Per-account locking

Previously, we were simply locking offlineimap whenever it was
running. Howver there is no reason why we shouldn't be able to invoke it
in parallel, e.g. to synchronize several accounts in one offlineimap
each.

This patch implements the locking per-account, so that it is possible to
sync different accounts at the same time. If in refresh mode, we will
attempt to loop three times before giving up.

This also fixes Debian bug #586655

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/accounts.py

@@ -25,6 +25,11 @@ import os
 from sys import exc_info
 import traceback
 
+try:
+    import fcntl
+except:
+    pass # ok if this fails, we can do without
+
 def getaccountlist(customconfig):
     return customconfig.getsectionlist('Account')
 
@@ -159,6 +164,35 @@ class SyncableAccount(Account):
     functions :meth:`syncrunner`, :meth:`sync`, :meth:`syncfolders`,
     used for syncing."""
 
+    def __init__(self, *args, **kwargs):
+        Account.__init__(self, *args, **kwargs)
+        self._lockfd = None
+        self._lockfilepath = os.path.join(self.config.getmetadatadir(),
+                                          "%s.lock" % self)
+
+    def lock(self):
+        """Lock the account, throwing an exception if it is locked already"""
+        self._lockfd = open(self._lockfilepath, 'w')
+        try:
+            fcntl.lockf(self._lockfd, fcntl.LOCK_EX|fcntl.LOCK_NB)
+        except NameError:
+            #fcntl not available (Windows), disable file locking... :(
+            pass
+        except IOError:
+            self._lockfd.close()
+            raise OfflineImapError("Could not lock account %s." % self,
+                                   OfflineImapError.ERROR.REPO)
+
+    def unlock(self):
+        """Unlock the account, deleting the lock file"""
+        #If we own the lock file, delete it
+        if self._lockfd and not self._lockfd.closed:
+            self._lockfd.close()
+            try:
+                os.unlink(self._lockfilepath)
+            except OSError:
+                pass #Failed to delete for some reason.
+
     def syncrunner(self):
         self.ui.registerthread(self.name)
         self.ui.acct(self.name)
@@ -175,6 +209,7 @@ class SyncableAccount(Account):
         while looping:
             try:
                 try:
+                    self.lock()
                     self.sync()
                 except (KeyboardInterrupt, SystemExit):
                     raise
@@ -194,6 +229,7 @@ class SyncableAccount(Account):
                     if self.refreshperiod:
                         looping = 3
             finally:
+                self.unlock()
                 if looping and self.sleeper() >= 2:
                     looping = 0                    
                 self.ui.acctdone(self.name)