Fast Pylance startup: don't block activation on environment enumeration (#26019)

## Related issue
Part of #26020

## Summary
Make Python interpreter resolution non-blocking so Pylance can start
immediately, even while the environment extension is doing a full
refresh on first run.

Previously, activation awaited interpreter auto-selection and
`getActiveInterpreter()` could block for the entire
environment-enumeration window (tens of seconds on large/conda setups),
which delayed the language server from starting.

## Changes
- **`interpreter/interpreterService.ts`** – `getActiveInterpreter()` now
races resolution against a ~100ms timeout. If discovery is slow it
returns the last-known/persisted interpreter immediately; the real
resolution still completes in the background and notifies listeners, so
the value self-corrects after a refresh. In-flight resolutions are
de-duplicated, and completion is reported via `onDidChange` so the
eventual interpreter propagates to Pylance without a server restart.
- **`activation/activationManager.ts`** – No longer blocks activation on
auto-select; it races a short timeout. Auto-selection still runs exactly
once and completes in the background.
- **`pythonEnvironments/.../conda.ts` + `pythonEnvironments/index.ts`**
– Add `Conda.setSkipDeepProbe(true)` when the environments extension
owns discovery, so startup no longer triggers slow sequential registry /
`conda info --json` probes (the env extension / PET already discovers
conda). `python.condaPath` remains an escape hatch.
- **`envExt/api.legacy.ts`** – De-duplicate in-flight active-interpreter
resolutions and keep reporting changes via
`reportActiveInterpreterChanged`.
- **`testing/testController/...`** – Defer project discovery and
self-heal on environment change instead of throwing `No Python
environment found` when an env hasn't been assigned yet during fast
startup.

## Tests
Updated/added unit tests for `activationManager`, `interpreterService`,
`conda`, the legacy env API, and the test controller.

## Notes
This is part of a cross-repo effort to speed up Pylance startup
(companion PRs in `vscode-python-environments` and the Pylance server).

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rchiodo

src/client/activation/activationManager.ts

@@ -10,13 +10,17 @@ import { IActiveResourceService, IDocumentManager, IWorkspaceService } from '../
 import { PYTHON_LANGUAGE } from '../common/constants';
 import { IFileSystem } from '../common/platform/types';
 import { IDisposable, IInterpreterPathService, Resource } from '../common/types';
-import { Deferred } from '../common/utils/async';
+import { Deferred, sleep } from '../common/utils/async';
 import { StopWatch } from '../common/utils/stopWatch';
 import { IInterpreterAutoSelectionService } from '../interpreter/autoSelection/types';
-import { traceDecoratorError } from '../logging';
+import { traceDecoratorError, traceError } from '../logging';
 import { sendActivationTelemetry } from '../telemetry/envFileTelemetry';
 import { IExtensionActivationManager, IExtensionActivationService, IExtensionSingleActivationService } from './types';
 
+// Upper bound on how long workspace activation waits for interpreter auto-selection before
+// proceeding. Auto-selection still completes in the background after this point.
+const AUTO_SELECT_INTERPRETER_TIMEOUT_MS = 100;
+
 @injectable()
 export class ExtensionActivationManager implements IExtensionActivationManager {
     public readonly activatedWorkspaces = new Set<string>();
@@ -94,7 +98,15 @@ export class ExtensionActivationManager implements IExtensionActivationManager {
 
         if (this.workspaceService.isTrusted) {
             // Do not interact with interpreters in a untrusted workspace.
-            await this.autoSelection.autoSelectInterpreter(resource);
+            // Don't block activation (and therefore language server startup) on auto-selection.
+            // On a cold start it can wait for a full environment refresh to complete, which would
+            // delay starting the language server. Let it finish in the background; the selected
+            // interpreter is reported to listeners (e.g. Pylance) via the environments API once it
+            // is ready. We still wait briefly so the common warm-start case is unchanged.
+            const autoSelection = this.autoSelection
+                .autoSelectInterpreter(resource)
+                .catch((ex) => traceError('Auto-selection of interpreter failed', ex));
+            await Promise.race([autoSelection, sleep(AUTO_SELECT_INTERPRETER_TIMEOUT_MS)]);
             await this.interpreterPathService.copyOldInterpreterStorageValuesToNew(resource);
         }
         await sendActivationTelemetry(this.fileSystem, this.workspaceService, resource);

src/client/envExt/api.legacy.ts

@@ -99,7 +99,9 @@ function toLegacyType(env: PythonEnvironment): PythonEnvironmentLegacy {
 }
 
 const previousEnvMap = new Map<string, PythonEnvironment | undefined>();
-export async function getActiveInterpreterLegacy(resource?: Uri): Promise<PythonEnvironmentLegacy | undefined> {
+const inFlightActiveInterpreter = new Map<string, Promise<PythonEnvironmentLegacy | undefined>>();
+
+async function resolveActiveInterpreterLegacy(resource?: Uri): Promise<PythonEnvironmentLegacy | undefined> {
     const api = await getEnvExtApi();
     const uri = resource ? api.getPythonProject(resource)?.uri : undefined;
 
@@ -117,7 +119,23 @@ export async function getActiveInterpreterLegacy(resource?: Uri): Promise<Python
         });
         previousEnvMap.set(uri?.fsPath || '', pythonEnv);
     }
-    return pythonEnv ? toLegacyType(pythonEnv) : undefined;
+    return newEnv;
+}
+
+export async function getActiveInterpreterLegacy(resource?: Uri): Promise<PythonEnvironmentLegacy | undefined> {
+    // De-duplicate concurrent resolutions for the same resource. The underlying
+    // `getEnvironment` call can block while the environments extension is performing a
+    // refresh, so multiple startup callers (e.g. the language server watcher and the
+    // configuration middleware) would otherwise each spawn their own blocking request.
+    const key = resource?.fsPath ?? '';
+    let inFlight = inFlightActiveInterpreter.get(key);
+    if (!inFlight) {
+        inFlight = resolveActiveInterpreterLegacy(resource).finally(() => {
+            inFlightActiveInterpreter.delete(key);
+        });
+        inFlightActiveInterpreter.set(key, inFlight);
+    }
+    return inFlight;
 }
 
 export async function setInterpreterLegacy(pythonPath: string, uri: Uri | undefined): Promise<void> {

src/client/interpreter/interpreterService.ts

@@ -18,6 +18,8 @@ import {
     IDisposableRegistry,
     IInstaller,
     IInterpreterPathService,
+    IPersistentState,
+    IPersistentStateFactory,
     Product,
 } from '../common/types';
 import { IServiceContainer } from '../ioc/types';
@@ -49,6 +51,15 @@ import { getActiveInterpreterLegacy } from '../envExt/api.legacy';
 
 type StoredPythonEnvironment = PythonEnvironment & { store?: boolean };
 
+// Upper bound on how long `getActiveInterpreter` may block its caller. Resolving the active
+// interpreter can wait on a full environment refresh (especially with the environments
+// extension enabled), which would otherwise delay language server startup.
+const GET_ACTIVE_INTERPRETER_TIMEOUT_MS = 100;
+const ACTIVE_INTERPRETER_TIMED_OUT = Symbol('activeInterpreterTimedOut');
+
+// Key prefix for the persisted (cross-restart) last-known active interpreter, one entry per workspace.
+const LAST_KNOWN_ACTIVE_INTERPRETER_KEY_PREFIX = 'lastKnownActiveInterpreter:';
+
 @injectable()
 export class InterpreterService implements Disposable, IInterpreterService {
     public async hasInterpreters(
@@ -100,6 +111,23 @@ export class InterpreterService implements Disposable, IInterpreterService {
         { path: string; workspaceFolder: WorkspaceFolder | undefined }
     >();
 
+    // Last successfully resolved active interpreter per workspace, served when a resolution
+    // doesn't complete within `GET_ACTIVE_INTERPRETER_TIMEOUT_MS`.
+    private readonly lastKnownActiveInterpreter = new Map<
+        string,
+        { resource: Uri | undefined; env: StoredPythonEnvironment | undefined }
+    >();
+
+    // In-flight active interpreter resolutions, de-duplicated per workspace.
+    private readonly inFlightActiveInterpreter = new Map<string, Promise<StoredPythonEnvironment | undefined>>();
+
+    // Persisted (cross-restart) last-known active interpreter, one persistent-state handle per workspace.
+    // Lets a warm start serve a real interpreter within the timeout budget before discovery completes.
+    private readonly persistedActiveInterpreter = new Map<
+        string,
+        IPersistentState<StoredPythonEnvironment | undefined>
+    >();
+
     constructor(
         @inject(IServiceContainer) private serviceContainer: IServiceContainer,
         @inject(IComponentAdapter) private readonly pyenvs: IComponentAdapter,
@@ -198,6 +226,13 @@ export class InterpreterService implements Disposable, IInterpreterService {
             }),
         );
         disposables.push(this.interpreterPathService.onDidChange((i) => this._onConfigChanged(i.uri)));
+        // Auto-selection completes in the background (it is no longer awaited before activation),
+        // which updates the python path setting once an interpreter is chosen. Re-run the config
+        // change handler so the newly selected interpreter is reported to listeners (e.g. Pylance).
+        // `_onConfigChanged` is gated on the python path actually changing, and
+        // `reportActiveInterpreterChanged` de-duplicates by path, so this is a no-op when nothing
+        // changed. The environments-extension path reports its own changes via the legacy API.
+        disposables.push(this.configService.onDidChange(() => this._onConfigChanged()));
     }
 
     public getInterpreters(resource?: Uri): PythonEnvironment[] {
@@ -219,6 +254,70 @@ export class InterpreterService implements Disposable, IInterpreterService {
     }
 
     public async getActiveInterpreter(resource?: Uri): Promise<PythonEnvironment | undefined> {
+        const workspaceService = this.serviceContainer.get<IWorkspaceService>(IWorkspaceService);
+        const key = workspaceService.getWorkspaceFolderIdentifier(resource);
+
+        // De-duplicate concurrent resolutions for the same workspace.
+        let resolution = this.inFlightActiveInterpreter.get(key);
+        if (!resolution) {
+            resolution = this.resolveActiveInterpreter(resource)
+                .then((env) => {
+                    this.lastKnownActiveInterpreter.set(key, { resource, env });
+                    if (env) {
+                        // Persist the resolved interpreter so a future window/session can serve a
+                        // real value within the timeout budget before discovery/refresh completes.
+                        this.getPersistedActiveInterpreterState(key)
+                            .updateValue(env)
+                            .catch((ex) => traceError('Failed to persist active interpreter', ex));
+                    }
+                    return env;
+                })
+                .catch((ex) => {
+                    traceError('Failed to get active interpreter', ex);
+                    return undefined;
+                })
+                .finally(() => {
+                    this.inFlightActiveInterpreter.delete(key);
+                });
+            this.inFlightActiveInterpreter.set(key, resolution);
+        }
+
+        // Don't block callers (notably language server startup) on a potentially slow
+        // environment discovery/refresh. If resolution doesn't complete promptly, return the
+        // last-known interpreter (or undefined on a cold start). The resolution promise is not
+        // abandoned, so listeners are still notified of the real value once it becomes available:
+        // the env-extension path reports via the environments API (see getActiveInterpreterLegacy),
+        // and the native path reports via `_onConfigChanged` once auto-selection completes.
+        const result = await Promise.race([
+            resolution,
+            sleep(GET_ACTIVE_INTERPRETER_TIMEOUT_MS).then(() => ACTIVE_INTERPRETER_TIMED_OUT),
+        ]);
+        if (result !== ACTIVE_INTERPRETER_TIMED_OUT) {
+            return result as StoredPythonEnvironment | undefined;
+        }
+        // Prefer the value resolved earlier this session; otherwise fall back to the persisted
+        // value from a previous session so warm restarts still serve a real interpreter quickly.
+        const cached = this.lastKnownActiveInterpreter.get(key);
+        if (cached) {
+            return cached.env;
+        }
+        return this.getPersistedActiveInterpreterState(key).value;
+    }
+
+    private getPersistedActiveInterpreterState(key: string): IPersistentState<StoredPythonEnvironment | undefined> {
+        let state = this.persistedActiveInterpreter.get(key);
+        if (!state) {
+            const factory = this.serviceContainer.get<IPersistentStateFactory>(IPersistentStateFactory);
+            state = factory.createWorkspacePersistentState<StoredPythonEnvironment | undefined>(
+                `${LAST_KNOWN_ACTIVE_INTERPRETER_KEY_PREFIX}${key}`,
+                undefined,
+            );
+            this.persistedActiveInterpreter.set(key, state);
+        }
+        return state;
+    }
+
+    private async resolveActiveInterpreter(resource?: Uri): Promise<StoredPythonEnvironment | undefined> {
         if (useEnvExtension()) {
             return getActiveInterpreterLegacy(resource);
         }

src/client/pythonEnvironments/common/environmentManagers/conda.ts

@@ -257,6 +257,15 @@ export class Conda {
      */
     private static condaPromise = new Map<string | undefined, Promise<Conda | undefined>>();
 
+    /**
+     * When another component (e.g. the Python Environments extension via pet) owns
+     * environment discovery, the legacy registry/known-path probing performed by
+     * locate() is redundant and was a significant startup cost (sequential
+     * `conda info --json` probes and reg.exe spawns). When set, locate() only honors
+     * the explicit `python.condaPath` setting and `conda` on PATH.
+     */
+    private static skipDeepProbe = false;
+
     private condaInfoCached = new Map<string | undefined, Promise<CondaInfo> | undefined>();
 
     /**
@@ -296,6 +305,18 @@ export class Conda {
         Conda.condaPromise.set(undefined, Promise.resolve(new Conda(condaPath)));
     }
 
+    /**
+     * Restrict {@link locate} to the explicit `python.condaPath` setting and `conda` on
+     * PATH, skipping the expensive registry/known-path filesystem probing. Used when
+     * environment discovery is delegated to another component (e.g. the Python
+     * Environments extension), which is the source of truth for locating conda.
+     */
+    public static setSkipDeepProbe(value: boolean): void {
+        Conda.skipDeepProbe = value;
+        // Drop any cached resolution so the new probing policy takes effect.
+        Conda.condaPromise = new Map<string | undefined, Promise<Conda | undefined>>();
+    }
+
     /**
      * Locates the preferred "conda" utility on this system by considering user settings,
      * binaries on PATH, Python interpreters in the registry, and known install locations.
@@ -314,13 +335,19 @@ export class Conda {
         const suffix = getOSType() === OSType.Windows ? 'Scripts\\conda.exe' : 'bin/conda';
 
         // Produce a list of candidate binaries to be probed by exec'ing them.
+        const skipDeepProbe = Conda.skipDeepProbe;
         async function* getCandidates() {
             if (customCondaPath && customCondaPath !== 'conda') {
                 // If user has specified a custom conda path, use it first.
                 yield customCondaPath;
             }
             // Check unqualified filename first, in case it's on PATH.
             yield 'conda';
+            if (skipDeepProbe) {
+                // Discovery is delegated to another component (e.g. the Python
+                // Environments extension); skip the costly registry/known-path probing.
+                return;
+            }
             if (getOSType() === OSType.Windows) {
                 yield* getCandidatesFromRegistry();
             }

src/client/pythonEnvironments/index.ts

@@ -45,6 +45,7 @@ import { getNativePythonFinder } from './base/locators/common/nativePythonFinder
 import { createNativeEnvironmentsApi } from './nativeAPI';
 import { useEnvExtension } from '../envExt/api.internal';
 import { createEnvExtApi } from '../envExt/envExtApi';
+import { Conda } from './common/environmentManagers/conda';
 
 const PYTHON_ENV_INFO_CACHE_KEY = 'PYTHON_ENV_INFO_CACHEv2';
 
@@ -61,6 +62,10 @@ export async function initialize(ext: ExtensionState): Promise<IDiscoveryAPI> {
     initializeLegacyExternalDependencies(ext.legacyIOC.serviceContainer);
 
     if (useEnvExtension()) {
+        // The Python Environments extension (via pet) owns environment discovery,
+        // including locating conda. Skip the legacy registry/known-path conda probing,
+        // which is redundant here and was a significant startup cost.
+        Conda.setSkipDeepProbe(true);
         const api = await createEnvExtApi(ext.disposables);
         registerNewDiscoveryForIOC(
             // These are what get wrapped in the legacy adapter.

src/client/testing/testController/common/testProjectRegistry.ts

@@ -152,7 +152,9 @@ export class TestProjectRegistry {
             for (const pythonProject of workspaceProjects) {
                 try {
                     const adapter = await this.createProjectAdapter(pythonProject, workspaceUri);
-                    adapters.push(adapter);
+                    if (adapter) {
+                        adapters.push(adapter);
+                    }
                 } catch (error) {
                     traceError(`[test-by-project] Failed to create adapter for ${pythonProject.uri.fsPath}:`, error);
                 }
@@ -178,16 +180,30 @@ export class TestProjectRegistry {
      * - **DiscoveryAdapter:** Discovers tests scoped to this project's root directory
      * - **ExecutionAdapter:** Runs tests for this project using its Python environment
      *
+     * Returns `undefined` when the Python Environments extension has not yet assigned an
+     * environment to the project. This is expected during startup: extension activation no longer
+     * waits for the environments extension's initial refresh to complete, so discovery can run
+     * before environments are resolved. The project is re-discovered once an environment is
+     * assigned (see the controller's environment-change subscription).
      */
-    private async createProjectAdapter(pythonProject: PythonProject, workspaceUri: Uri): Promise<ProjectAdapter> {
+    private async createProjectAdapter(
+        pythonProject: PythonProject,
+        workspaceUri: Uri,
+    ): Promise<ProjectAdapter | undefined> {
         const projectId = getProjectId(pythonProject.uri);
         traceInfo(`[test-by-project] Creating adapter for: ${pythonProject.name} at ${projectId}`);
 
         // Resolve Python environment
         const envExtApi = await getEnvExtApi();
         const pythonEnvironment = await envExtApi.getEnvironment(pythonProject.uri);
         if (!pythonEnvironment) {
-            throw new Error(`No Python environment found for project ${projectId}`);
+            // Not an error: the environments extension may not have assigned an environment to
+            // this project yet. Defer it; the controller re-discovers the workspace when an
+            // environment is later assigned (onDidChangeEnvironment).
+            traceInfo(
+                `[test-by-project] No Python environment resolved yet for project ${projectId}; deferring until one is assigned`,
+            );
+            return undefined;
         }
 
         // Create test infrastructure