Implement more granular control over port number allocation

which can be achieved by passing a custom port allocator,
a callable function taking number of try as an argument and
returning next port candidate to try.

Provide a reference implementation of the RTP port allocator,
which takes range of ports and allocates even numbers in this
range.

sippy/Network_server.py

@@ -27,11 +27,12 @@
 from typing import Optional, Tuple, List, Union
 from queue import Queue
 from abc import ABC, abstractmethod
-from random import random
+from random import random, randint
 
 from sippy.Core.Exceptions import dump_exception
 from sippy.Time.MonoTime import MonoTime
 from sippy.Time.Timeout import Timeout
+from sippy.SipConf import MyPort
 
 class Remote_address():
     transport: str
@@ -47,7 +48,7 @@ def __str__(self):
         return f'{self.transport}:{self.address[0]}:{self.address[1]}'
 
 class Network_server_opts():
-    laddress: Optional[Tuple[str, int]] = None
+    laddress: Optional[Tuple[str, Union[int, callable, MyPort]]] = None
     data_callback: Optional[callable] = None
     direct_dispatch: bool = False
     ploss_out_rate: float = 0.0
@@ -124,3 +125,24 @@ def shutdown(self):
         self.sendqueue.put(None)
         self.join()
         self.uopts.data_callback = None
+
+class PortAllocationError(Exception): pass
+
+class RTP_port_allocator():
+    min_port: int
+    max_port: int
+
+    def __init__(self, min_port:int = 1024, max_port:int = 65535):
+        if min_port % 2 != 0:
+            min_port += 1
+        assert min_port <= max_port, f'min_port={min_port} > max_port={max_port}'
+        self.min_port = min_port
+        self.max_port = max_port
+
+    def __call__(self, ntry: int) -> int:
+        rlen = self.max_port - self.min_port
+        if ntry > (rlen // 2):
+            raise PortAllocationError(f'No free ports available after {ntry} tries')
+        port = self.min_port + (randint(0, rlen // 2) * 2)
+        assert port <= self.max_port, f'port={port} > self.max_port={self.max_port}'
+        return port

sippy/Udp_server.py

@@ -32,7 +32,7 @@ class BrokenPipeError(Exception):
         pass
 
 from errno import ECONNRESET, ENOTCONN, ESHUTDOWN, EWOULDBLOCK, ENOBUFS, EAGAIN, \
-  EINTR, EBADF
+  EINTR, EBADF, EADDRINUSE
 from datetime import datetime
 from time import sleep, time
 from threading import Thread
@@ -41,7 +41,8 @@ class BrokenPipeError(Exception):
 
 from sippy.Core.EventDispatcher import ED2
 from sippy.Core.Exceptions import dump_exception
-from sippy.Network_server import Network_server_opts, Network_server, Remote_address
+from sippy.Network_server import Network_server_opts, Network_server, Remote_address, \
+  RTP_port_allocator
 from sippy.Time.Timeout import Timeout
 from sippy.Time.MonoTime import MonoTime
 from sippy.SipConf import MyPort
@@ -191,7 +192,19 @@ def __init__(self, global_config, uopts):
                 # TypeError: 'MyPort' object cannot be interpreted as an integer
                 # might be something inside socket.bind?
                 address = (address[0], int(address[1]))
-            self.skt.bind(address)
+            if not callable(address[1]):
+                self.skt.bind(address)
+            else:
+                ntry = -1
+                for ntry in iter(lambda: ntry + 1, -1):
+                    try_address = (address[0], address[1](ntry))
+                    try: self.skt.bind(try_address)
+                    except OSError as ex:
+                        if ex.errno != EADDRINUSE:
+                            raise
+                    else:
+                        self.uopts.laddress = try_address
+                        break
             if self.uopts.laddress[1] == 0:
                 self.uopts.laddress = self.skt.getsockname()
         self.asenders = []
@@ -278,28 +291,27 @@ def pong_received(self, data, ra, udp_server, rtime):
             ED2.breakLoop()
 
     def run(self):
+        palloc = RTP_port_allocator()
         local_host = '127.0.0.1'
         local_host6 = '[::1]'
-        remote_host = local_host
-        remote_host6 = local_host6
         self.ping_laddr = (local_host, 12345)
-        self.pong_laddr = (local_host, 54321)
-        self.ping_laddr6 = (local_host6, 12345)
+        self.pong_laddr = (local_host, palloc)
+        self.ping_laddr6 = (local_host6, 0)
         self.pong_laddr6 = (local_host6, 54321)
-        self.ping_raddr = (remote_host, 12345)
-        self.pong_raddr = (remote_host, 54321)
-        self.ping_raddr6 = (remote_host6, 12345)
-        self.pong_raddr6 = (remote_host6, 54321)
         uopts_ping = Udp_server_opts(self.ping_laddr, self.ping_received)
         uopts_ping6 = Udp_server_opts(self.ping_laddr6, self.ping_received)
         uopts_pong = Udp_server_opts(self.pong_laddr, self.pong_received)
         uopts_pong6 = Udp_server_opts(self.pong_laddr6, self.pong_received)
         udp_server_ping = Udp_server({}, uopts_ping)
         udp_server_pong = Udp_server({}, uopts_pong)
-        udp_server_pong.send_to(self.ping_data, self.ping_laddr)
+        self.ping_raddr = udp_server_ping.getSIPaddr()[0]
+        self.pong_raddr = udp_server_pong.getSIPaddr()[0]
+        udp_server_pong.send_to(self.ping_data, self.ping_raddr)
         udp_server_ping6 = Udp_server({}, uopts_ping6)
         udp_server_pong6 = Udp_server({}, uopts_pong6)
-        udp_server_pong6.send_to(self.ping_data6, self.ping_laddr6)
+        self.ping_raddr6 = udp_server_ping6.getSIPaddr()[0]
+        self.pong_raddr6 = udp_server_pong6.getSIPaddr()[0]
+        udp_server_pong6.send_to(self.ping_data6, self.ping_raddr6)
         ED2.loop()
         udp_server_ping.shutdown()
         udp_server_pong.shutdown()