fix(driver): fail commands instead of hanging when the Dart VM connection closes

driver/lib/sessions/isolate_socket.ts

@@ -23,6 +23,10 @@ export class IsolateSocket extends Client {
 
     // @ts-ignore - runtime call is valid; types are provided by the package
     super(address as any);
+    // rpc-websockets keeps a pending call until a reply or its timeout arrives, and
+    // executeSocketCommand sets no timeout. Reject pending calls when the Dart VM connection
+    // closes (e.g. the app crashed or relaunched) so the command fails instead of hanging.
+    this.on(`close`, () => this.rejectPendingCalls());
   }
 
   public async executeSocketCommand(args: ExecuteArgs) {
@@ -35,4 +39,14 @@ export class IsolateSocket extends Client {
       response: any;
     }>;
   }
+
+  private rejectPendingCalls() {
+    const queue: Record<string, {promise: [unknown, (err: Error) => void]; timeout?: NodeJS.Timeout}> =
+      (this as any).queue ?? {};
+    for (const [id, {promise, timeout}] of Object.entries(queue)) {
+      clearTimeout(timeout);
+      delete queue[id];
+      promise[1](new Error(`The Dart VM connection was closed before it replied`));
+    }
+  }
 }

driver/lib/sessions/observatory.ts

@@ -70,7 +70,7 @@ export async function connectSocket(
           // `await` is needed so that rejected promise will be thrown and caught
           return await originalSocketCall.apply(socket, args);
         } catch (e) {
-          this.log.errorWithException(new Error(JSON.stringify(e)));
+          this.log.errorWithException(e instanceof Error ? e : new Error(JSON.stringify(e)));
         }
       };
       this.log.info(`Connecting to Dart Observatory: ${dartObservatoryURL}`);
@@ -183,6 +183,11 @@ export async function executeElementCommand(
   this.log.debug(`>>> ${JSON.stringify(serializedCommand)}`);
   const data = await (this.socket as IsolateSocket).executeSocketCommand(serializedCommand);
   this.log.debug(`<<< ${JSON.stringify(data)} | previous command ${command}`);
+  if (!data) {
+    throw new Error(
+      `Cannot execute command ${command}, no response from the Dart VM. Check the server log for the connection error`,
+    );
+  }
   if (data.isError) {
     throw new Error(`Cannot execute command ${command}, server response ${JSON.stringify(data, null, 2)}`);
   }

driver/test/isolate-socket.test.mjs

@@ -0,0 +1,63 @@
+import assert from 'node:assert/strict';
+import {after, before, it} from 'node:test';
+
+import {Server} from 'rpc-websockets';
+
+import {IsolateSocket} from '../build/lib/sessions/isolate_socket.js';
+import {connectSocket, executeElementCommand} from '../build/lib/sessions/observatory.js';
+
+const noopLog = {info() {}, debug() {}, warn() {}, error() {}, errorWithException() {}};
+const sockets = [];
+let server;
+let url;
+
+// A fake Dart VM service that drops the connection while a Flutter Driver command is pending,
+// like an app that crashes or relaunches in the middle of `waitForAbsent`.
+before(async () => {
+  server = new Server({host: '127.0.0.1', port: 0});
+  await new Promise((resolve) => server.on('listening', resolve));
+  url = `ws://127.0.0.1:${server.wss.address().port}/ws`;
+  server.register('getIsolate', () => ({extensionRPCs: ['ext.flutter.driver']}), '/ws');
+  server.register(
+    'ext.flutter.driver',
+    () => {
+      for (const client of server.wss.clients) {
+        client.terminate();
+      }
+      return new Promise(() => {});
+    },
+    '/ws',
+  );
+});
+
+after(async () => {
+  for (const socket of sockets) {
+    socket.reconnect = false;
+    socket.close();
+  }
+  await server.close();
+});
+
+it('rejects a pending command when the Dart VM connection closes', {timeout: 5000}, async () => {
+  const socket = new IsolateSocket(url);
+  sockets.push(socket);
+  await new Promise((resolve) => socket.once('open', resolve));
+
+  await assert.rejects(socket.executeSocketCommand({command: 'waitForAbsent'}), /connection was closed/);
+});
+
+it('fails an element command instead of hanging when the Dart VM connection closes', {timeout: 5000}, async () => {
+  const logged = [];
+  const driver = {log: {...noopLog, errorWithException: (err) => logged.push(err.message)}};
+  driver.socket = await connectSocket.call(driver, url, {isolateId: 'isolates/1'});
+  sockets.push(driver.socket);
+  const finder = Buffer.from(
+    JSON.stringify({finderType: 'ByValueKey', keyValueString: 'my_key', keyValueType: 'String'}),
+  ).toString('base64url');
+
+  await assert.rejects(
+    executeElementCommand.call(driver, 'waitForAbsent', finder),
+    /Cannot execute command waitForAbsent, no response from the Dart VM/,
+  );
+  assert.deepEqual(logged, ['The Dart VM connection was closed before it replied']);
+});