -146,7 +176,7 @@ fn convert(args: &[String]) -> Result<(), String> {
             json!({"command": "document.export", "params": {"path": output, "scale": scale, "artboard": artboard, "range": range, "outlineText": outline_text}}),
         )
         .map_err(|e| format!("export {output}: {e}"))?;
-    println!("{r}");
+    outln!("{r}");
     Ok(())
 }
 
@@ -162,7 +192,7 @@ fn info(args: &[String]) -> Result<(), String> {
     let artboards: Vec<Value> =
         doc.artboards.iter().map(|a| json!({"name": a.name, "rect": [a.rect.x0, a.rect.y0, a.rect.width(), a.rect.height()]})).collect();
     let v = json!({"file": file, "info": base, "artboards": artboards, "kinds": kinds, "fonts": fonts});
-    println!("{}", serde_json::to_string_pretty(&v).unwrap_or_default());
+    outln!("{}", serde_json::to_string_pretty(&v).unwrap_or_default());
     Ok(())
 }
 
@@ -252,7 +282,7 @@ fn bench(args: &[String]) -> Result<(), String> {
         * vectorcraft_geom::Affine::scale(z)
         * vectorcraft_geom::Affine::translate(-b.center().to_vec2());
     let opts = vectorcraft_render::RenderOptions::default();
-    println!("{file}: {} nodes, {w}x{h}", doc.layers.iter().map(|l| l.count()).sum::<usize>());
+    outln!("{file}: {} nodes, {w}x{h}", doc.layers.iter().map(|l| l.count()).sum::<usize>());
     for threads in [vectorcraft_render::default_threads(), 0] {
         let mut r = vectorcraft_render::Renderer::new();
         r.threads = threads;
@@ -261,7 +291,7 @@ fn bench(args: &[String]) -> Result<(), String> {
         for _ in 0..iters {
             r.render(&doc, w, h, view, &opts);
         }
-        println!(
+        outln!(
             "  threads {threads}: {:.1} ms/frame (drawn {}, culled {})",
             t.elapsed().as_secs_f64() * 1000.0 / iters as f64,
             r.stats.drawn,

apps/vectorcraft-cli/src/perf.rs

@@ -81,14 +81,14 @@ pub fn run(args: &[String]) -> Result<(), String> {
     }
     let cores = std::thread::available_parallelism().map_or(1, |c| c.get());
     let load = load_average();
-    println!("VectorCraft performance budgets — {n} paths, {cores} cores, load average {}", load.map_or("?".into(), |l| format!("{l:.1}")));
+    outln!("VectorCraft performance budgets — {n} paths, {cores} cores, load average {}", load.map_or("?".into(), |l| format!("{l:.1}")));
     let noisy = load.is_some_and(|l| l > cores as f64 * 0.75);
     if noisy {
-        println!("WARNING: the machine is busy; wall-clock timings below are not trustworthy.");
+        outln!("WARNING: the machine is busy; wall-clock timings below are not trustworthy.");
     }
     let t = Instant::now();
     let doc = synthetic(n)?;
-    println!("  (built in {:.0} ms)", t.elapsed().as_secs_f64() * 1000.0);
+    outln!("  (built in {:.0} ms)", t.elapsed().as_secs_f64() * 1000.0);
 
     let mut rows: Vec<(&str, f64, f64)> = vec![];
     let opts = RenderOptions::default();
@@ -150,16 +150,16 @@ pub fn run(args: &[String]) -> Result<(), String> {
     }
 
     let mut over = 0;
-    println!("  {:<40} {:>10} {:>10}", "", "measured", "budget");
+    outln!("  {:<40} {:>10} {:>10}", "", "measured", "budget");
     for (name, ms, budget) in &rows {
         let ok = ms <= budget;
         over += usize::from(!ok);
-        println!("  {name:<40} {ms:>8.2} ms {budget:>7.0} ms  {}", if ok { "ok" } else { "OVER" });
+        outln!("  {name:<40} {ms:>8.2} ms {budget:>7.0} ms  {}", if ok { "ok" } else { "OVER" });
     }
     match (over, noisy) {
         (0, _) => Ok(()),
         (_, true) => {
-            println!("{over} over budget, but the machine is busy: re-run when idle.");
+            outln!("{over} over budget, but the machine is busy: re-run when idle.");
             Ok(())
         }
         _ => Err(format!("{over} budget(s) exceeded")),

apps/vectorcraft-cli/tests/cli.rs

@@ -82,3 +82,17 @@ fn mcp_headless_over_stdio() {
     assert_eq!(replies[2]["result"]["isError"], false);
     assert_eq!(replies[3]["result"]["content"][0]["type"], "image");
 }
+
+/// `vectorcraft-cli commands | head -1` panicked with "failed printing to
+/// stdout: Broken pipe (os error 32)" and exit status 101.
+#[test]
+fn closed_stdout_ends_quietly() {
+    use std::io::Read;
+    let mut child = Command::new(BIN).arg("commands").stdout(Stdio::piped()).stderr(Stdio::piped()).spawn().unwrap();
+    let mut first = [0u8; 16];
+    child.stdout.as_mut().unwrap().read_exact(&mut first).unwrap();
+    drop(child.stdout.take());
+    let out = child.wait_with_output().unwrap();
+    let err = String::from_utf8_lossy(&out.stderr);
+    assert!(out.status.success() && !err.contains("panicked"), "{:?}: {err}", out.status);
+}