IMAP: don't spam the logs about non-SSL connections to localhost

When using IMAP as auth module every single request causes a warning to
be logged just because we're not encrypting traffic sent to another
process on the *same* machine.

This change recognizes that while some people might consider this
undesirable, others might have made this a conscious choice and *don't*
wish to be spammed for it. As such now only a single warning is logged
(the first time in the server's lifetime that a user logs in).

Signed-off-by: Giel van Schijndel <me@mortis.eu>

radicale/auth/IMAP.py

@@ -40,9 +40,12 @@ IMAP_SERVER = config.get("auth", "imap_hostname")
 IMAP_SERVER_PORT = config.getint("auth", "imap_port")
 IMAP_USE_SSL = config.getboolean("auth", "imap_ssl")
 
+IMAP_WARNED_UNENCRYPTED = False
 
 def is_authenticated(user, password):
     """Check if ``user``/``password`` couple is valid."""
+    global IMAP_WARNED_UNENCRYPTED
+
     if not user or not password:
         return False
 
@@ -72,7 +75,8 @@ def is_authenticated(user, password):
                 "IMAP server at %s failed to accept TLS connection "
                 "because of: %s" % (IMAP_SERVER, exception))
 
-    if server_is_local and not connection_is_secure:
+    if server_is_local and not connection_is_secure and not IMAP_WARNED_UNENCRYPTED:
+        IMAP_WARNED_UNENCRYPTED = True
         log.LOGGER.warning(
             "IMAP server is local. "
             "Will allow transmitting unencrypted credentials.")