Leaving and re-entering a room can cause incorrect room state

[dmckeone]

It seems there is an issue with BaseManager.pending_removals getting stale and causing the removal of a user from a room they should be in.

Example (Imagine the various leave/enter/emit calls being spread out over time and multiple requests):

server = ... # Socket.IO Server
room = 'my room'
session_id = ... # Current session id for one user

# Enter room normally
server.enter_room(session_id, room)  

 # Leave room normally, server.manager.pending_removals populated
server.leave_room(session_id, room) 

# Re-enter room, server.manager.pending_removals still populated
server.enter_room(session_id, room)     

# Message emitted, followed by server.manager.pending_removals being processed internally
server.emit('something', 'data', room=room)     

# Message *NOT* emitted, because the room was removed after the previous emit
server.emit('something else', 'data', room=room)

The workaround was to do:

server = ... # Socket.IO Server
room = 'my room'
session_id = ... # Current session id for one user

# Enter room normally
server.enter_room(session_id, room)  

 # Leave room normally, server.manager.pending_removals populated
server.leave_room(session_id, room) 

# WORKAROUND: Force clean-up of old removals before entering the new room
server.manager._clean_rooms()

# Re-enter room, server.manager.pending_removals is now clear
server.enter_room(session_id, room)     

# Message emitted normally
server.emit('something', 'data', room=room)     

# Message emitted normally
server.emit('something else', 'data', room=room)

This leads me to believe that the fix is likely something that needs to be added to enter_room( ... ) that reconciles the new subscriptions to a given room with the previous, perhaps unreconciled, room evictions in BaseManager.pending_removals.