Use a lock to prevent concurrent native calls.

The lock is static to ensure that if the TorService instance is destroyed and a new instance is created (eg when unbinding and then re-binding), the background threads created by the two instances don't concurrently call into the process's Tor library.

tor-android-binary/src/main/java/org/torproject/jni/TorService.java

@@ -7,6 +7,7 @@ import android.os.Binder;
 import android.os.FileObserver;
 import android.os.IBinder;
 import android.os.Process;
+import android.util.Log;
 
 import net.freehaven.tor.control.RawEventListener;
 import net.freehaven.tor.control.TorControlCommands;
@@ -24,6 +25,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.ReentrantLock;
 
 import androidx.annotation.Nullable;
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
@@ -167,6 +169,12 @@ public class TorService extends Service {
 
     private TorControlConnection torControlConnection;
 
+    /**
+     * This lock must be acquired before calling createTorConfiguration() and
+     * held until mainConfigurationFree() has been called.
+     */
+    private static final ReentrantLock runLock = new ReentrantLock();
+
     private native String apiGetProviderVersion();
 
     private native boolean createTorConfiguration();
@@ -314,6 +322,12 @@ public class TorService extends Service {
                         httpTunnelPort = Integer.toString(8118);
                     }
 
+                    if (runLock.isLocked()) {
+                        Log.i(TAG, "Waiting for lock");
+                    }
+                    runLock.lock();
+                    Log.i(TAG, "Acquired lock");
+                    try {
                         createTorConfiguration();
                         ArrayList<String> lines = new ArrayList<>(Arrays.asList("tor", "--verify-config", // must always be here
                                 "--RunAsDaemon", "0",
@@ -359,6 +373,11 @@ public class TorService extends Service {
                         if (runMain() != 0) {
                             throw new IllegalStateException("Tor could not start!");
                         }
+                    } finally {
+                        mainConfigurationFree();
+                        Log.i(TAG, "Releasing lock");
+                        runLock.unlock();
+                    }
 
                 } catch (IllegalStateException | IllegalArgumentException | InterruptedException e) {
                     e.printStackTrace();
@@ -377,7 +396,6 @@ public class TorService extends Service {
             torControlConnection.removeRawEventListener(startedEventListener);
         }
         shutdownTor(TorControlCommands.SIGNAL_SHUTDOWN);
-        mainConfigurationFree();
         broadcastStatus(TorService.this, STATUS_OFF);
     }