import/delegate-to foolscap's allocate_tcp_port

(instead of using a copy). Foolscap-0.12.3 fixes a problem with
allocate_tcp_port() that was causing intermittent test failures. I think
it makes more sense to use Foolscap's copy (and fixes) than to keep
re-copying it into Tahoe each time it changes.

If/when we manage to stop depending upon foolscap for server RPC, we can
re-copy this back into tahoe's source tree.

refs ticket:2795

src/allmydata/_auto_deps.py

@@ -39,7 +39,8 @@ install_requires = [
     # * foolscap 0.8.0 generates 2048-bit RSA-with-SHA-256 signatures,
     #   rather than 1024-bit RSA-with-MD5. This also allows us to work
     #   with a FIPS build of OpenSSL.
-    # * foolscap >= 0.12.3 provides tcp/tor/i2p connection handlers we need
+    # * foolscap >= 0.12.3 provides tcp/tor/i2p connection handlers we need,
+    #   and allocate_tcp_port
     "foolscap >= 0.12.3",
 
     # Needed for SFTP.

src/allmydata/util/iputil.py

@@ -1,5 +1,5 @@
 # from the Python Standard Library
-import os, sys, re, socket, subprocess, errno
+import os, re, socket, subprocess, errno
 
 from sys import platform
 
@@ -8,9 +8,10 @@ from twisted.internet import defer, threads, reactor
 from twisted.internet.protocol import DatagramProtocol
 from twisted.internet.error import CannotListenError
 from twisted.python.procutils import which
-from twisted.python.runtime import platformType
 from twisted.python import log
 
+from foolscap.util import allocate_tcp_port # re-exported
+
 try:
     import resource
     def increase_rlimits():
@@ -237,70 +238,9 @@ def _cygwin_hack_find_addresses():
 
     return defer.succeed(addresses)
 
-def allocate_tcp_port():
-    """Return an (integer) available TCP port on localhost. This briefly
-    listens on the port in question, then closes it right away."""
-
-    # Making this work correctly on multiple OSes is non-trivial:
-    # * on OS-X:
-    #   * Binding the test socket to 127.0.0.1 lets the kernel give us a
-    #     LISTEN port that some other process is using, if they bound it to
-    #     ANY (0.0.0.0). These will fail when we attempt to
-    #     listen(bind=0.0.0.0) ourselves
-    #   * Binding the test socket to 0.0.0.0 lets the kernel give us LISTEN
-    #     ports bound to 127.0.0.1, although then our subsequent listen()
-    #     call usually succeeds.
-    #   * In both cases, the kernel can give us a port that's in use by the
-    #     near side of an ESTABLISHED socket. If the process which owns that
-    #     socket is not owned by the same user as us, listen() will fail.
-    #   * Doing a listen() right away (on the kernel-allocated socket)
-    #     succeeds, but a subsequent listen() on a new socket (bound to
-    #     the same port) will fail.
-    # * on Linux:
-    #   * The kernel never gives us a port in use by a LISTEN socket, whether
-    #     we bind the test socket to 127.0.0.1 or 0.0.0.0
-    #   * Binding it to 127.0.0.1 does let the kernel give us ports used in
-    #     an ESTABLISHED connection. Our listen() will fail regardless of who
-    #     owns that socket. (note that we are using SO_REUSEADDR but not
-    #     SO_REUSEPORT, which would probably affect things).
-    #
-    # So to make this work properly everywhere, allocate_tcp_port() needs two
-    # phases: first we allocate a port (with 0.0.0.0), then we close that
-    # socket, then we open a second socket, bind the second socket to the
-    # same port, then try to listen. If the listen() fails, we loop back and
-    # try again.
-
-    # Ideally we'd refrain from doing listen(), to minimize impact on the
-    # system, and we'd bind the port to 127.0.0.1, to avoid making it look
-    # like we're accepting data from the outside world (in situations where
-    # we're going to end up binding the port to 127.0.0.1 anyways). But for
-    # the above reasons, neither would work. We *do* add SO_REUSEADDR, to
-    # make sure our lingering socket won't prevent our caller from opening it
-    # themselves in a few moments (note that Twisted's
-    # tcp.Port.createInternetSocket sets SO_REUSEADDR, among other flags).
-
-    count = 0
-    while True:
-        s = _make_socket()
-        s.bind(("0.0.0.0", 0))
-        port = s.getsockname()[1]
-        s.close()
-
-        s = _make_socket()
-        try:
-            s.bind(("0.0.0.0", port))
-            s.listen(5) # this is what sometimes fails
-            s.close()
-            return port
-        except socket.error:
-            s.close()
-            count += 1
-            if count > 100:
-                raise
-            # try again
-
-def _make_socket():
-    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-    if platformType == "posix" and sys.platform != "cygwin":
-        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-    return s
+__all__ = ["allocate_tcp_port",
+           "increase_rlimits",
+           "get_local_addresses_sync",
+           "get_local_addresses_async",
+           "get_local_ip_for",
+           ]