[be][snap] Warm up workers when in watch + filter mode

compiler/packages/snap/src/runner-watch.ts

@@ -114,7 +114,6 @@ function subscribeFixtures(
   onChange: (state: RunnerState) => void
 ) {
   // Watch the fixtures directory for changes
-  /* const fileSubscription = */
   watcher.subscribe(FIXTURES_PATH, async (err, _events) => {
     if (err) {
       console.error(err);
@@ -127,6 +126,7 @@ function subscribeFixtures(
     const isRealUpdate = performance.now() - state.lastUpdate > 5000;
     if (isRealUpdate) {
       // Fixtures changed, re-run tests
+      state.mode.action = RunnerAction.Test;
       onChange(state);
     }
   });
@@ -136,23 +136,20 @@ function subscribeFilterFile(
   state: RunnerState,
   onChange: (state: RunnerState) => void
 ) {
-  const filterSubscription = watcher.subscribe(
-    process.cwd(),
-    async (err, events) => {
-      if (err) {
-        console.error(err);
-        process.exit(1);
-      } else if (
-        events.findIndex((event) => event.path.includes(FILTER_FILENAME)) !== -1
-      ) {
-        state.filter = await readTestFilter();
-        if (state.mode.filter) {
-          state.mode.action = RunnerAction.Test;
-          onChange(state);
-        }
+  watcher.subscribe(process.cwd(), async (err, events) => {
+    if (err) {
+      console.error(err);
+      process.exit(1);
+    } else if (
+      events.findIndex((event) => event.path.includes(FILTER_FILENAME)) !== -1
+    ) {
+      state.filter = await readTestFilter();
+      if (state.mode.filter) {
+        state.mode.action = RunnerAction.Test;
+        onChange(state);
       }
     }
-  );
+  });
 }
 
 function subscribeTsc(
@@ -172,11 +169,7 @@ function subscribeTsc(
         state.compilerVersion++;
       }
       state.isCompilerBuildValid = isSuccess;
-      if (state.filter) {
-        state.mode.action = RunnerAction.Test;
-      } else {
-        state.mode.action = RunnerAction.Test;
-      }
+      state.mode.action = RunnerAction.Test;
       onChange(state);
     }
   );
@@ -194,6 +187,7 @@ function subscribeKeyEvents(
       process.exit(0);
     } else if (key.name === "f") {
       state.mode.filter = !state.mode.filter;
+      state.mode.action = RunnerAction.Test;
     } else {
       // any other key re-runs tests
       state.mode.action = RunnerAction.Test;

compiler/packages/snap/src/runner.ts

@@ -6,6 +6,7 @@
  */
 
 import { Worker } from "jest-worker";
+import { cpus } from "os";
 import process from "process";
 import * as readline from "readline";
 import ts from "typescript";
@@ -23,6 +24,7 @@ import {
 import * as runnerWorker from "./runner-worker";
 
 const WORKER_PATH = require.resolve("./runner-worker.js");
+const NUM_WORKERS = cpus().length - 1;
 
 readline.emitKeypressEvents(process.stdin);
 
@@ -163,12 +165,40 @@ async function onChange(
 export async function main(opts: RunnerOptions): Promise<void> {
   const worker: Worker & typeof runnerWorker = new Worker(WORKER_PATH, {
     enableWorkerThreads: opts.workerThreads,
+    numWorkers: NUM_WORKERS,
   }) as any;
   worker.getStderr().pipe(process.stderr);
   worker.getStdout().pipe(process.stdout);
 
   if (opts.watch) {
     makeWatchRunner((state) => onChange(worker, state), opts.filter);
+    if (opts.filter) {
+      /**
+       * Warm up wormers when in watch mode. Loading the Forget babel plugin
+       * and all of its transitive dependencies takes 1-3s (per worker) on a M1.
+       * As jest-worker dispatches tasks using a round-robin strategy, we can
+       * avoid an additional 1-3s wait on the first num_workers runs by warming
+       * up workers eagerly.
+       */
+      for (let i = 0; i < NUM_WORKERS - 1; i++) {
+        worker.transformFixture(
+          {
+            fixturePath: "tmp",
+            snapshotPath: "./tmp.expect.md",
+            inputPath: "./tmp.js",
+            input: `
+            function Foo(props) {
+              return identity(props);
+            }
+            `,
+            snapshot: null,
+          },
+          0,
+          false,
+          false
+        );
+      }
+    }
   } else {
     // Non-watch mode. For simplicity we re-use the same watchSrc() function.
     // After the first build completes run tests and exit