P0.2a: finish file output after stdout closes

apps/deckcraft-cli/src/help.rs

@@ -180,7 +180,10 @@ A presentation is edited by running commands on it:
   deckcraft-cli run --cmd 'slide.new={{\"title\":\"Q3 results\"}}' --save q3.deckcraft
 
 `deckcraft-cli help SUBCOMMAND`, or `deckcraft-cli SUBCOMMAND --help`, explains one subcommand;
-`deckcraft-cli help run` explains how to write a command.",
+`deckcraft-cli help run` explains how to write a command.
+
+If an output reader closes early, run, render, render-transition and convert continue; their exit
+status reflects the work, not output delivery. Stop the process to cancel them.",
         list.join("\n")
     )
 }

apps/deckcraft-cli/src/main.rs

@@ -7,23 +7,38 @@ mod automation_args;
 mod help;
 
 use std::process::ExitCode;
+use std::sync::atomic::{AtomicBool, Ordering};
 
 use deckcraft_engine::Session;
 use serde_json::{Value, json};
 
+static STDOUT_CLOSED: AtomicBool = AtomicBool::new(false);
+
 macro_rules! outln {
     ($($arg:tt)*) => {{
-        use std::io::Write as _;
-        if let Err(e) = writeln!(std::io::stdout(), $($arg)*) {
+        if !STDOUT_CLOSED.load(Ordering::Relaxed) && let Err(e) = stdout_line(format_args!($($arg)*)) {
             if e.kind() == std::io::ErrorKind::BrokenPipe {
-                std::process::exit(0);
+                STDOUT_CLOSED.store(true, Ordering::Relaxed);
+            } else {
+                eprintln!("deckcraft-cli: can't write to stdout: {e}");
+                std::process::exit(1);
             }
-            eprintln!("deckcraft-cli: can't write to stdout: {e}");
-            std::process::exit(1);
         }
     }};
 }
 
+fn stdout_line(args: std::fmt::Arguments<'_>) -> std::io::Result<()> {
+    use std::io::Write as _;
+    #[cfg(unix)]
+    let mut stdout = {
+        use std::os::fd::AsFd as _;
+        std::fs::File::from(std::io::stdout().as_fd().try_clone_to_owned()?)
+    };
+    #[cfg(not(unix))]
+    let mut stdout = std::io::stdout();
+    stdout.write_all(format!("{args}\n").as_bytes())
+}
+
 fn main() -> ExitCode {
     let args: Vec<String> = std::env::args().skip(1).collect();
     let rest = args.get(1..).unwrap_or(&[]);