tunnel: replace CompletableFuture with GhettoCompletableFuture

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tunnel/build.gradle

@@ -25,7 +25,6 @@ android {
 
 dependencies {
     api "net.sourceforge.streamsupport:android-retrostreams:$streamsupportVersion"
-    implementation "net.sourceforge.streamsupport:android-retrofuture:$streamsupportVersion"
     implementation "androidx.annotation:annotation:$annotationsVersion"
     implementation "androidx.collection:collection:$collectionVersion"
     implementation "com.google.code.findbugs:jsr305:$jsr305Version"

tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java

@@ -24,18 +24,20 @@ import com.wireguard.util.NonNullForAll;
 import java.net.InetAddress;
 import java.util.Collections;
 import java.util.Set;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 
 import androidx.annotation.Nullable;
 import androidx.collection.ArraySet;
-import java9.util.concurrent.CompletableFuture;
 
 @NonNullForAll
 public final class GoBackend implements Backend {
     private static final String TAG = "WireGuard/" + GoBackend.class.getSimpleName();
     @Nullable private static AlwaysOnCallback alwaysOnCallback;
-    private static CompletableFuture<VpnService> vpnService = new CompletableFuture<>();
+    private static GhettoCompletableFuture<VpnService> vpnService = new GhettoCompletableFuture<>();
     private final Context context;
     @Nullable private Config currentConfig;
     @Nullable private Tunnel currentTunnel;
@@ -247,6 +249,35 @@ public final class GoBackend implements Backend {
         void alwaysOnTriggered();
     }
 
+    // TODO: When we finally drop API 21 and move to API 24, delete this and replace with the ordinary CompletableFuture.
+    private static final class GhettoCompletableFuture<V> {
+        private final LinkedBlockingQueue<V> completion = new LinkedBlockingQueue<>(1);
+        private final FutureTask<V> result = new FutureTask<>(completion::peek);
+
+        public boolean complete(final V value) {
+            final boolean offered = completion.offer(value);
+            if (offered)
+                result.run();
+            return offered;
+        }
+
+        public V get() throws ExecutionException, InterruptedException {
+            return result.get();
+        }
+
+        public V get(final long timeout, final TimeUnit unit) throws ExecutionException, InterruptedException, TimeoutException {
+            return result.get(timeout, unit);
+        }
+
+        public boolean isDone() {
+            return !completion.isEmpty();
+        }
+
+        public GhettoCompletableFuture<V> newIncompleteFuture() {
+            return new GhettoCompletableFuture<>();
+        }
+    }
+
     public static class VpnService extends android.net.VpnService {
         @Nullable private GoBackend owner;