Fix issues with DH parameters

- registry: make --dh mandatory
- node: retry if the registry returns nothing (instead of writing an empty file)

re6st/cache.py

-import json, logging, os, sqlite3, socket, subprocess, time, zlib
+import json, logging, os, sqlite3, socket, subprocess, sys, time, zlib
 from .registry import RegistryClient
 from . import utils, version, x509
 
@@ -129,16 +129,23 @@ class Cache(object):
                             " you should update.")
 
     def getDh(self, path):
+        # We'd like to do a full check here but
+        #   from OpenSSL import SSL
+        #   SSL.Context(SSL.TLSv1_METHOD).load_tmp_dh(path)
+        # segfaults if file is corrupted.
         if not os.path.exists(path):
             retry = 1
             while True:
                 try:
                     dh = self._registry.getDh(self._prefix)
+                    if dh:
                         break
-                except socket.error, e:
+                    e = None
+                except socket.error:
+                    e = sys.exc_info()
                 logging.warning(
                     "Failed to get DH parameters from the registry."
-                        " Will retry in %s seconds", retry, exc_info=1)
+                    " Will retry in %s seconds", retry, exc_info=e)
                 time.sleep(retry)
                 retry = min(60, retry * 2)
             with open(path, "wb") as f:

re6st/cli/registry.py

@@ -71,7 +71,7 @@ def main():
     _('--db', default='/var/lib/re6stnet/registry.db',
         help="Path to SQLite database file. It is automatically initialized"
              " if the file does not exist.")
-    _('--dh',
+    _('--dh', required=True,
         help="File containing Diffie-Hellman parameters in .pem format."
              " To generate them, you can use something like:\n"
              "openssl dhparam -out dh2048.pem 2048")