Wait until process exit in esbuild.stop()

This changes `esbuild.stop()` to only resolve the returned promise when the child process has actually exited.

This is useful for when you have some resource sanitization in place that checks that the child process has actually been closed.

lib/npm/node.ts

@@ -220,10 +220,9 @@ export let analyzeMetafileSync: typeof types.analyzeMetafileSync = (metafile, op
   return result!
 }
 
-export const stop = () => {
-  if (stopService) stopService()
+export const stop = async () => {
+  if (stopService) await stopService()
   if (workerThreadService) workerThreadService.stop()
-  return Promise.resolve()
 }
 
 let initializeWasCalled = false
@@ -249,7 +248,7 @@ interface Service {
 
 let defaultWD = process.cwd()
 let longLivedService: Service | undefined
-let stopService: (() => void) | undefined
+let stopService: (() => Promise<void>) | undefined
 
 let ensureServiceIsRunning = (): Service => {
   if (longLivedService) return longLivedService
@@ -285,14 +284,25 @@ let ensureServiceIsRunning = (): Service => {
   stdout.on('data', readFromStdout)
   stdout.on('end', afterClose)
 
-  stopService = () => {
+  stopService = async () => {
+    const exited = new Promise<void>(resolve => {
+      if (child.exitCode !== null || child.signalCode !== null) {
+        resolve()
+      } else {
+        // Re-ref the child so that this process's event loop doesn't starve
+        // if the user waits for the `stop` promise to resolve.
+        refs.ref()
+        child.on('exit', resolve)
+      }
+    })
     // Close all resources related to the subprocess.
     stdin.destroy()
     stdout.destroy()
     child.kill()
     initializeWasCalled = false
     longLivedService = undefined
     stopService = undefined
+    await exited
   }
 
   let refCount = 0