test: Split the test suite into isolation groups for faster runs (#2613)

package.json

@@ -188,7 +188,7 @@
     "format-check": "prettier --check .",
     "rollup": "rollup -c",
     "prune": "knip",
-    "test": "vitest run",
+    "test": "VITEST_GROUP=shared vitest run && VITEST_GROUP=isolated vitest run",
     "coverage": "vitest run --coverage"
   },
   "volta": {

vite.config.ts

@@ -1,3 +1,6 @@
+import { readdirSync, readFileSync } from 'fs';
+import { join } from 'path';
+
 import { defineConfig } from 'vitest/config';
 
 import { svgPath } from './scripts/svg-path-plugin.js';
@@ -19,7 +22,48 @@ const EXCLUSIONS = [
   'src/patches/**/*.ts',
 ];
 
-const INCLUSIONS = ['tests/**/*.test.ts'];
+const TEST_DIRECTORY = 'tests';
+const INCLUSIONS = [`${TEST_DIRECTORY}/**/*.test.ts`];
+
+// For test performance reasons, tests are split into two groups:
+//
+// - `shared`: run non-isolated, so they share one loaded copy of the source
+//   tree instead of each re-importing it.
+// - `isolated`: given their own module registry per file, because sharing one
+//   would change their behaviour. A file calling `vi.mock()` cannot replace a
+//   module an earlier file already loaded unmocked; a file loading the template
+//   renderer reads browser globals as the renderer loads; and a file needing a
+//   DOM leaves modules holding a `window` that is torn down when it finishes,
+//   which breaks any later file that reaches one of those modules.
+//
+// Absent this variable every test file runs isolated, which is both the safe
+// default and what coverage requires (istanbul only counts a module's top-level
+// code the first time it runs).
+const GROUP = process.env.VITEST_GROUP;
+
+const findTestFiles = (directory: string): string[] => {
+  const files: string[] = [];
+  for (const entry of readdirSync(directory, { withFileTypes: true })) {
+    const path = join(directory, entry.name);
+    if (entry.isDirectory()) {
+      files.push(...findTestFiles(path));
+    } else if (entry.name.endsWith('.test.ts')) {
+      files.push(path);
+    }
+  }
+  return files;
+};
+
+const REQUIRE_ISOLATION_REGEXP =
+  /\bvi\.(do)?mock\(|loadRenderer|stubConnectedHomeAssistant|@vitest-environment\s+jsdom/;
+
+const getGroup = (file: string): string =>
+  REQUIRE_ISOLATION_REGEXP.test(readFileSync(file, 'utf-8')) ? 'isolated' : 'shared';
+
+const getInclusions = (): string[] =>
+  GROUP
+    ? findTestFiles(TEST_DIRECTORY).filter((file) => getGroup(file) === GROUP)
+    : INCLUSIONS;
 
 export default defineConfig({
   plugins: [svgPath()],
@@ -31,7 +75,13 @@ export default defineConfig({
         inline: ['ha-nunjucks', 'ts-py-datetime'],
       },
     },
-    include: INCLUSIONS,
+    include: getInclusions(),
+
+    // Forked child processes start and tear down faster here than worker
+    // threads, which matters when every test file needs a fresh one.
+    pool: 'forks',
+
+    isolate: !GROUP || GROUP === 'isolated',
 
     // Hide console writing to keep output clean, usual sources of noise:
     // - Unnecessary Lit dev-mode warnings.