fix: only check for watchman existence once per process (#14826)

Author: SimenB

CHANGELOG.md

@@ -50,6 +50,7 @@
 ### Performance
 
 - `[*]` [**BREAKING**] Bundle all of Jest's modules into `index.js` ([#12348](https://github.com/jestjs/jest/pull/12348), [#14550](https://github.com/jestjs/jest/pull/14550) & [#14661](https://github.com/jestjs/jest/pull/14661))
+- `[*]` [jest-haste-map] Only spawn one process to check for `watchman` installation ([#14826](https://github.com/jestjs/jest/pull/14826))
 
 ### Chore & Maintenance
 

packages/jest-core/src/getChangedFilesPromise.ts

@@ -18,16 +18,10 @@ export default function getChangedFilesPromise(
   configs: Array<Config.ProjectConfig>,
 ): ChangedFilesPromise | undefined {
   if (globalConfig.onlyChanged) {
-    const allRootsForAllProjects = configs.reduce<Array<string>>(
-      (roots, config) => {
-        if (config.roots) {
-          roots.push(...config.roots);
-        }
-        return roots;
-      },
-      [],
+    const allRootsForAllProjects = new Set(
+      configs.flatMap(config => config.roots || []),
     );
-    return getChangedFilesForRoots(allRootsForAllProjects, {
+    return getChangedFilesForRoots([...allRootsForAllProjects], {
       changedSince: globalConfig.changedSince,
       lastCommit: globalConfig.lastCommit,
       withAncestor: globalConfig.changedFilesWithAncestor,

packages/jest-haste-map/src/index.ts

@@ -113,6 +113,8 @@ type Watcher = {
 
 type HasteWorker = typeof import('./worker');
 
+let isWatchmanInstalledPromise: Promise<boolean> | undefined;
+
 export const ModuleMap = HasteModuleMap as {
   create: (rootPath: string) => IModuleMap;
 };
@@ -216,7 +218,6 @@ class HasteMap extends EventEmitter implements IHasteMap {
   private _cachePath = '';
   private _changeInterval?: ReturnType<typeof setInterval>;
   private readonly _console: Console;
-  private _isWatchmanInstalledPromise: Promise<boolean> | null = null;
   private readonly _options: InternalOptions;
   private _watchers: Array<Watcher> = [];
   private _worker: JestWorkerFarm<HasteWorker> | HasteWorker | null = null;
@@ -1108,10 +1109,10 @@ class HasteMap extends EventEmitter implements IHasteMap {
     if (!this._options.useWatchman) {
       return false;
     }
-    if (!this._isWatchmanInstalledPromise) {
-      this._isWatchmanInstalledPromise = isWatchmanInstalled();
+    if (!isWatchmanInstalledPromise) {
+      isWatchmanInstalledPromise = isWatchmanInstalled();
     }
-    return this._isWatchmanInstalledPromise;
+    return isWatchmanInstalledPromise;
   }
 
   private _createEmptyMap(): InternalHasteMap {