SSH holds onto some sessions.

Summary

If you open and close a connection really fast it seems to hold onto lots of them in IoServiceListenerSupport.
In the wild this is sometimes caused by haproxy health checks.

Steps to Reproduce

The problem can be reproduced running this python script:

import socket
import sys

TCP_IP = '127.0.0.1'
TCP_PORT = 7999

def makeTestConnection():
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.connect((TCP_IP, TCP_PORT))
        s.close()

numConnections = 10000
if len(sys.argv) == 2:
        numConnections = int(sys.argv[1])

for i in range(numConnections):
        print(str(i))
        makeTestConnection()

Expected Results

The session object number is small.

Actual Results

If you take a heap dump afterwards you will see lots of session objects.
There seems to be a 2 minute auth timeout that gets hit and some of the sessions are removed but the majority are not removed.
Creating 10000 connections in seems to result in around 7000 of them hanging around.

Eclipse Memory Analyzer output:

Class Name                                      | Objects | Shallow Heap | Retained Heap
-----------------------------------------------------------------------------------------
org.apache.sshd.server.session.ServerSession    |   6,841 |    1,860,752 |              
org.apache.sshd.common.io.nio2.Nio2Session      |   6,841 |      492,552 |              
org.apache.sshd.common.future.DefaultCloseFuture|  13,685 |      437,920 |              
org.apache.sshd.common.util.Buffer              |   6,841 |      164,184 |              
-----------------------------------------------------------------------------------------

This condition could lead to an OutOfMemoryError.

This problem has been raised against Apache SSHD-417 - Memory leak in IoServiceListenerSupport.managedSessions when clients cut connections quickly, which has now been resolved.