Ensure project_eval result is received by the parent (#45)

Currently the child does `process.send` and immediately `process.exit`.
My understanding is that `process.send` is async and it is not
guaranteed that the message is sent by the time the child exists. To
guarantee it, we make the parent send an acknowledgement before exiting.
See nodejs/node#6767.

Author: jonatanklosko

src/evaluation/code_executor.ts

@@ -32,6 +32,8 @@ export async function executeIsolated(request: EvaluationRequest): Promise<Evalu
         const { data, success } = msg;
         evaluation.result = data;
         evaluation.success = success;
+        // Acknowledge the result and tell the child to exit gracefully.
+        child.send({ type: 'finish' });
       }
     });
 
@@ -58,6 +60,6 @@ export async function executeIsolated(request: EvaluationRequest): Promise<Evalu
 
     child.on('exit', () => clearTimeout(timeoutId));
 
-    child.send(request);
+    child.send({ type: 'evaluate', request: request });
   });
 }

src/evaluation/eval_worker.ts

@@ -1,28 +1,38 @@
 import type { EvaluationRequest } from '../core';
 
-process.on('message', async ({ code, args }: EvaluationRequest) => {
+type WorkerMessage = { type: 'evaluate'; request: EvaluationRequest } | { type: 'finish' };
+
+process.on('message', async (message: WorkerMessage) => {
   if (!process.send) {
     console.error('[Tidewave] Unable to establish communication channel with code-executor.');
     process.exit(1);
   }
 
-  try {
-    const AsyncFunction = Object.getPrototypeOf(async () => {}).constructor;
-    const fn = new AsyncFunction(code);
-    const result = await fn(...args);
+  if (message.type === 'evaluate') {
+    const { code, args } = message.request;
 
-    process.send({
-      type: 'result',
-      success: true,
-      data: (result ?? null) && result,
-    });
-  } catch (error) {
-    process.send({
-      type: 'result',
-      success: false,
-      data: new String(error),
-    });
-  }
+    try {
+      const AsyncFunction = Object.getPrototypeOf(async () => {}).constructor;
+      const fn = new AsyncFunction(code);
+      const result = await fn(...args);
 
-  process.exit(0);
+      process.send({
+        type: 'result',
+        success: true,
+        data: (result ?? null) && result,
+      });
+    } catch (error) {
+      process.send({
+        type: 'result',
+        success: false,
+        data: new String(error),
+      });
+    }
+  } else if (message.type === 'finish') {
+    // Note that process.send is async [1], so we wait for the parent
+    // to receive the result and then tell us to exit.
+    //
+    // [1]: https://github.com/nodejs/node/issues/6767
+    process.exit(0);
+  }
 });