Configurable watch root

packages/core/core/src/Parcel.js

@@ -389,7 +389,7 @@ export default class Parcel {
     let resolvedOptions = nullthrows(this.#resolvedOptions);
     let opts = getWatcherOptions(resolvedOptions);
     let sub = await resolvedOptions.inputFS.watch(
-      resolvedOptions.projectRoot,
+      resolvedOptions.watchDir,
       (err, events) => {
         if (err) {
           this.#watchEvents.emit({error: err});

packages/core/core/src/RequestTracker.js

@@ -1217,7 +1217,7 @@ async function loadRequestGraph(options): Async<RequestGraph> {
     let snapshotKey = hashString(`${cacheKey}:snapshot`);
     let snapshotPath = path.join(options.cacheDir, snapshotKey + '.txt');
     let events = await options.inputFS.getEventsSince(
-      options.projectRoot,
+      options.watchDir,
       snapshotPath,
       opts,
     );

packages/core/core/src/resolveOptions.js

@@ -97,6 +97,14 @@ export default async function resolveOptions(
       ? path.resolve(outputCwd, initialOptions.cacheDir)
       : path.resolve(projectRoot, DEFAULT_CACHE_DIRNAME);
 
+  // Make the root watch directory configurable. This is useful in some cases
+  // where symlinked dependencies outside the project root need to trigger HMR
+  // updates. Default to the project root if not provided.
+  let watchDir =
+    initialOptions.watchDir != null
+      ? path.resolve(initialOptions.watchDir)
+      : projectRoot;
+
   let cache =
     initialOptions.cache ??
     (outputFS instanceof NodeFS
@@ -180,6 +188,7 @@ export default async function resolveOptions(
     shouldProfile: initialOptions.shouldProfile ?? false,
     shouldTrace: initialOptions.shouldTrace ?? false,
     cacheDir,
+    watchDir,
     entries: entries.map(e => toProjectPath(projectRoot, e)),
     targets: initialOptions.targets,
     logLevel: initialOptions.logLevel ?? 'info',

packages/core/core/src/types.js

@@ -266,6 +266,7 @@ export type ParcelOptions = {|
 
   shouldDisableCache: boolean,
   cacheDir: FilePath,
+  watchDir: FilePath,
   mode: BuildMode,
   hmrOptions: ?HMROptions,
   shouldContentHash: boolean,

packages/core/core/test/test-utils.js

@@ -17,6 +17,7 @@ cache.ensure();
 
 export const DEFAULT_OPTIONS: ParcelOptions = {
   cacheDir: path.join(__dirname, '.parcel-cache'),
+  watchDir: __dirname,
   entries: [],
   logLevel: 'info',
   targets: undefined,

packages/core/integration-tests/test/monorepos.js

@@ -6,6 +6,7 @@ import {
   assertBundles,
   inputFS,
   outputFS,
+  fsFixture,
   ncp,
   run,
   overlayFS,
@@ -875,4 +876,71 @@ describe('monorepos', function () {
       inputFS.chdir(oldcwd);
     }
   });
+
+  // This test ensures that workspace linked dependency changes are correctly
+  // detected in watch mode when `watchDir` is set to the monorepo root.
+  it('should correctly detect changes when watchDir is higher up in a project-only lockfile monorepo', async () => {
+    const dir = path.join(__dirname, 'project-specific-lockfiles');
+    overlayFS.mkdirp(dir);
+
+    await fsFixture(overlayFS, dir)`
+      packages
+        app
+          package.json:
+            { "name": "app" }
+          pnpm-lock.yaml:
+            lockfileVersion: 5.4
+          index.js:
+            import {msg} from 'lib';
+            console.log(msg);
+          node_modules
+            lib -> ${path.join(
+              __dirname,
+              'project-specific-lockfiles',
+              'packages',
+              'lib',
+            )}
+        lib
+          package.json:
+            { "name": "lib" }
+          pnpm-lock.yaml:
+            lockfileVersion: 5.4
+          index.js:
+            export const msg = "initial";`;
+
+    let b = await bundler(path.join(dir, 'packages', 'app', 'index.js'), {
+      inputFS: overlayFS,
+      watchDir: path.join(dir),
+    });
+
+    let builds = 0;
+
+    return new Promise((resolve, reject) => {
+      // 1. Increment the build counter and modify `packages/lib/index.js` which
+      //    should trigger a subsquent build.
+      //
+      // 2. Ensure the changed asset was detected and built
+      b.watch(async (err, buildEvent) => {
+        builds++;
+
+        if (builds < 2) {
+          await overlayFS.writeFile(
+            path.join(dir, 'packages', 'lib', 'index.js'),
+            'export const msg = "changed-NcMB9nA7"',
+          );
+        } else {
+          const values = buildEvent?.changedAssets?.values();
+          if (values != null) {
+            const code = await Array.from(values)[0].getCode();
+            assert(code.includes('changed-NcMB9nA7'));
+            resolve();
+          } else {
+            reject(new Error('Changed assets missing.'));
+          }
+        }
+      }).then(sub => {
+        subscription = sub;
+      });
+    });
+  });
 });

packages/core/integration-tests/test/project-specific-lockfiles/packages/app/README.md

@@ -0,0 +1,3 @@
+This directory exists for the `should correctly detect changes when watchDir is higher up in a project-only lockfile monorepo` test.
+
+Without it I was getting a `Uncaught Error: No such file or directory` error stemming from [this line](https://github.com/parcel-bundler/parcel/blob/3b798e0456bbef951c684d43f96fda1fea386f62/packages/core/fs/src/OverlayFS.js#L375). The test itself doesn't care about watching the readable file system but it was necessary because `.watch` on `OverlayFS` subscribes to both the readable and writable filesystems.

packages/core/parcel/src/cli.js

@@ -73,6 +73,8 @@ const commonOptions = {
   '--config <path>':
     'specify which config to use. can be a path or a package name',
   '--cache-dir <path>': 'set the cache directory. defaults to ".parcel-cache"',
+  '--watch-dir <path>':
+    'set the root watch directory. defaults to nearest lockfile or source control dir.',
   '--no-source-maps': 'disable sourcemaps',
   '--target [name]': [
     'only build given target(s)',
@@ -473,6 +475,7 @@ async function normalizeOptions(
   return {
     shouldDisableCache: command.cache === false,
     cacheDir: command.cacheDir,
+    watchDir: command.watchDir,
     config: command.config,
     mode,
     hmrOptions,

packages/core/types/index.js

@@ -294,6 +294,7 @@ export type InitialParcelOptions = {|
 
   +shouldDisableCache?: boolean,
   +cacheDir?: FilePath,
+  +watchDir?: FilePath,
   +mode?: BuildMode,
   +hmrOptions?: ?HMROptions,
   +shouldContentHash?: boolean,