fix: unexpected null pointer exception due to race condition

.changeset/shiny-kids-check.md

@@ -0,0 +1,5 @@
+---
+"synckit": patch
+---
+
+fix: unexpected null pointer exception due to race condition

package.json

@@ -141,7 +141,7 @@
     "detail": true,
     "ignoreAsAssertion": true,
     "ignoreFiles": [
-      "**/*.d.ts"
+      "lib"
     ],
     "ignoreNonNullAssertion": true,
     "showRelativePath": true,

src/index.ts

@@ -657,11 +657,13 @@ function startWorkerThread<T extends AnyFn, R = Awaited<ReturnType<T>>>(
       throw new Error('Internal error: Atomics.wait() failed: ' + status)
     }
 
-    const { id, ...message } = (
-      receiveMessageOnPort(mainPort) as { message: WorkerToMainMessage<R> }
-    ).message
+    const result = receiveMessageOnPort(mainPort) as
+      | { message: WorkerToMainMessage<R> }
+      | undefined
 
-    if (id < expectedId) {
+    const msg = result?.message
+
+    if (msg?.id == null || msg.id < expectedId) {
       const waitingTime = Date.now() - start
       return receiveMessageWithId(
         port,
@@ -670,6 +672,8 @@ function startWorkerThread<T extends AnyFn, R = Awaited<ReturnType<T>>>(
       )
     }
 
+    const { id, ...message } = msg
+
     if (expectedId !== id) {
       throw new Error(
         `Internal error: Expected id ${expectedId} but got id ${id}`,

test/reliability.spec.ts

@@ -0,0 +1,22 @@
+import path from 'node:path'
+
+import { _dirname, testIf } from './helpers.js'
+
+const times = 1e6
+
+const test = testIf(!!process.env.CI)
+
+test(`Reliability (${times} runs)`, async () => {
+  const { createSyncFn } = await import('synckit')
+  const identity = createSyncFn(path.resolve(_dirname, `./worker-identity.js`))
+
+  for (let index = 0; index < times; index++) {
+    try {
+      // eslint-disable-next-line jest/no-standalone-expect
+      expect(identity(index)).toBe(index)
+    } catch (error) {
+      console.error(`Failed on ${index + 1}/${times} run.`)
+      throw error
+    }
+  }
+})

test/worker-identity.js

@@ -0,0 +1,3 @@
+import { runAsWorker } from 'synckit'
+
+runAsWorker(value => value)