fix: handle closed stdout pipes in CLI command listing

apps/solvecraft-cli/src/main.rs

@@ -53,8 +53,17 @@ fn main() -> ExitCode {
         Some("snapshot") => cmd_snapshot(&args[1..]),
         Some("exec") => cmd_exec(&args[1..]),
         Some("commands") => {
+            use std::io::Write as _;
             let mut s = Session::default();
-            s.execute("engine.commands", &json!({})).map(|v| println!("{}", pretty(&v))).map_err(|e| e.to_string())
+            s.execute("engine.commands", &json!({})).map_err(|e| e.to_string()).and_then(|v| {
+                // A pipeline consumer may stop early (e.g. `commands | head`).
+                // Unlike println!, writing explicitly lets us handle that without a panic.
+                match writeln!(std::io::stdout().lock(), "{}", pretty(&v)) {
+                    Ok(()) => Ok(()),
+                    Err(e) if e.kind() == std::io::ErrorKind::BrokenPipe => Ok(()),
+                    Err(e) => Err(format!("stdout: {e}")),
+                }
+            })
         }
         Some("recipe") => args.get(1).ok_or_else(|| USAGE.to_string()).and_then(|p| {
             let v = read_json(p)?;

apps/solvecraft-cli/tests/commands_output.rs

@@ -0,0 +1,26 @@
+//! The registry remains valid JSON and a closed pipeline reader must not cause a panic.
+
+use std::process::{Command, Stdio};
+
+#[test]
+fn registry_output_is_complete_json() {
+    let output = Command::new(env!("CARGO_BIN_EXE_solvecraft-cli")).arg("commands").output().unwrap();
+    assert!(output.status.success(), "{}", String::from_utf8_lossy(&output.stderr));
+    let registry: serde_json::Value = serde_json::from_slice(&output.stdout).unwrap();
+    let commands = registry.as_array().unwrap();
+    assert!(commands.iter().any(|command| command["id"] == "solid.box"));
+    assert!(commands.iter().any(|command| command["id"] == "engine.commands"));
+    assert!(output.stdout.ends_with(b"\n"));
+    assert!(output.stderr.is_empty());
+}
+
+#[test]
+fn closed_registry_pipe_exits_without_panicking() {
+    let mut child = Command::new(env!("CARGO_BIN_EXE_solvecraft-cli")).arg("commands").stdout(Stdio::piped()).stderr(Stdio::piped()).spawn().unwrap();
+    // Close our read end before waiting. No sleeps or timing threshold are needed:
+    // the registry is larger than the pipe buffer, so it cannot all be written first.
+    drop(child.stdout.take());
+    let output = child.wait_with_output().unwrap();
+    assert!(output.status.success(), "status: {}; stderr: {}", output.status, String::from_utf8_lossy(&output.stderr));
+    assert!(output.stderr.is_empty(), "{}", String::from_utf8_lossy(&output.stderr));
+}