Prevent activity loops when pressing back button while unlocking

This solution even works when "don't keep activities" is active

briar-android/src/main/java/org/briarproject/briar/android/activity/BriarActivity.java

@@ -56,22 +56,32 @@ public abstract class BriarActivity extends BaseActivity {
 	@Override
 	protected void onActivityResult(int request, int result, Intent data) {
 		super.onActivityResult(request, result, data);
-		if (request == REQUEST_PASSWORD) {
-			if (result == RESULT_OK) briarController.startAndBindService();
-			else supportFinishAfterTransition();
-		} else if (request == REQUEST_UNLOCK) {
-			// if we don't finish here, we will enter onStart()
-			if (result != RESULT_OK) supportFinishAfterTransition();
+		if (request == REQUEST_PASSWORD && result == RESULT_OK) {
+			// PasswordActivity finishes when password was entered correctly.
+			// When back button is pressed there, it will bring itself back,
+			// so that we never arrive here with a result that is not OK.
+			briarController.startAndBindService();
+		} else if (request == REQUEST_UNLOCK && result != RESULT_OK) {
+			// We arrive here, if the user presses 'back'
+			// in the Keyguard unlock screen, because UnlockActivity finishes.
+			// If we don't finish here, isFinishing will be false in onResume()
+			// and we launch a new UnlockActivity causing a loop.
+			supportFinishAfterTransition();
+			// If the result is OK, we don't need to do anything here
+			// and can resume normally.
 		}
 	}
 
 	@Override
-	public void onStart() {
-		super.onStart();
-		if (!briarController.accountSignedIn() && !isFinishing()) {
+	public void onResume() {
+		super.onResume();
+		if (!briarController.accountSignedIn()) {
 			Intent i = new Intent(this, PasswordActivity.class);
 			startActivityForResult(i, REQUEST_PASSWORD);
-		} else if (lockManager.isLocked()) {
+		} else if (lockManager.isLocked() && !isFinishing()) {
+			// Also check that the activity isn't finishing already.
+			// This is possible if finishing in onActivityResult().
+			// Failure to do this check would cause an UnlockActivity loop.
 			Intent i = new Intent(this, UnlockActivity.class);
 			startActivityForResult(i, REQUEST_UNLOCK);
 		} else if (SDK_INT >= 23) {

briar-android/src/main/java/org/briarproject/briar/android/login/PasswordActivity.java

@@ -23,8 +23,6 @@ import org.briarproject.briar.api.android.AndroidNotificationManager;
 
 import javax.inject.Inject;
 
-import static android.content.Intent.ACTION_MAIN;
-import static android.content.Intent.CATEGORY_HOME;
 import static android.content.Intent.FLAG_ACTIVITY_CLEAR_TASK;
 import static android.content.Intent.FLAG_ACTIVITY_NEW_TASK;
 import static android.view.View.INVISIBLE;
@@ -109,10 +107,9 @@ public class PasswordActivity extends BaseActivity {
 
 	@Override
 	public void onBackPressed() {
-		// Show the home screen rather than another password prompt
-		Intent intent = new Intent(ACTION_MAIN);
-		intent.addCategory(CATEGORY_HOME);
-		startActivity(intent);
+		// Move task and activity to the background instead of showing another
+		// password prompt. onActivityResult() won't be called in BriarActivity
+		moveTaskToBack(true);
 	}
 
 	private void deleteAccount() {

briar-android/src/main/java/org/briarproject/briar/android/login/UnlockActivity.java

@@ -20,6 +20,7 @@ import javax.inject.Inject;
 
 import static android.os.Build.VERSION.SDK_INT;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_KEYGUARD_UNLOCK;
+import static org.briarproject.briar.android.util.UiUtils.hasScreenLock;
 
 @RequiresApi(21)
 @MethodsNotNullByDefault
@@ -27,7 +28,7 @@ import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_KEYGU
 public class UnlockActivity extends BaseActivity {
 
 	private static final Logger LOG =
-			Logger.getLogger(UnlockActivity.class.getSimpleName());
+			Logger.getLogger(UnlockActivity.class.getName());
 
 	@Inject
 	LockManager lockManager;
@@ -44,13 +45,6 @@ public class UnlockActivity extends BaseActivity {
 
 		Button button = findViewById(R.id.unlock);
 		button.setOnClickListener(view -> requestKeyguardUnlock());
-
-		requestKeyguardUnlock();
-	}
-
-	@Override
-	public void onBackPressed() {
-		moveTaskToBack(true);
 	}
 
 	@Override
@@ -63,6 +57,22 @@ public class UnlockActivity extends BaseActivity {
 		}
 	}
 
+	@Override
+	protected void onResume() {
+		super.onResume();
+		// Show keyguard after onActivityResult() as been called.
+		// Check if app is still locked, lockable
+		// and not finishing (which is possible if recreated)
+		if (lockManager.isLocked() && hasScreenLock(this) && !isFinishing()) {
+			requestKeyguardUnlock();
+		}
+	}
+
+	@Override
+	public void onBackPressed() {
+		moveTaskToBack(true);
+	}
+
 	private void requestKeyguardUnlock() {
 		KeyguardManager keyguardManager =
 				(KeyguardManager) getSystemService(KEYGUARD_SERVICE);