From f137d8424e84591f37532d7e7c2dd7e5daedcd42 Mon Sep 17 00:00:00 2001
From: Alex Zorin <alex@zorin.id.au>
Date: Sat, 19 Jun 2021 09:55:02 +1000
Subject: [PATCH] acme.standalone: expose original socket.error

---
 acme/acme/standalone.py       | 12 ++++++++++--
 acme/tests/standalone_test.py | 16 +++++++++++-----
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/acme/acme/standalone.py b/acme/acme/standalone.py
index eda45304c..e2672fb94 100644
--- a/acme/acme/standalone.py
+++ b/acme/acme/standalone.py
@@ -8,6 +8,7 @@ import socket
 import socketserver
 import threading
 from typing import List
+from typing import Optional
 
 from acme import challenges
 from acme import crypto_util
@@ -66,6 +67,9 @@ class BaseDualNetworkedServers:
         self.threads: List[threading.Thread] = []
         self.servers: List[socketserver.BaseServer] = []
 
+        # Preserve socket error for re-raising, if no servers can be started
+        last_socket_err: Optional[socket.error] = None
+
         # Must try True first.
         # Ubuntu, for example, will fail to bind to IPv4 if we've already bound
         # to IPv6. But that's ok, since it will accept IPv4 connections on the IPv6
@@ -82,7 +86,8 @@ class BaseDualNetworkedServers:
                 logger.debug(
                     "Successfully bound to %s:%s using %s", new_address[0],
                     new_address[1], "IPv6" if ip_version else "IPv4")
-            except socket.error:
+            except socket.error as e:
+                last_socket_err = e
                 if self.servers:
                     # Already bound using IPv6.
                     logger.debug(
@@ -101,7 +106,10 @@ class BaseDualNetworkedServers:
                 # bind to the same port for both servers.
                 port = server.socket.getsockname()[1]
         if not self.servers:
-            raise socket.error("Could not bind to IPv4 or IPv6.")
+            if last_socket_err:
+                raise last_socket_err
+            else: # pragma: no cover
+                raise socket.error("Could not bind to IPv4 or IPv6.")
 
     def serve_forever(self):
         """Wraps socketserver.TCPServer.serve_forever"""
diff --git a/acme/tests/standalone_test.py b/acme/tests/standalone_test.py
index 17d73dba8..e0aa5aa22 100644
--- a/acme/tests/standalone_test.py
+++ b/acme/tests/standalone_test.py
@@ -190,12 +190,18 @@ class BaseDualNetworkedServersTest(unittest.TestCase):
 
     @mock.patch("socket.socket.bind")
     def test_fail_to_bind(self, mock_bind):
-        mock_bind.side_effect = socket.error
+        from errno import EADDRINUSE
         from acme.standalone import BaseDualNetworkedServers
-        self.assertRaises(socket.error, BaseDualNetworkedServers,
-                          BaseDualNetworkedServersTest.SingleProtocolServer,
-                          ('', 0),
-                          socketserver.BaseRequestHandler)
+
+        mock_bind.side_effect = socket.error(EADDRINUSE, "Fake addr in use error")
+
+        with self.assertRaises(socket.error) as em:
+            BaseDualNetworkedServers(
+                BaseDualNetworkedServersTest.SingleProtocolServer,
+                ('', 0), socketserver.BaseRequestHandler)
+
+        self.assertEqual(em.exception.errno, EADDRINUSE)
+
 
     def test_ports_equal(self):
         from acme.standalone import BaseDualNetworkedServers