[flutter_tools] check if process manager can find adb (#74672)

packages/flutter_tools/lib/src/android/android_device_discovery.dart

@@ -64,7 +64,7 @@ class AndroidDevices extends PollingDeviceDiscovery {
 
   @override
   Future<List<Device>> pollingGetDevices({ Duration timeout }) async {
-    if (_androidSdk == null || _androidSdk.adbPath == null) {
+    if (_doesNotHaveAdb()) {
       return <AndroidDevice>[];
     }
     String text;
@@ -88,7 +88,7 @@ class AndroidDevices extends PollingDeviceDiscovery {
 
   @override
   Future<List<String>> getDiagnostics() async {
-    if (_androidSdk == null || _androidSdk.adbPath == null) {
+    if (_doesNotHaveAdb()) {
       return <String>[];
     }
 
@@ -104,6 +104,12 @@ class AndroidDevices extends PollingDeviceDiscovery {
     return diagnostics;
   }
 
+  bool _doesNotHaveAdb() {
+    return _androidSdk == null ||
+      _androidSdk.adbPath == null ||
+      !_processManager.canRun(_androidSdk.adbPath);
+  }
+
   // 015d172c98400a03       device usb:340787200X product:nakasi model:Nexus_7 device:grouper
   static final RegExp _kDeviceRegex = RegExp(r'^(\S+)\s+(\S+)(.*)');
 

packages/flutter_tools/test/general.shard/android/android_device_discovery_test.dart

@@ -44,6 +44,25 @@ void main() {
     expect(await androidDevices.getDiagnostics(), isEmpty);
   });
 
+  testWithoutContext('AndroidDevices returns empty device list and diagnostics when adb cannot be run', () async {
+    final AndroidDevices androidDevices = AndroidDevices(
+      androidSdk: FakeAndroidSdk(null),
+      logger: BufferLogger.test(),
+      androidWorkflow: AndroidWorkflow(
+        androidSdk: FakeAndroidSdk('adb'),
+        featureFlags: TestFeatureFlags(),
+      ),
+      // Will throw an exception if anything other than canRun is invoked
+      processManager: FakeProcessManger(),
+      fileSystem: MemoryFileSystem.test(),
+      platform: FakePlatform(),
+      userMessages: UserMessages(),
+    );
+
+    expect(await androidDevices.pollingGetDevices(), isEmpty);
+    expect(await androidDevices.getDiagnostics(), isEmpty);
+  });
+
   testWithoutContext('AndroidDevices returns empty device list and diagnostics on null Android SDK', () async {
     final AndroidDevices androidDevices = AndroidDevices(
       androidSdk: null,
@@ -218,3 +237,10 @@ class FakeAndroidSdk extends Fake implements AndroidSdk {
   @override
   final String adbPath;
 }
+
+class FakeProcessManger extends Fake implements ProcessManager {
+  @override
+  bool canRun(dynamic executable, {String workingDirectory}) {
+    return false;
+  }
+}