Implements a getFeatureFlag to avoid option drilling (#9618)

* Implement a simple feature-flag client to avoid option drilling

* Ensure feature flag client is initialised in workers

* Rename featureFlags -> featureFlagValues

* Avoid depending on ephemeral flag in test

packages/core/core/src/Parcel.js

@@ -53,6 +53,7 @@ import {
   fromProjectPathRelative,
 } from './projectPath';
 import {tracer} from '@parcel/profiler';
+import {setFeatureFlags} from '@parcel/feature-flags';
 
 registerCoreWithSerializer();
 
@@ -108,6 +109,8 @@ export default class Parcel {
     let {config} = await loadParcelConfig(resolvedOptions);
     this.#config = new ParcelConfig(config, resolvedOptions);
 
+    setFeatureFlags(resolvedOptions.featureFlags);
+
     if (this.#initialOptions.workerFarm) {
       if (this.#initialOptions.workerFarm.ending) {
         throw new Error('Supplied WorkerFarm is ending');

packages/core/core/src/worker.js

@@ -26,6 +26,7 @@ import {clearBuildCaches} from './buildCache';
 import {init as initSourcemaps} from '@parcel/source-map';
 import {init as initRust} from '@parcel/rust';
 import WorkerFarm from '@parcel/workers';
+import {setFeatureFlags} from '@parcel/feature-flags';
 
 import '@parcel/cache'; // register with serializer
 import '@parcel/package-manager';
@@ -78,6 +79,9 @@ async function loadConfig(cachePath, options) {
   );
   config = new ParcelConfig(processedConfig, options);
   parcelConfigCache.set(cachePath, config);
+
+  setFeatureFlags(options.featureFlags);
+
   return config;
 }
 

packages/core/feature-flags/src/index.js

@@ -9,3 +9,13 @@ export const DEFAULT_FEATURE_FLAGS: FeatureFlags = {
   exampleFeature: false,
   configKeyInvalidation: false,
 };
+
+let featureFlagValues: FeatureFlags = {...DEFAULT_FEATURE_FLAGS};
+
+export function setFeatureFlags(flags: FeatureFlags) {
+  featureFlagValues = flags;
+}
+
+export function getFeatureFlag(flagName: $Keys<FeatureFlags>): boolean {
+  return featureFlagValues[flagName];
+}

packages/core/feature-flags/test/feature-flags.test.js

@@ -0,0 +1,21 @@
+// @flow strict
+import assert from 'assert';
+import {getFeatureFlag, DEFAULT_FEATURE_FLAGS, setFeatureFlags} from '../src';
+
+describe('feature-flag test', () => {
+  beforeEach(() => {
+    setFeatureFlags(DEFAULT_FEATURE_FLAGS);
+  });
+
+  it('has defaults', () => {
+    assert.equal(
+      getFeatureFlag('exampleFeature'),
+      DEFAULT_FEATURE_FLAGS.exampleFeature,
+    );
+  });
+
+  it('can override', () => {
+    setFeatureFlags({...DEFAULT_FEATURE_FLAGS, exampleFeature: true});
+    assert.equal(getFeatureFlag('exampleFeature'), true);
+  });
+});

packages/core/integration-tests/test/feature-flags.js

@@ -28,7 +28,15 @@ describe('feature flags', () => {
                 '*.js': ['./transformer.js', '...']
             },
         }
-    
+
+    .parcelrc-2:
+      {
+          extends: "@parcel/config-default",
+          transformers: {
+              '*.js': ['./transformer-client.js', '...']
+          },
+      }        
+
     transformer.js:
         const {Transformer} = require('@parcel/plugin');
         module.exports = new Transformer({
@@ -40,6 +48,19 @@ describe('feature flags', () => {
                 return [asset];
             }
         });
+
+    transformer-client.js:
+        const {Transformer} = require('@parcel/plugin');
+        const {getFeatureFlag} = require('@parcel/feature-flags');
+        module.exports = new Transformer({
+            async transform({asset, options}) {
+                const code = await asset.getCode();
+                if (code.includes('MARKER') && getFeatureFlag('exampleFeature')) {
+                    asset.setCode(code.replace('MARKER', 'REPLACED'));
+                }
+                return [asset];
+            }
+        });
 `;
   });
 
@@ -98,4 +119,20 @@ describe('feature flags', () => {
       `Expected ${output} to NOT contain 'REPLACED'`,
     );
   });
+
+  it('flag should be available in plugins via client', async () => {
+    await overlayFS.mkdirp(dir);
+
+    const b = await bundle(path.join(dir, 'index.js'), {
+      inputFS: overlayFS,
+      featureFlags: {exampleFeature: true},
+      config: path.join(dir, '.parcelrc-2'),
+    });
+    const output = await run(b);
+
+    assert(
+      output.includes('REPLACED'),
+      `Expected ${output} to contain 'REPLACED'`,
+    );
+  });
 });