Prompt to restart extension on xcode-select

If the user performs an `xcode-select` and doesn't have a `swift.path`
set in their VS Code settings, prompt them to restart the extension to
pick up the new Xcode path. This only applies on macOS where
`xcode-select` is possible.

src/extension.ts

@@ -38,6 +38,7 @@ import { showReloadExtensionNotification } from "./ui/ReloadExtension";
 import { checkAndWarnAboutWindowsSymlinks } from "./ui/win32";
 import { SwiftEnvironmentVariablesManager, SwiftTerminalProfileProvider } from "./terminal";
 import { resolveFolderDependencies } from "./commands/dependencies/resolve";
+import { SelectedXcodeWatcher } from "./toolchain/SelectedXcodeWatcher";
 
 /**
  * External API as exposed by the extension. Can be queried by other extensions
@@ -125,6 +126,7 @@ export async function activate(context: vscode.ExtensionContext): Promise<Api> {
         context.subscriptions.push(...commands.register(workspaceContext));
         context.subscriptions.push(workspaceContext);
         context.subscriptions.push(registerDebugger(workspaceContext));
+        context.subscriptions.push(new SelectedXcodeWatcher(outputChannel));
 
         // listen for workspace folder changes and active text editor changes
         workspaceContext.setupEventListeners();

src/toolchain/SelectedXcodeWatcher.ts

@@ -0,0 +1,93 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the VS Code Swift open source project
+//
+// Copyright (c) 2021-2024 the VS Code Swift project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of VS Code Swift project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import * as fs from "fs/promises";
+import * as vscode from "vscode";
+import { SwiftOutputChannel } from "../ui/SwiftOutputChannel";
+import { showReloadExtensionNotification } from "../ui/ReloadExtension";
+import configuration from "../configuration";
+
+export class SelectedXcodeWatcher implements vscode.Disposable {
+    private xcodePath: string | undefined;
+    private disposed: boolean = false;
+    private interval: NodeJS.Timeout | undefined;
+    private checkIntervalMs: number;
+    private xcodeSymlink: () => Promise<string | undefined>;
+
+    private static DEFAULT_CHECK_INTERVAL_MS = 2000;
+    private static XCODE_SYMLINK_LOCATION = "/var/select/developer_dir";
+
+    constructor(
+        private outputChannel: SwiftOutputChannel,
+        testDependencies?: {
+            checkIntervalMs?: number;
+            xcodeSymlink?: () => Promise<string | undefined>;
+        }
+    ) {
+        this.checkIntervalMs =
+            testDependencies?.checkIntervalMs || SelectedXcodeWatcher.DEFAULT_CHECK_INTERVAL_MS;
+        this.xcodeSymlink =
+            testDependencies?.xcodeSymlink ||
+            (async () => {
+                try {
+                    return await fs.readlink(SelectedXcodeWatcher.XCODE_SYMLINK_LOCATION);
+                } catch (e) {
+                    return undefined;
+                }
+            });
+
+        if (!this.isValidXcodePlatform()) {
+            return;
+        }
+
+        // Deliberately not awaiting this, as we don't want to block the extension activation.
+        this.setup();
+    }
+
+    dispose() {
+        this.disposed = true;
+        clearInterval(this.interval);
+    }
+
+    /**
+     * Polls the Xcode symlink location checking if it has changed.
+     * If the user has `swift.path` set in their settings this check is skipped.
+     */
+    private async setup() {
+        this.xcodePath = await this.xcodeSymlink();
+        this.interval = setInterval(async () => {
+            if (this.disposed) {
+                return clearInterval(this.interval);
+            }
+
+            const newXcodePath = await this.xcodeSymlink();
+            if (!configuration.path && newXcodePath && this.xcodePath !== newXcodePath) {
+                this.outputChannel.appendLine(
+                    `Selected Xcode changed from ${this.xcodePath} to ${newXcodePath}`
+                );
+                showReloadExtensionNotification(
+                    "The Swift Extension has detected a change in the selected Xcode. Please reload the extension to apply the changes."
+                );
+                this.xcodePath = newXcodePath;
+            }
+        }, this.checkIntervalMs);
+    }
+
+    /**
+     * Xcode selection is a macOS only concept.
+     */
+    private isValidXcodePlatform() {
+        return process.platform === "darwin";
+    }
+}

test/unit-tests/toolchain/SelectedXcodeWatcher.test.ts

@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the VS Code Swift open source project
+//
+// Copyright (c) 2024 the VS Code Swift project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of VS Code Swift project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import * as vscode from "vscode";
+import { expect } from "chai";
+import { SelectedXcodeWatcher } from "../../../src/toolchain/SelectedXcodeWatcher";
+import { SwiftOutputChannel } from "../../../src/ui/SwiftOutputChannel";
+import { instance, MockedObject, mockFn, mockGlobalObject, mockObject } from "../../MockUtils";
+
+suite("Selected Xcode Watcher", () => {
+    const mockedVSCodeWindow = mockGlobalObject(vscode, "window");
+    let mockOutputChannel: MockedObject<SwiftOutputChannel>;
+
+    setup(() => {
+        mockOutputChannel = mockObject<SwiftOutputChannel>({
+            appendLine: mockFn(),
+        });
+    });
+
+    async function run(symLinksOnCallback: (string | undefined)[]) {
+        return new Promise<void>(resolve => {
+            let ctr = 0;
+            const watcher = new SelectedXcodeWatcher(instance(mockOutputChannel), {
+                checkIntervalMs: 1,
+                xcodeSymlink: async () => {
+                    if (ctr >= symLinksOnCallback.length) {
+                        watcher.dispose();
+                        resolve();
+                        return;
+                    }
+                    const response = symLinksOnCallback[ctr];
+                    ctr += 1;
+                    return response;
+                },
+            });
+        });
+    }
+
+    test("Does nothing when the symlink is undefined", async () => {
+        await run([undefined, undefined]);
+
+        expect(mockedVSCodeWindow.showWarningMessage).to.have.not.been.called;
+    });
+
+    test("Does nothing when the symlink is identical", async () => {
+        await run(["/foo", "/foo"]);
+
+        expect(mockedVSCodeWindow.showWarningMessage).to.have.not.been.called;
+    });
+
+    test("Prompts to restart when the symlink changes", async () => {
+        await run(["/foo", "/bar"]);
+
+        expect(mockedVSCodeWindow.showWarningMessage).to.have.been.calledOnceWithExactly(
+            "The Swift Extension has detected a change in the selected Xcode. Please reload the extension to apply the changes.",
+            "Reload Extensions"
+        );
+    });
+});