Cherry pick fix for hasInterpreters and update change log

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Fixes
 
+1. Double-check for interpreters when running diagnostics before displaying the "Python is not installed" message.
+   ([#11870](https://github.com/Microsoft/vscode-python/issues/11870))
 1. Ensure user cannot belong to all experiments in an experiment group.
    ([#11943](https://github.com/Microsoft/vscode-python/issues/11943))
 1. Ensure extension features are started when in `Deprecate PythonPath` experiment and opening a file without any folder opened.

src/client/application/diagnostics/checks/pythonInterpreter.ts

@@ -67,7 +67,14 @@ export class InvalidPythonInterpreterService extends BaseDiagnosticsService {
         }
 
         const interpreterService = this.serviceContainer.get<IInterpreterService>(IInterpreterService);
-        const hasInterpreters = await interpreterService.hasInterpreters;
+        // hasInterpreters being false can mean one of 2 things:
+        // 1. getInterpreters hasn't returned any interpreters;
+        // 2. getInterpreters hasn't run yet.
+        // We want to make sure that false comes from 1, so we're adding this fix until we refactor interpreter discovery.
+        // Also see https://github.com/microsoft/vscode-python/issues/3023.
+        const hasInterpreters =
+            (await interpreterService.hasInterpreters) ||
+            (await interpreterService.getInterpreters(resource)).length > 0;
 
         if (!hasInterpreters) {
             return [new InvalidPythonInterpreterDiagnostic(DiagnosticCodes.NoPythonInterpretersDiagnostic, resource)];

src/test/application/diagnostics/checks/pythonInterpreter.unit.test.ts

@@ -29,7 +29,12 @@ import { CommandsWithoutArgs } from '../../../../client/common/application/comma
 import { IPlatformService } from '../../../../client/common/platform/types';
 import { IConfigurationService, IDisposableRegistry, IPythonSettings } from '../../../../client/common/types';
 import { noop } from '../../../../client/common/utils/misc';
-import { IInterpreterHelper, IInterpreterService, InterpreterType } from '../../../../client/interpreter/contracts';
+import {
+    IInterpreterHelper,
+    IInterpreterService,
+    InterpreterType,
+    PythonInterpreter
+} from '../../../../client/interpreter/contracts';
 import { IServiceContainer } from '../../../../client/ioc/types';
 
 suite('Application Diagnostics - Checks Python Interpreter', () => {
@@ -129,7 +134,7 @@ suite('Application Diagnostics - Checks Python Interpreter', () => {
             expect(diagnostics).to.be.deep.equal([]);
             settings.verifyAll();
         });
-        test('Should return diagnostics if there are no interpreters', async () => {
+        test('Should return diagnostics if there are no interpreters after double-checking', async () => {
             settings
                 .setup((s) => s.disableInstallationChecks)
                 .returns(() => false)
@@ -138,6 +143,10 @@ suite('Application Diagnostics - Checks Python Interpreter', () => {
                 .setup((i) => i.hasInterpreters)
                 .returns(() => Promise.resolve(false))
                 .verifiable(typemoq.Times.once());
+            interpreterService
+                .setup((i) => i.getInterpreters(undefined))
+                .returns(() => Promise.resolve([]))
+                .verifiable(typemoq.Times.once());
 
             const diagnostics = await diagnosticService.diagnose(undefined);
             expect(diagnostics).to.be.deep.equal(
@@ -147,6 +156,34 @@ suite('Application Diagnostics - Checks Python Interpreter', () => {
             settings.verifyAll();
             interpreterService.verifyAll();
         });
+        test('Should return empty diagnostics if there are interpreters after double-checking', async () => {
+            const interpreter: PythonInterpreter = { type: InterpreterType.Unknown } as any;
+
+            settings
+                .setup((s) => s.disableInstallationChecks)
+                .returns(() => false)
+                .verifiable(typemoq.Times.once());
+            interpreterService
+                .setup((i) => i.hasInterpreters)
+                .returns(() => Promise.resolve(false))
+                .verifiable(typemoq.Times.once());
+            interpreterService
+                .setup((i) => i.getInterpreters(undefined))
+                .returns(() => Promise.resolve([interpreter]))
+                .verifiable(typemoq.Times.once());
+            interpreterService
+                .setup((i) => i.getActiveInterpreter(typemoq.It.isAny()))
+                .returns(() => {
+                    return Promise.resolve(interpreter);
+                })
+                .verifiable(typemoq.Times.once());
+
+            const diagnostics = await diagnosticService.diagnose(undefined);
+
+            expect(diagnostics).to.be.deep.equal([], 'not the same');
+            settings.verifyAll();
+            interpreterService.verifyAll();
+        });
         test('Should return invalid diagnostics if there are interpreters but no current interpreter', async () => {
             settings
                 .setup((s) => s.disableInstallationChecks)