[PATCH v4 4/4] python/aqmp: add socket bind step to legacy.py

[John Snow <jsnow@redhat.com>]

The synchronous QMP library would bind to the server address during
__init__(). The new library delays this to the accept() call, because
binding occurs inside of the call to start_[unix_]server(), which is an
async method -- so it cannot happen during __init__ anymore.

Python 3.7+ adds the ability to create the server (and thus the bind()
call) and begin the active listening in separate steps, but we don't
have that functionality in 3.6, our current minimum.

Therefore ... Add a temporary workaround that allows the synchronous
version of the client to bind the socket in advance, guaranteeing that
there will be a UNIX socket in the filesystem ready for the QEMU client
to connect to without a race condition.

(Yes, it's a bit ugly. Fixing it more nicely will have to wait until our
minimum Python version is 3.7+.)

Signed-off-by: John Snow <jsnow@redhat.com>
---
 python/qemu/aqmp/legacy.py   |  3 +++
 python/qemu/aqmp/protocol.py | 41 +++++++++++++++++++++++++++++++++---
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/python/qemu/aqmp/legacy.py b/python/qemu/aqmp/legacy.py
index 0890f95b16..6baa5f3409 100644
--- a/python/qemu/aqmp/legacy.py
+++ b/python/qemu/aqmp/legacy.py
@@ -56,6 +56,9 @@ def __init__(self, address: SocketAddrT,
         self._address = address
         self._timeout: Optional[float] = None
 
+        if server:
+            self._aqmp._bind_hack(address)  # pylint: disable=protected-access
+
     _T = TypeVar('_T')
 
     def _sync(
diff --git a/python/qemu/aqmp/protocol.py b/python/qemu/aqmp/protocol.py
index 50e973c2f2..33358f5cd7 100644
--- a/python/qemu/aqmp/protocol.py
+++ b/python/qemu/aqmp/protocol.py
@@ -15,6 +15,7 @@
 from enum import Enum
 from functools import wraps
 import logging
+import socket
 from ssl import SSLContext
 from typing import (
     Any,
@@ -238,6 +239,9 @@ def __init__(self, name: Optional[str] = None) -> None:
         self._runstate = Runstate.IDLE
         self._runstate_changed: Optional[asyncio.Event] = None
 
+        # Workaround for bind()
+        self._sock: Optional[socket.socket] = None
+
     def __repr__(self) -> str:
         cls_name = type(self).__name__
         tokens = []
@@ -427,6 +431,34 @@ async def _establish_connection(
         else:
             await self._do_connect(address, ssl)
 
+    def _bind_hack(self, address: Union[str, Tuple[str, int]]) -> None:
+        """
+        Used to create a socket in advance of accept().
+
+        This is a workaround to ensure that we can guarantee timing of
+        precisely when a socket exists to avoid a connection attempt
+        bouncing off of nothing.
+
+        Python 3.7+ adds a feature to separate the server creation and
+        listening phases instead, and should be used instead of this
+        hack.
+        """
+        if isinstance(address, tuple):
+            family = socket.AF_INET
+        else:
+            family = socket.AF_UNIX
+
+        sock = socket.socket(family, socket.SOCK_STREAM)
+        sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+
+        try:
+            sock.bind(address)
+        except:
+            sock.close()
+            raise
+
+        self._sock = sock
+
     @upper_half
     async def _do_accept(self, address: SocketAddrT,
                          ssl: Optional[SSLContext] = None) -> None:
@@ -464,24 +496,27 @@ async def _client_connected_cb(reader: asyncio.StreamReader,
         if isinstance(address, tuple):
             coro = asyncio.start_server(
                 _client_connected_cb,
-                host=address[0],
-                port=address[1],
+                host=None if self._sock else address[0],
+                port=None if self._sock else address[1],
                 ssl=ssl,
                 backlog=1,
                 limit=self._limit,
+                sock=self._sock,
             )
         else:
             coro = asyncio.start_unix_server(
                 _client_connected_cb,
-                path=address,
+                path=None if self._sock else address,
                 ssl=ssl,
                 backlog=1,
                 limit=self._limit,
+                sock=self._sock,
             )
 
         server = await coro     # Starts listening
         await connected.wait()  # Waits for the callback to fire (and finish)
         assert server is None
+        self._sock = None
 
         self.logger.debug("Connection accepted.")
 
-- 
2.31.1