Refactor CLI to use spawn for better signal handling in watch mode (#7844)

* Refactor CLI to use spawn for better signal handling in watch mode

* Add changelog

Author: nojaf

CHANGELOG.md

@@ -35,6 +35,7 @@
 #### :nail_care: Polish
 
 - Improve error message for trying to define a type inside a function. https://github.com/rescript-lang/rescript/pull/7843
+- Refactor CLI to use spawn for better signal handling in watch mode. https://github.com/rescript-lang/rescript/pull/7844
 
 #### :house: Internal
 

cli/rescript.js

@@ -7,14 +7,85 @@ import { rescript_exe } from "./common/bins.js";
 
 const args = process.argv.slice(2);
 
-try {
-  child_process.execFileSync(rescript_exe, args, {
-    stdio: "inherit",
-  });
-} catch (err) {
-  if (err.status !== undefined) {
-    process.exit(err.status); // Pass through the exit code
+// We intentionally use spawn (async) instead of execFileSync (sync) here.
+// Rationale:
+// - execFileSync blocks Node's event loop, so Ctrl+C (SIGINT) causes Node to
+//   exit immediately without giving us a chance to forward the signal to the
+//   child and wait for its cleanup. In watch mode, the Rust watcher prints
+//   "Exiting..." on SIGINT and performs cleanup; with execFileSync that output
+//   may appear after the shell prompt and sometimes requires an extra keypress.
+// - spawn lets us install signal handlers, forward them to the child, and then
+//   exit the parent with the correct status only after the child has exited.
+const child = child_process.spawn(rescript_exe, args, {
+  stdio: "inherit",
+});
+
+// Map POSIX signal names to conventional exit status numbers so we can
+// reproduce the usual 128 + signal behavior when exiting due to a signal.
+/** @type {Record<string, number>} */
+const signalToNumber = { SIGINT: 2, SIGTERM: 15, SIGHUP: 1, SIGQUIT: 3 };
+
+let forwardedSignal = false;
+/**
+ * @param {NodeJS.Signals} signal
+ */
+const handleSignal = (signal) => {
+  // Intercept the signal in the parent, forward it to the child, and let the
+  // child perform its own cleanup. This ensures ordered shutdown in watch mode.
+  // Guard against double-forwarding since terminals or OSes can deliver
+  // multiple signals (e.g., repeated Ctrl+C).
+  // Prevent Node from exiting immediately; forward to child first
+  if (forwardedSignal) return;
+  forwardedSignal = true;
+  try {
+    if (child.exitCode === null && child.signalCode == null) {
+      child.kill(signal);
+    }
+  } catch {
+    // best effort
+  }
+};
+
+process.on("SIGINT", handleSignal);
+process.on("SIGTERM", handleSignal);
+process.on("SIGHUP", handleSignal);
+process.on("SIGQUIT", handleSignal);
+
+// Cross-platform note:
+// - On Unix, Ctrl+C sends SIGINT to the process group; we also explicitly
+//   forward it to the child to be robust.
+// - On Windows, Node maps kill('SIGINT'/'SIGTERM') to console control events;
+//   the Rust watcher (via the ctrlc crate) handles these and exits cleanly.
+
+// Ensure no orphaned process if parent exits unexpectedly
+process.on("exit", () => {
+  if (child.exitCode === null && child.signalCode == null) {
+    try {
+      child.kill("SIGTERM");
+    } catch {
+      // ignore
+    }
+  }
+});
+
+child.on("exit", (code, signal) => {
+  process.removeListener("SIGINT", handleSignal);
+  process.removeListener("SIGTERM", handleSignal);
+  process.removeListener("SIGHUP", handleSignal);
+  process.removeListener("SIGQUIT", handleSignal);
+
+  // If the child exited due to a signal, emulate the conventional exit status
+  // (128 + signalNumber). Otherwise, pass through the child's numeric exit code.
+  if (signal) {
+    const n = signalToNumber[signal];
+    process.exit(typeof n === "number" ? 128 + n : 1);
   } else {
-    process.exit(1); // Generic error
+    process.exit(typeof code === "number" ? code : 0);
   }
-}
+});
+
+// Surface spawn errors (e.g., executable not found) and exit with failure.
+child.on("error", (err) => {
+  console.error(err?.message ?? String(err));
+  process.exit(1);
+});