Two phones on one phone's hotspot could not find or reach each other.
Nothing was wrong with the Wi-Fi. There were three faults, and the first
one is the whole bug.
A hotspot has no internet behind it, and Android takes that literally:
the Wi-Fi network fails its connectivity check, stays unvalidated, and
mobile data remains the *default* network. Every socket opened without
naming a network is therefore routed out over cellular — where
192.168.43.1 means nothing. The join socket is now bound to the Wi-Fi
network before it connects, and the search for that network deliberately
does not ask whether it has internet, because asking is what breaks it.
Second, the manifest has asked for CHANGE_WIFI_MULTICAST_STATE since the
beginning, with a comment saying discovery needs it, and nothing ever
took the lock. Without it the radio drops multicast frames not addressed
to this device before the system sees them, and mDNS is multicast. Both
advertising and discovering now hold it.
Third, and not fixable: a phone sharing its own hotspot does not
reliably advertise over the tethering interface, whatever the other end
does. So the host now shows where it is and the joiner can type it in.
That path is offered always rather than kept back for when discovery has
failed, because discovery here does not fail loudly — it just never
finds anything, and a screen saying only "no games found" leaves nowhere
to go. Hosts take a known port when they can get one, so what gets typed
is an address and not an address and five more digits.
The lobby scrolls instead of filling. Its lazy lists took the whole
screen for a handful of rows and pushed the Start button off the bottom;
they would have done the same to the address box.
Parsing what somebody types off another phone's screen is the one pure
piece of this and is tested, including that what a host displays is
exactly what the other end accepts. The rest needs two phones and a
hotspot, which CI does not have.
Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0132V9gE6DF9itz2z3Hn7BK9
Claude
f8f02dd93e