Speed up unit tests by reusing Vitest workers

packages/prettier-plugin-liquid/vitest.config.mjs

@@ -7,7 +7,7 @@ export default defineConfig({
     exclude: [...configDefaults.exclude],
     pool: 'forks',
     maxWorkers: 1,
-    isolate: true,
+    isolate: false,
     globalSetup: ['./src/test/test-setup.js'],
   },
 });

packages/theme-check-common/src/test/test-setup.ts

@@ -20,7 +20,21 @@ chaiAssertions.forEach(({ name, fn }) => {
  */
 expect.extend({ containOffense });
 
-process.on('unhandledRejection', (reason) => {
+/**
+ * Vitest runs setup files before every test file, and test files can share one
+ * process. Mark the process so each worker installs the logger only once.
+ */
+const unhandledRejectionLoggerInstalled = Symbol.for('theme-tools.unhandledRejectionLogger');
+installUnhandledRejectionLogger();
+
+function installUnhandledRejectionLogger() {
+  if (Reflect.get(process, unhandledRejectionLoggerInstalled)) return;
+
+  Reflect.set(process, unhandledRejectionLoggerInstalled, true);
+  process.on('unhandledRejection', logUnhandledRejection);
+}
+
+function logUnhandledRejection(reason: unknown) {
   console.error(reason);
   debugger;
-});
+}

vitest.config.mjs

@@ -6,16 +6,32 @@ const CI = !!process.env.CI;
 const alwaysExclude = ['**/browser/test/**', '**/test/browser/**'];
 /** In CI prettier plugin tests are covered by a different run command */
 const ciExclude = ['./packages/prettier-plugin-liquid'];
+/**
+ * Test files that need a fresh module graph. Most mock a dependency of a module
+ * that another test file already loaded. ObjectCompletionProvider depends on a
+ * module-level memo in TypeSystem.
+ */
+const isolatedTests = [
+  'packages/release-orchestrator/src/steps/getPackageJsonRecord.spec.ts',
+  'packages/theme-check-common/src/checks/asset-size-css/index.spec.ts',
+  'packages/theme-check-common/src/checks/asset-size-javascript/index.spec.ts',
+  'packages/theme-language-server-common/src/completions/providers/ContentForParameterCompletionProvider.spec.ts',
+  'packages/theme-language-server-common/src/completions/providers/ObjectCompletionProvider.spec.ts',
+];
 
 export default defineConfig({
   test: {
     exclude: [...configDefaults.exclude, ...alwaysExclude, ...(CI ? ciExclude : [])],
     pool: 'forks',
     maxWorkers: 1,
-    isolate: true,
+    isolate: false,
     setupFiles: [
       './packages/theme-check-common/src/test/test-setup.ts',
       './packages/theme-language-server-common/src/test/test-setup.ts',
     ],
+    projects: [
+      { extends: true, test: { name: 'shared', exclude: isolatedTests } },
+      { extends: true, test: { name: 'isolated', include: isolatedTests, isolate: true } },
+    ],
   },
 });