On API < 29 wait for Wi-Fi p2p broadcast before creating group

app/src/main/java/org/briarproject/hotspot/HotspotManager.java

 package org.briarproject.hotspot;
 
+import android.content.BroadcastReceiver;
 import android.content.Context;
+import android.content.Intent;
+import android.content.IntentFilter;
 import android.net.wifi.WifiManager;
 import android.net.wifi.p2p.WifiP2pConfig;
 import android.net.wifi.p2p.WifiP2pGroup;
@@ -23,8 +26,12 @@ import static android.net.wifi.WifiManager.WIFI_MODE_FULL_HIGH_PERF;
 import static android.net.wifi.p2p.WifiP2pConfig.GROUP_OWNER_BAND_2GHZ;
 import static android.net.wifi.p2p.WifiP2pManager.BUSY;
 import static android.net.wifi.p2p.WifiP2pManager.ERROR;
+import static android.net.wifi.p2p.WifiP2pManager.EXTRA_WIFI_STATE;
 import static android.net.wifi.p2p.WifiP2pManager.NO_SERVICE_REQUESTS;
 import static android.net.wifi.p2p.WifiP2pManager.P2P_UNSUPPORTED;
+import static android.net.wifi.p2p.WifiP2pManager.WIFI_P2P_STATE_CHANGED_ACTION;
+import static android.net.wifi.p2p.WifiP2pManager.WIFI_P2P_STATE_DISABLED;
+import static android.net.wifi.p2p.WifiP2pManager.WIFI_P2P_STATE_ENABLED;
 import static android.os.Build.VERSION.SDK_INT;
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Logger.getLogger;
@@ -103,8 +110,10 @@ class HotspotManager implements ActionListener {
 						.setPassphrase(passphrase)
 						.build();
 				wifiP2pManager.createGroup(channel, config, this);
-			} else {
-				wifiP2pManager.createGroup(channel, this);
+			} else if (!receiverRegistered) {
+				ctx.registerReceiver(receiver, new IntentFilter(
+						WIFI_P2P_STATE_CHANGED_ACTION));
+				receiverRegistered = true;
 			}
 		} catch (SecurityException e) {
 			// this should never happen, because we request permissions before
@@ -112,6 +121,46 @@ class HotspotManager implements ActionListener {
 		}
 	}
 
+	// Only used on API < 29
+	private boolean receiverRegistered = false;
+	final BroadcastReceiver receiver = new BroadcastReceiver() {
+
+		@Override
+		@UiThread
+		public void onReceive(Context context, Intent intent) {
+			if (intent.getAction() == null ||
+					!intent.getAction().equals(WIFI_P2P_STATE_CHANGED_ACTION)) {
+				return;
+			}
+			int state = intent.getIntExtra(EXTRA_WIFI_STATE,
+					WIFI_P2P_STATE_DISABLED);
+			if (state == WIFI_P2P_STATE_ENABLED) {
+				try {
+					wifiP2pManager.createGroup(channel, HotspotManager.this);
+				} catch (SecurityException e) {
+					// this should never happen, because we request permissions before
+					throw new AssertionError(e);
+				}
+				unregisterReceiver();
+			}
+		}
+
+	};
+
+	private void unregisterReceiver() {
+		if (!receiverRegistered)
+			return;
+		try {
+			ctx.unregisterReceiver(receiver);
+			receiverRegistered = false;
+		} catch (IllegalArgumentException e) {
+			// This gets thrown when we try to unregister an already unregistered
+			// receiver. We do keep a flag whether the receiver is actually
+			// registered, so it should not happen.
+			throw new AssertionError(e);
+		}
+	}
+
 	@RequiresApi(29)
 	private String getNetworkName() {
 		return "DIRECT-" + getRandomString(2) + "-" +
@@ -122,7 +171,10 @@ class HotspotManager implements ActionListener {
 		return getRandomString(8);
 	}
 
+	@UiThread
 	void stopWifiP2pHotspot() {
+		if (SDK_INT < 29)
+			unregisterReceiver();
 		if (channel == null) return;
 		wifiP2pManager.removeGroup(channel, new ActionListener() {