Allow to specify remote hostname even for the Gmail case

Previously we hard-coded the imap server name in the case of Gmail
repositories, but often we need a different host name. So, allow people
to specify the hostname via the regular "remotehosteval" and
"remotehost" settings, and only falling back to imap.gmail.com when
nothing has been specified.

Cache the hostname, so we don't evaluate the whole thing each time we
query the host name.

Make the remotehosteval processing more robust, by catching any
Exceptions that occur, and throw a OfflineImapError, that explains where
exactly the error had occured. You can test this, e.g. by setting
remotehosteval to 1/"n" or some other invalid expression.

The whole IMAP.gethost() function has been documented code wise while
going through.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/IMAP.py

@@ -17,7 +17,7 @@
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 
 from offlineimap.repository.Base import BaseRepository
-from offlineimap import folder, imaputil, imapserver
+from offlineimap import folder, imaputil, imapserver, OfflineImapError
 from offlineimap.folder.UIDMaps import MappedIMAPFolder
 from offlineimap.threadutil import ExitNotifyThread
 from threading import Event
@@ -32,6 +32,7 @@ class IMAPRepository(BaseRepository):
         """Initialize an IMAPRepository object."""
         BaseRepository.__init__(self, reposname, account)
         # self.ui is being set by the BaseRepository
+        self._host = None
         self.imapserver = imapserver.ConfigedIMAPServer(self)
         self.folders = None
         self.nametrans = lambda foldername: foldername
@@ -94,17 +95,34 @@ class IMAPRepository(BaseRepository):
         return self.imapserver.delim
 
     def gethost(self):
-        host = None
-        localeval = self.localeval
+        """Return the configured hostname to connect to
 
+        :returns: hostname as string or throws Exception"""
+        if self._host:  # use cached value if possible
+            return self._host
+
+        # 1) check for remotehosteval setting
         if self.config.has_option(self.getsection(), 'remotehosteval'):
             host = self.getconf('remotehosteval')
+            try:
+                host = self.localeval.eval(host)
+            except Exception, e:
+                raise OfflineImapError("remotehosteval option for repository "\
+                                       "'%s' failed:\n%s" % (self, e),
+                                       OfflineImapError.ERROR.REPO)
+            if host:
+                self._host = host
+                return self._host
+        # 2) check for plain remotehost setting
+        host = self.getconf('remotehost', None)
         if host != None:
-            return localeval.eval(host)
+            self._host = host
+            return self._host
 
-        host = self.getconf('remotehost')
-        if host != None:
-            return host
+        # no success
+        raise OfflineImapError("No remote host for repository "\
+                                   "'%s' specified." % self,
+                               OfflineImapError.ERROR.REPO)
 
     def getuser(self):
         user = None