feat(debug): enable Python debug workflow and debug server integration

Adds an end-to-end Python debug workflow for the extension.
Includes a new prepare-debug.ps1 script and a VS Code task.
Extends the Python server and extension to coordinate a debug session and safe startup.

- Add prepare-debug.ps1 and a VS Code task to build the debug bundle
- Enable Python debug wiring in the server and tests
- Ensure a single stable Python debug session during startup

client/package-lock.json

@@ -14,6 +14,7 @@
         "vscode-languageclient": "^9.0.1"
       },
       "devDependencies": {
+        "@types/fs-extra": "^11.0.4",
         "@types/node": "^22.10.5",
         "@types/vscode": "^1.96.0"
       },
@@ -21,6 +22,27 @@
         "vscode": "^1.96.0"
       }
     },
+    "node_modules/@types/fs-extra": {
+      "version": "11.0.4",
+      "resolved": "https://registry.npmjs.org/@types/fs-extra/-/fs-extra-11.0.4.tgz",
+      "integrity": "sha512-yTbItCNreRooED33qjunPthRcSjERP1r4MqCZc7wv0u2sUkzTFp45tgUfS5+r7FrZPdmCCNflLhVSP/o+SemsQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/jsonfile": "*",
+        "@types/node": "*"
+      }
+    },
+    "node_modules/@types/jsonfile": {
+      "version": "6.1.4",
+      "resolved": "https://registry.npmjs.org/@types/jsonfile/-/jsonfile-6.1.4.tgz",
+      "integrity": "sha512-D5qGUYwjvnNNextdU59/+fI+spnwtTFmyQP0h+PfIOSkNfpU6AOICUOkm4i0OnSk+NyjdPJrxCDro0sJsWlRpQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/node": {
       "version": "22.10.5",
       "resolved": "https://registry.npmjs.org/@types/node/-/node-22.10.5.tgz",

client/package.json

@@ -12,6 +12,7 @@
     "vscode-languageclient": "^9.0.1"
   },
   "devDependencies": {
+    "@types/fs-extra": "^11.0.4",
     "@types/node": "^22.10.5",
     "@types/vscode": "^1.96.0"
   }

client/src/common/server.ts

@@ -39,13 +39,26 @@ async function createServer(
     initializationOptions: IInitOptions
 ): Promise<LanguageClient> {
     const command = settings.interpreter[0]
+    if (!command) {
+        throw new Error("No Python interpreter is configured for the language server.")
+    }
     const cwd = settings.cwd
 
     // Set debugger path needed for debugging python code.
     const newEnv = { ...process.env }
-    const debuggerPath = await getDebuggerPath()
     const isDebugScript = await fsapi.pathExists(DEBUG_SERVER_SCRIPT_PATH)
-    if (newEnv.USE_DEBUGPY && debuggerPath) {
+    const debugRequested = newEnv.USE_DEBUGPY?.toLowerCase() === "true"
+    if (debugRequested && !isDebugScript) {
+        throw new Error(`Python debug bootstrap not found: ${DEBUG_SERVER_SCRIPT_PATH}`)
+    }
+
+    const debuggerPath = debugRequested ? await getDebuggerPath() : undefined
+    if (debugRequested && !debuggerPath) {
+        throw new Error(
+            "Python debugging was requested, but the Python Debugger extension did not provide debugpy."
+        )
+    }
+    if (debugRequested && debuggerPath) {
         newEnv.DEBUGPY_PATH = debuggerPath
     } else {
         newEnv.USE_DEBUGPY = "False"
@@ -57,10 +70,13 @@ async function createServer(
     // Set notification type
     newEnv.LS_SHOW_NOTIFICATION = settings.showNotifications
 
-    const args =
-        newEnv.USE_DEBUGPY === "False" || !isDebugScript
-            ? settings.interpreter.slice(1).concat([SERVER_SCRIPT_PATH])
-            : settings.interpreter.slice(1).concat([DEBUG_SERVER_SCRIPT_PATH])
+    const serverScript = debugRequested ? DEBUG_SERVER_SCRIPT_PATH : SERVER_SCRIPT_PATH
+    const interpreterArgs = settings.interpreter.slice(1)
+    if (debugRequested && !interpreterArgs.includes("-Xfrozen_modules=off")) {
+        interpreterArgs.push("-Xfrozen_modules=off")
+    }
+    const args = interpreterArgs.concat([serverScript])
+    traceInfo(`Python debug mode: ${debugRequested ? "enabled" : "disabled"}`)
     traceInfo(`Server run command: ${[command, ...args].join(" ")}`)
 
     const serverOptions: ServerOptions = {
@@ -108,35 +124,40 @@ export async function restartServer(
     const projectRoot = await getProjectRoot()
     const workspaceSetting = await getWorkspaceSettings(serverId, projectRoot, true)
 
-    const newLSClient = await createServer(workspaceSetting, serverId, serverName, outputChannel, {
-        settings: await getExtensionSettings(serverId, true),
-        globalSettings: await getGlobalSettings(serverId, false)
-    })
-    traceInfo(`Server: Start requested.`)
-    _disposables.push(
-        newLSClient.onDidChangeState((e) => {
-            switch (e.newState) {
-                case State.Stopped:
-                    traceVerbose(`Server State: Stopped`)
-                    break
-                case State.Starting:
-                    traceVerbose(`Server State: Starting`)
-                    break
-                case State.Running:
-                    traceVerbose(`Server State: Running`)
-                    break
-            }
-        })
-    )
     try {
+        const newLSClient = await createServer(
+            workspaceSetting,
+            serverId,
+            serverName,
+            outputChannel,
+            {
+                settings: await getExtensionSettings(serverId, true),
+                globalSettings: await getGlobalSettings(serverId, false)
+            }
+        )
+        traceInfo(`Server: Start requested.`)
+        _disposables.push(
+            newLSClient.onDidChangeState((e) => {
+                switch (e.newState) {
+                    case State.Stopped:
+                        traceVerbose(`Server State: Stopped`)
+                        break
+                    case State.Starting:
+                        traceVerbose(`Server State: Starting`)
+                        break
+                    case State.Running:
+                        traceVerbose(`Server State: Running`)
+                        break
+                }
+            })
+        )
         await newLSClient.start()
+        const level = getLSClientTraceLevel(outputChannel.logLevel, env.logLevel)
+        await newLSClient.setTrace(level)
+        return newLSClient
     } catch (ex) {
         traceError(`Server: Start failed: ${ex}`)
         disposeServerResources()
         return undefined
     }
-
-    const level = getLSClientTraceLevel(outputChannel.logLevel, env.logLevel)
-    await newLSClient.setTrace(level)
-    return newLSClient
 }

client/src/extension.ts

@@ -38,6 +38,7 @@ export async function activate(context: vscode.ExtensionContext) {
     const serverInfo = loadServerDefaults()
     const serverName = serverInfo.name
     const serverId = serverInfo.module
+    const pythonDebugMode = process.env.USE_DEBUGPY?.toLowerCase() === "true"
 
     // Setup logging
     const outputChannel = createOutputChannel(serverName)
@@ -101,10 +102,15 @@ export async function activate(context: vscode.ExtensionContext) {
         return runServerQueue
     }
 
+    if (!pythonDebugMode) {
+        context.subscriptions.push(
+            onDidChangePythonInterpreter(async () => {
+                await runServer()
+            })
+        )
+    }
+
     context.subscriptions.push(
-        onDidChangePythonInterpreter(async () => {
-            await runServer()
-        }),
         onDidChangeConfiguration(async (e: vscode.ConfigurationChangeEvent) => {
             if (checkIfConfigurationChanged(e, serverId)) {
                 await runServer()
@@ -116,6 +122,15 @@ export async function activate(context: vscode.ExtensionContext) {
     )
 
     setImmediate(async () => {
+        if (pythonDebugMode) {
+            // A debugpy listen session is attached to exactly one process. Do not
+            // subscribe to interpreter changes during startup, as the Python
+            // extension can emit a duplicate event and restart that process.
+            traceLog("Python debug mode: starting one stable server session")
+            await runServer()
+            return
+        }
+
         const interpreter = getInterpreterFromSetting(serverId)
         if (interpreter === undefined || interpreter.length === 0) {
             traceLog(`Python extension loading`)