fix(codeUtil): keep a user-supplied EXTENSION_DEV_PATH when coverage is off

docs-site/src/content/docs/guides/test-setup.md

@@ -23,7 +23,11 @@ The ExTester offers both CLI and API to perform all the setup actions. That way
 - `HTTP_PROXY` - route http requests through a proxy when downloading VS Code and ChromeDriver (see also `HTTPS_PROXY`).
 - `HTTPS_PROXY` / `NO_PROXY` - proxy for the https downloads of VS Code and ChromeDriver, and the usual no-proxy exception list.
 - `EXTENSIONS_FOLDER` - configuring the [extension path](https://code.visualstudio.com/docs/configure/extensions/extension-marketplace#_where-are-extensions-installed) where extensions are installed/loaded.
-- `EXTENSION_DEV_PATH` - The [developer extension](https://vscode-docs.readthedocs.io/en/stable/extensions/debugging-extensions/) that is loaded under development.
+- `EXTENSION_DEV_PATH` - The [developer extension](https://vscode-docs.readthedocs.io/en/stable/extensions/debugging-extensions/) that is loaded under development. ExTester passes it to VS Code as `--extensionDevelopmentPath`, so the extension is loaded from that folder instead of from an installed `.vsix`. Every test run honours it; `extest setup-and-run --coverage` overrides it with the current working directory, because that is the extension being instrumented.
+
+  ```shell
+  export EXTENSION_DEV_PATH="$(pwd)"
+  ```
 - `HTTPS_TLS_REJECT_UNAUTHORIZED` - Disable TLS check when downloading VSCode and Chromium driver. '0' is disabled and '1' is enabled, this setting aligns with [`NODE_TLS_REJECT_UNAUTHORIZED`](https://nodejs.org/api/cli.html#node_tls_reject_unauthorizedvalue).
 
   ```shell

packages/extester/src/test/codeUtil.test.ts

@@ -32,10 +32,43 @@ import {
 	flagConflictWarnings,
 	findShadowedSettings,
 	resolveMacExecutable,
+	applyExtensionDevPath,
 } from '../util/codeUtil';
 import { ReleaseQuality } from '../util/codeUtil';
 import { Download } from '../util/download';
 
+describe('applyExtensionDevPath', () => {
+	it('keeps a user-supplied EXTENSION_DEV_PATH when coverage is off', () => {
+		const env: NodeJS.ProcessEnv = { EXTENSION_DEV_PATH: '/home/me/my-extension' };
+		applyExtensionDevPath(false, '/tmp/instrumented', env);
+		assert.strictEqual(env.EXTENSION_DEV_PATH, '/home/me/my-extension');
+	});
+
+	it('leaves EXTENSION_DEV_PATH absent when neither coverage nor the user set it', () => {
+		const env: NodeJS.ProcessEnv = {};
+		applyExtensionDevPath(undefined, '/tmp/instrumented', env);
+		assert.ok(!('EXTENSION_DEV_PATH' in env));
+	});
+
+	it('overrides a user-supplied EXTENSION_DEV_PATH with the working directory when coverage is on', () => {
+		const env: NodeJS.ProcessEnv = { EXTENSION_DEV_PATH: '/home/me/my-extension' };
+		applyExtensionDevPath(true, '/tmp/instrumented', env);
+		assert.strictEqual(env.EXTENSION_DEV_PATH, '/tmp/instrumented');
+	});
+
+	it('sets EXTENSION_DEV_PATH from the working directory when coverage is on and the user set nothing', () => {
+		const env: NodeJS.ProcessEnv = {};
+		applyExtensionDevPath(true, '/tmp/instrumented', env);
+		assert.strictEqual(env.EXTENSION_DEV_PATH, '/tmp/instrumented');
+	});
+
+	it('mutates only EXTENSION_DEV_PATH', () => {
+		const env: NodeJS.ProcessEnv = { EXTENSION_DEV_PATH: '/home/me/my-extension', TEST_RESOURCES: '/tmp/test-resources' };
+		applyExtensionDevPath(false, '/tmp/instrumented', env);
+		assert.deepStrictEqual(env, { EXTENSION_DEV_PATH: '/home/me/my-extension', TEST_RESOURCES: '/tmp/test-resources' });
+	});
+});
+
 describe('CodeUtil.packageExtension', () => {
 	let originalCreateVSIX: typeof vsce.createVSIX;
 	let capturedOptions: IPackageOptions | undefined;

packages/extester/src/util/codeUtil.ts

@@ -314,6 +314,29 @@ export function validateUserDataDirLength(settingsDir: string, platform: NodeJS.
 	return undefined;
 }
 
+/**
+ * Applies the `EXTENSION_DEV_PATH` environment variable for a test run.
+ *
+ * `EXTENSION_DEV_PATH` is a documented, user-facing variable that makes ExTester
+ * launch VS Code with `--extensionDevelopmentPath`, which loads an extension straight
+ * from a folder instead of from an installed `.vsix`.
+ *
+ * When coverage is enabled the instrumented extension is whatever lives in the current
+ * working directory, so that value has to win. In every other case the variable is left
+ * exactly as the caller exported it: deleting it here silently dropped
+ * `--extensionDevelopmentPath` from the launch arguments, so a user-supplied path had no
+ * effect on the run.
+ *
+ * @param coverage whether the run instruments the extension in the current working directory
+ * @param cwd the directory of the extension being instrumented
+ * @param env the environment to mutate, defaults to `process.env`
+ */
+export function applyExtensionDevPath(coverage: boolean | undefined, cwd: string, env: NodeJS.ProcessEnv = process.env): void {
+	if (coverage) {
+		env.EXTENSION_DEV_PATH = cwd;
+	}
+}
+
 /** defaults for the [[RunOptions]] */
 export const DEFAULT_RUN_OPTIONS = {
 	vscodeVersion: 'latest',
@@ -694,11 +717,7 @@ export class CodeUtil {
 		} else {
 			delete process.env.EXTENSIONS_FOLDER;
 		}
-		if (this.coverage) {
-			process.env.EXTENSION_DEV_PATH = process.cwd();
-		} else {
-			delete process.env.EXTENSION_DEV_PATH;
-		}
+		applyExtensionDevPath(this.coverage, process.cwd());
 		const runner = new VSRunner(
 			this.getExecutablePath(),
 			literalVersion,