Make GmailFolder sync GMail labels

When synclabels config flag is set to "yes" for the GMail repo,
offlineimap fetches the message labels along with the messages, and
embeds them into the body under the header X-Keywords (or whatever
'labelsheader' was set to), as a comma separated list.

It also adds an extra pass to savemessageto, that performs label
synchronization on existing messages from GMail to local, the same way
it is done with flags.

We also introduce GmailMaildir repository that adds functionality to
change message labels.  It keeps track of messages modification time,
so one can quickly detect when the labels may have changed.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/Maildir.py

@@ -19,6 +19,7 @@ import socket
 import time
 import re
 import os
+import tempfile
 from .Base import BaseFolder
 from threading import Lock
 
@@ -234,7 +235,7 @@ class MaildirFolder(BaseFolder):
         filepath = os.path.join(self.getfullname(), filename)
         return os.path.getmtime(filepath)
 
-    def __new_message_filename(self, uid, flags=set()):
+    def new_message_filename(self, uid, flags=set()):
         """Creates a new unique Maildir filename
 
         :param uid: The UID`None`, or a set of maildir flags
@@ -245,6 +246,52 @@ class MaildirFolder(BaseFolder):
             (timeval, timeseq, os.getpid(), socket.gethostname(),
              uid, self._foldermd5, self.infosep, ''.join(sorted(flags)))
 
+
+    def save_to_tmp_file(self, filename, content):
+        """
+        Saves given content to the named temporary file in the
+        'tmp' subdirectory of $CWD.
+
+        Arguments:
+        - filename: name of the temporary file;
+        - content: data to be saved.
+
+        Returns: relative path to the temporary file
+        that was created.
+
+        """
+
+        tmpname = os.path.join('tmp', filename)
+        # open file and write it out
+        tries = 7
+        while tries:
+            tries = tries - 1
+            try:
+                fd = os.open(os.path.join(self.getfullname(), tmpname),
+                             os.O_EXCL|os.O_CREAT|os.O_WRONLY, 0o666)
+                break
+            except OSError as e:
+                if e.errno == e.EEXIST:
+                    if tries:
+                        time.slep(0.23)
+                        continue
+                    severity = OfflineImapError.ERROR.MESSAGE
+                    raise OfflineImapError("Unique filename %s already exists." % \
+                      filename, severity)
+                else:
+                    raise
+
+        fd = os.fdopen(fd, 'wt')
+        fd.write(content)
+        # Make sure the data hits the disk
+        fd.flush()
+        if self.dofsync:
+            os.fsync(fd)
+        fd.close()
+
+        return tmpname
+
+
     # Interface from BaseFolder
     def savemessage(self, uid, content, flags, rtime):
         """Writes a new message, with the specified uid.
@@ -267,33 +314,12 @@ class MaildirFolder(BaseFolder):
         # Otherwise, save the message in tmp/ and then call savemessageflags()
         # to give it a permanent home.
         tmpdir = os.path.join(self.getfullname(), 'tmp')
-        messagename = self.__new_message_filename(uid, flags)
-        # open file and write it out
-        try:
-            fd = os.open(os.path.join(tmpdir, messagename),
-                           os.O_EXCL|os.O_CREAT|os.O_WRONLY, 0o666)
-        except OSError as e:
-            if e.errno == 17:
-                #FILE EXISTS ALREADY
-                severity = OfflineImapError.ERROR.MESSAGE
-                raise OfflineImapError("Unique filename %s already existing." %\
-                                           messagename, severity)
-            else:
-                raise
-
-        file = os.fdopen(fd, 'wt')
-        file.write(content)
-        # Make sure the data hits the disk
-        file.flush()
-        if self.dofsync:
-            os.fsync(fd)
-        file.close()
-
+        messagename = self.new_message_filename(uid, flags)
+        tmpname = self.save_to_tmp_file(messagename, content)
         if rtime != None:
-            os.utime(os.path.join(tmpdir, messagename), (rtime, rtime))
+            os.utime(os.path.join(self.getfullname(), tmpname), (rtime, rtime))
 
-        self.messagelist[uid] = {'flags': flags,
-                                 'filename': os.path.join('tmp', messagename)}
+        self.messagelist[uid] = {'flags': flags, 'filename': tmpname}
         # savemessageflags moves msg to 'cur' or 'new' as appropriate
         self.savemessageflags(uid, flags)
         self.ui.debug('maildir', 'savemessage: returning uid %d' % uid)
@@ -357,7 +383,7 @@ class MaildirFolder(BaseFolder):
         dir_prefix, filename = os.path.split(oldfilename)
         flags = self.getmessageflags(uid)
         newfilename = os.path.join(dir_prefix,
-          self.__new_message_filename(new_uid, flags))
+          self.new_message_filename(new_uid, flags))
         os.rename(os.path.join(self.getfullname(), oldfilename),
                   os.path.join(self.getfullname(), newfilename))
         self.messagelist[new_uid] = self.messagelist[uid]