fix(android): cast from the Wi-Fi the phone is on, not its hotspotHEADmain

Android 17 lists the phone's own hotspot as a Wi-Fi network; taken first,
the television was handed an address it cannot reach.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

3 files changed, 61 insertions, 4 deletions

diff --git a/packages/meshbay-android/app/src/main/kotlin/org/meshbay/client/cast/CastChannels.kt b/packages/meshbay-android/app/src/main/kotlin/org/meshbay/client/cast/CastChannels.kt
index edaec67a..ced0ed3b 100644
--- a/packages/meshbay-android/app/src/main/kotlin/org/meshbay/client/cast/CastChannels.kt
+++ b/packages/meshbay-android/app/src/main/kotlin/org/meshbay/client/cast/CastChannels.kt
@@ -134,19 +134,38 @@ class CastChannels(
      * The Wi-Fi (or Ethernet) address, never the mobile network's: a TV cannot
      * be reached there. Nor a VPN's: a VPN network carries the transports of
      * the network under it, Wi-Fi included, and its address is a tunnel the
-     * television cannot route to.
+     * television cannot route to. Nor the phone's own hotspot when it is
+     * joined to a Wi-Fi: Android 17 lists the hotspot to applications as a
+     * Wi-Fi network of its own, local only, and taken first it handed the
+     * television an address on a network it is not on (`pickLan`).
      */
     private fun lanAddress(): InetAddress? {
         val cm = context.getSystemService(ConnectivityManager::class.java)
+        val active = cm.activeNetwork
+        val found = ArrayList<Lan>()
         for (network in cm.allNetworks) {
             val caps = cm.getNetworkCapabilities(network) ?: continue
             if (caps.hasTransport(NetworkCapabilities.TRANSPORT_VPN)) continue
             if (!caps.hasTransport(NetworkCapabilities.TRANSPORT_WIFI) &&
                 !caps.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET)) continue
             val addr = cm.getLinkProperties(network)?.linkAddresses?.map { it.address }
-                ?.firstOrNull { it is Inet4Address && !it.isLoopbackAddress }
-            if (addr != null) return addr
+                ?.firstOrNull { it is Inet4Address && !it.isLoopbackAddress } ?: continue
+            found.add(Lan(addr, network == active,
+                caps.hasCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)))
         }
-        return null
+        return pickLan(found)
+    }
+
+    /** A candidate network for the relay: its address, and what says it is the one the phone is on. */
+    class Lan(val address: InetAddress, val active: Boolean, val internet: Boolean)
+
+    companion object {
+        /**
+         * The network the phone uses, then one with Internet, then any: a
+         * home Wi-Fi without Internet still casts, a hotspot is the last resort.
+         */
+        fun pickLan(candidates: List<Lan>): InetAddress? =
+            (candidates.firstOrNull { it.active } ?: candidates.firstOrNull { it.internet }
+                ?: candidates.firstOrNull())?.address
     }
 }
diff --git a/packages/meshbay-android/app/src/test/kotlin/org/meshbay/client/CastLanTest.kt b/packages/meshbay-android/app/src/test/kotlin/org/meshbay/client/CastLanTest.kt
new file mode 100644
index 00000000..8bdeed15
--- /dev/null
+++ b/packages/meshbay-android/app/src/test/kotlin/org/meshbay/client/CastLanTest.kt
@@ -0,0 +1,29 @@
+package org.meshbay.client
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import org.meshbay.client.cast.CastChannels.Companion.pickLan
+import org.meshbay.client.cast.CastChannels.Lan
+import java.net.InetAddress
+
+/** Which address the relay is offered on, when the phone has more than one Wi-Fi. */
+class CastLanTest {
+    private val hotspot = Lan(InetAddress.getByName("10.105.183.74"), active = false, internet = false)
+    private val home = Lan(InetAddress.getByName("192.168.1.13"), active = true, internet = true)
+
+    @Test fun theWifiThePhoneIsOnWinsOverItsOwnHotspotListedFirst() =
+        assertEquals(home.address, pickLan(listOf(hotspot, home)))
+
+    @Test fun onMobileDataAWifiWithInternetWins() {
+        val wifi = Lan(InetAddress.getByName("192.168.1.13"), active = false, internet = true)
+        assertEquals(wifi.address, pickLan(listOf(hotspot, wifi)))
+    }
+
+    @Test fun aWifiWithoutInternetStillCasts() {
+        val offline = Lan(InetAddress.getByName("192.168.50.2"), active = false, internet = false)
+        assertEquals(offline.address, pickLan(listOf(offline)))
+    }
+
+    @Test fun noWifiNoRelay() = assertNull(pickLan(emptyList()))
+}
diff --git a/packages/meshbay-hub/tests/test_android_cast.py b/packages/meshbay-hub/tests/test_android_cast.py
index 82b36d61..da8b912b 100644
--- a/packages/meshbay-hub/tests/test_android_cast.py
+++ b/packages/meshbay-hub/tests/test_android_cast.py
@@ -93,6 +93,15 @@ def test_the_relay_is_on_wifi_never_the_mobile_network():
     assert "TRANSPORT_CELLULAR" not in lan
 
 
+def test_the_relay_prefers_the_wifi_the_phone_is_on():
+    """Android 17 lists the phone's own hotspot as a Wi-Fi network; taken first,
+    the television was handed an address on a network it is not on."""
+    channels = _read(CAST / "CastChannels.kt")
+    lan = channels.split("private fun lanAddress()", 1)[1]
+    assert "activeNetwork" in lan and "NET_CAPABILITY_INTERNET" in lan
+    assert "return pickLan(found)" in lan
+
+
 def test_the_receiver_is_pointed_at_the_relay_and_nothing_else():
     channels = _read(CAST / "CastChannels.kt")
     check = channels.split("private fun relayUrl(", 1)[1].split("\n    }", 1)[0]