Probe socket errors via SO_ERROR in TCPSocketProbe

bundler/lib/bundler/mirror.rb

@@ -160,18 +160,18 @@ def replies?(mirror)
 
       def wait_for_writtable_socket(socket, address, timeout)
         if IO.select(nil, [socket], nil, timeout)
-          probe_writtable_socket(socket, address)
+          probe_writtable_socket(socket)
         else # TCP Handshake timed out, or there is something dropping packets
           false
         end
       end
 
-      def probe_writtable_socket(socket, address)
-        socket.connect_nonblock(address)
-      rescue Errno::EISCONN
-        true
-      rescue StandardError # Connection failed
-        false
+      def probe_writtable_socket(socket)
+        # Check the pending error on the socket rather than calling
+        # +connect_nonblock+ a second time. On BSD-based systems such as macOS a
+        # second connect returns +EISCONN+ even when the asynchronous connect
+        # failed, which would make a down mirror look reachable.
+        socket.getsockopt(Socket::SOL_SOCKET, Socket::SO_ERROR).int == 0
       end
     end
   end