Remove signal event listeners from process on finish (#512)

src/flow-control/kill-on-signal.spec.ts

@@ -32,7 +32,7 @@ it('returns commands that map SIGINT to exit code 0', () => {
 
     const callback = jest.fn();
     newCommands[0].close.subscribe(callback);
-    process.emit('SIGINT');
+    process.emit('SIGINT', 'SIGINT');
 
     // A fake command's .kill() call won't trigger a close event automatically...
     commands[0].close.next(createFakeCloseEvent({ exitCode: 1 }));
@@ -56,7 +56,7 @@ it('returns commands that keep non-SIGINT exit codes', () => {
 describe.each(['SIGINT', 'SIGTERM', 'SIGHUP'])('on %s', (signal) => {
     it('kills all commands', () => {
         controller.handle(commands);
-        process.emit(signal);
+        process.emit(signal, signal);
 
         expect(process.listenerCount(signal)).toBe(1);
         expect(commands[0].kill).toHaveBeenCalledWith(signal);
@@ -65,8 +65,14 @@ describe.each(['SIGINT', 'SIGTERM', 'SIGHUP'])('on %s', (signal) => {
 
     it('sends abort signal', () => {
         controller.handle(commands);
-        process.emit(signal);
+        process.emit(signal, signal);
 
         expect(abortController.signal.aborted).toBe(true);
     });
+
+    it('removes event listener on finish', () => {
+        const { onFinish } = controller.handle(commands);
+        onFinish();
+        expect(process.listenerCount(signal)).toBe(0);
+    });
 });

src/flow-control/kill-on-signal.ts

@@ -4,6 +4,8 @@ import { map } from 'rxjs/operators';
 import { Command } from '../command';
 import { FlowController } from './flow-controller';
 
+const SIGNALS = ['SIGINT', 'SIGTERM', 'SIGHUP'] as const;
+
 /**
  * Watches the main concurrently process for signals and sends the same signal down to each spawned
  * command.
@@ -25,13 +27,12 @@ export class KillOnSignal implements FlowController {
 
     handle(commands: Command[]) {
         let caughtSignal: NodeJS.Signals;
-        (['SIGINT', 'SIGTERM', 'SIGHUP'] as NodeJS.Signals[]).forEach((signal) => {
-            this.process.on(signal, () => {
-                caughtSignal = signal;
-                this.abortController?.abort();
-                commands.forEach((command) => command.kill(signal));
-            });
-        });
+        const signalListener = (signal: NodeJS.Signals) => {
+            caughtSignal = signal;
+            this.abortController?.abort();
+            commands.forEach((command) => command.kill(signal));
+        };
+        SIGNALS.forEach((signal) => this.process.on(signal, signalListener));
 
         return {
             commands: commands.map((command) => {
@@ -50,6 +51,10 @@ export class KillOnSignal implements FlowController {
                     },
                 });
             }),
+            onFinish: () => {
+                // Avoids MaxListenersExceededWarning when running programmatically
+                SIGNALS.forEach((signal) => this.process.off(signal, signalListener));
+            },
         };
     }
 }