make portalocker really optional

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

offlineimap/accounts.py

@@ -35,6 +35,9 @@ SYNC_MUTEXES_LOCK = Lock()
 
 try:
     import portalocker
+except:
+    try:
+        import fcntl
     except:
         pass # Ok if this fails, we can do without.
 
@@ -234,8 +237,11 @@ class SyncableAccount(Account):
         try:
             portalocker.lock(self._lockfd, portalocker.LOCK_EX)
         except NameError:
-            #fcntl not available (Windows), disable file locking... :(
+            # portalocker not available for Windows.
-            pass
+            try:
+                fcntl.lockf(self._lockfd, fcntl.LOCK_EX|fcntl.LOCK_NB)
+            except NameError:
+                pass # fnctl not available, disable file locking... :(
         except IOError:
             self._lockfd.close()
             six.reraise(OfflineImapError,
@@ -250,7 +256,10 @@ class SyncableAccount(Account):
 
         #If we own the lock file, delete it
         if self._lockfd and not self._lockfd.closed:
+            try:
                 portalocker.unlock(self._lockfd)
+            except NameError:
+                pass
             self._lockfd.close()
             try:
                 os.unlink(self._lockfilepath)

requirements.txt

@@ -1,4 +1,4 @@
 # Requirements
 six
 gssapi[kerberos]
-portalocker
+portalocker[cygwin]