watch: debounce restart in watch mode

lib/internal/main/watch_mode.js

@@ -6,6 +6,7 @@ const {
   ArrayPrototypeMap,
   ArrayPrototypePushApply,
   ArrayPrototypeSlice,
+  PromiseResolve,
   StringPrototypeStartsWith,
 } = primordials;
 
@@ -110,16 +111,62 @@ async function restart() {
   start();
 }
 
+function debounce(fn, delay) {
+  let state = 'idle';
+  let timer;
+  let promise = PromiseResolve();
+
+  const afterDelay = () => {
+    state = 'running';
+    promise = promise
+      .then(() => fn())
+      .catch((error) => {
+        triggerUncaughtException(error, true /* fromPromise */);
+      })
+      .finally(() => {
+        if (state === 'running') {
+          state = 'idle';
+        } else if (state === 'pending') {
+          state = 'debouncing';
+          timer = setTimeout(afterDelay, delay);
+        }
+      });
+  };
+
+  return debouncedFn;
+
+  function debouncedFn(destroy = false) {
+    if (destroy) {
+      clearTimeout(timer);
+      state = 'destroyed';
+    } else if (state === 'idle') {
+      state = 'debouncing';
+      timer = setTimeout(afterDelay, delay);
+    } else if (state === 'debouncing') {
+      clearTimeout(timer);
+      timer = setTimeout(afterDelay, delay);
+    } else if (state === 'running') {
+      state = 'pending';
+    }
+
+    return promise;
+  }
+}
+
 (async () => {
   emitExperimentalWarning('Watch mode');
 
   try {
     start();
 
+    const debouncedRestart = debounce(restart, 200);
+
     // eslint-disable-next-line no-unused-vars
     for await (const _ of on(watcher, 'changed')) {
-      await restart();
+      debouncedRestart();
     }
+
+    await debouncedRestart(true);
   } catch (error) {
     triggerUncaughtException(error, true /* fromPromise */);
   }