flexnbd: Move building the allocation map to before server socket bind()

Building the allocation map takes time, which scales with the size of the disc
being presented. By building that map in the space between bind() and accept(),
we leave the process in a useless state after the only good signal we have for
"we are ready" and the state where it is actually ready. This was breaking
migrations of large files.

tests/acceptance/flexnbd.rb

@@ -314,9 +314,16 @@ module FlexNBD
 
       while !File.socket?(ctrl)
         pid, status = Process.wait2(@pid, Process::WNOHANG)
-        raise "server did not start (#{cmd})" if pid
+        raise "server did not start (#{cmd}) - UNIX socket didn't appear" if pid
         sleep 0.1
       end
+
+      while !socket_open?
+        pid, status = Process.wait2(@pid, Process::WNOHANG)
+        raise "server did not start (#{cmd}) - TCP socket didn't appear" if pid
+        sleep 0.1
+      end
+
       at_exit { kill }
     end
     private :run_serve_cmd
@@ -511,7 +518,15 @@ module FlexNBD
       hsh
     end
 
+    def socket_open?
+      sock = (TCPSocket.new(@ip, @port) rescue nil)
+      !!sock
+    ensure
+      sock.close rescue nil if sock
+    end
+
 
   end
 
 end
+