Give the hotspot's owner a route to its guests too

Binding the join socket to the Wi-Fi network covers a phone that has
joined a hotspot. It does nothing for the phone that is sharing one:
that phone has no Wi-Fi *client* network, so there was nothing to bind
to and the code gave up — leaving the socket on the default network,
which is mobile data, and a connection to a guest going out over
cellular to die.

ConnectivityManager does not offer the tethering interface as a Network,
so there is no network to name. Binding this end of the socket to one of
our own addresses on the guest's subnet picks the route instead, which
is the only lever available. The subnet test assumes a /24, which every
Android hotspot hands out, and getting it wrong costs a bind that would
not have helped anyway.

This is the direction that was left broken: the owner of the hotspot
trying to join a game hosted by one of its guests.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0132V9gE6DF9itz2z3Hn7BK9

app/src/main/java/org/prolibertate/games/net/LanTransport.kt

@@ -201,11 +201,11 @@ class LanTransport(private val context: Context) : Transport {
     override suspend fun join(host: DiscoveredHost, scope: CoroutineScope): Connection =
         withContext(Dispatchers.IO) {
             val socket = Socket()
-            // Before connecting, and this is the whole fix for hotspot play: a
+            // Before connecting, and this is the fix for hotspot play: a
             // hotspot has no internet behind it, so Android leaves mobile data
             // as the default network and an unbound socket goes out over
             // cellular looking for a 192.168 address.
-            bindToWifi(context, socket)
+            bindForTarget(context, socket, host.address)
             socket.connect(InetSocketAddress(host.address, host.port), CONNECT_TIMEOUT_MS)
             socket.tcpNoDelay = true
             StreamConnection(

app/src/main/java/org/prolibertate/games/net/LocalNetwork.kt

@@ -6,6 +6,8 @@ import android.net.Network
 import android.net.NetworkCapabilities
 import android.net.wifi.WifiManager
 import java.net.Inet4Address
+import java.net.InetAddress
+import java.net.InetSocketAddress
 import java.net.NetworkInterface
 import java.net.Socket
 
@@ -40,14 +42,42 @@ fun wifiNetwork(context: Context): Network? {
 }
 
 /**
- * Sends this socket out over the Wi-Fi rather than wherever the system would
- * otherwise send it. Silent if there is no Wi-Fi network to bind to: a phone
- * running the hotspot itself has no Wi-Fi *client* network, and reaches its own
- * guests over the tethering interface's own route without any help.
+ * Points this socket at the network [target] is actually on, before it
+ * connects.
+ *
+ * Two different problems wearing the same hat, depending on which end of the
+ * hotspot this phone is.
+ *
+ * A phone that has *joined* a hotspot has a Wi-Fi network, and naming it is
+ * enough. A phone that is *sharing* one does not: its guests live on the
+ * tethering interface, and ConnectivityManager does not offer that as a
+ * [Network] anything can bind to — so the socket falls back to the default
+ * network, which is mobile data, and a connection to a guest goes out over
+ * cellular and dies. Binding this end to an address on the guest's own subnet
+ * picks the route instead, which is the only lever available.
  */
-fun bindToWifi(context: Context, socket: Socket) {
-    val network = wifiNetwork(context) ?: return
+fun bindForTarget(context: Context, socket: Socket, target: String) {
+    val network = wifiNetwork(context)
+    if (network != null) {
         runCatching { network.bindSocket(socket) }
+        return
+    }
+    val source = localIpv4Addresses().firstOrNull { sameSubnet(it, target) } ?: return
+    runCatching { socket.bind(InetSocketAddress(InetAddress.getByName(source), 0)) }
+}
+
+/**
+ * Whether two IPv4 addresses are on the same /24.
+ *
+ * A guess, but a safe one here: every Android hotspot hands out a /24, and the
+ * only thing this decides is which of our own addresses to send from. Getting
+ * it wrong costs a bind that would not have helped anyway.
+ */
+fun sameSubnet(a: String, b: String): Boolean {
+    val left = a.split('.')
+    val right = b.split('.')
+    if (left.size != 4 || right.size != 4) return false
+    return left.subList(0, 3) == right.subList(0, 3)
 }
 
 /**

app/src/test/java/org/prolibertate/games/net/ManualAddressTest.kt

@@ -1,7 +1,9 @@
 package org.prolibertate.games.net
 
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
 import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
 import org.junit.Test
 
 /**
@@ -103,4 +105,32 @@ class ManualAddressTest {
         val shown = HostEndpoint(listOf("192.168.43.1"), 47654).primary!!
         assertEquals(ManualAddress("192.168.43.1", 47654), parseManualAddress(shown))
     }
+
+    // -- Choosing which of our own addresses to send from --------------------
+
+    /**
+     * The phone sharing the hotspot has no Wi-Fi network to bind to, so it
+     * picks a source address on the guest's subnet instead. Getting this wrong
+     * sends the connection out over mobile data.
+     */
+    @Test
+    fun `addresses on one hotspot subnet match`() {
+        assertTrue(sameSubnet("192.168.43.1", "192.168.43.117"))
+        assertTrue(sameSubnet("192.168.1.20", "192.168.1.20"))
+    }
+
+    @Test
+    fun `addresses on different subnets do not`() {
+        assertFalse(sameSubnet("192.168.43.1", "192.168.1.5"))
+        assertFalse(sameSubnet("192.168.43.1", "10.0.0.5"))
+    }
+
+    /** Anything that is not a dotted quad is not a match, rather than a crash. */
+    @Test
+    fun `nonsense is not a match`() {
+        assertFalse(sameSubnet("", "192.168.43.1"))
+        assertFalse(sameSubnet("192.168.43", "192.168.43.1"))
+        assertFalse(sameSubnet("pixel-9.local", "192.168.43.1"))
+        assertFalse(sameSubnet("192.168.43.1.7", "192.168.43.1"))
+    }
 }