Fix deadlock for IMAP folder synced in singlethreaded mode

The problem lies in the fact that offlineimap.folder.Base's method
syncmessagesto_copy() uses threaded code everytime it is suggested by
the derived class's suggeststhreads() (currently, only IMAP does this
suggestion), but offlineimap/init.py will not spawn the
exitnotifymonitorloop() from offlineimap.threadutil.

The root cause is that ExitNotifyThread-derived threads need
offlineimap.threadutil's exitnotifymonitorloop() to be running the
cleaner for the exitthreads Queue(), because it fills the queue via
the run() method from this class: it wants to put() itself to the
Queue on exit, so when no exitnotifymonitorloop() is running, the
queue will fill up.  And if this thread is an instance of
InstanceLimitedThread that hits the limit on the number of threads,
then it will hold the instancelimitedsems[] semaphore will prevent
other InstanceLimitedThread()s of the same name to pass its start()
method.

The fix is to avoid using threaded code if we're running
single-threaded.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Obtained-from: X-Ryl669 <boite.pour.spam@gmail.com>

Changelog.rst

@@ -7,6 +7,8 @@ ChangeLog
 WIP (add new stuff for the next release)
 ========================================
 
+* Avoid lockups for IMAP synchronizations running with the
+  "-1" command-line switch (X-Ryl669 <boite.pour.spam@gmail.com>)
 * Dump stacktrace for all threads on SIGQUIT: ease debugging
   of threading and other issues
 * SIGHUP is now handled as the termination notification rather than

offlineimap/accounts.py

@@ -321,13 +321,16 @@ class SyncableAccount(Account):
                     self.ui.debug('', "Not syncing filtered folder '%s'"
                                  "[%s]" % (localfolder, localfolder.repository))
                     continue # Ignore filtered folder
-                thread = InstanceLimitedThread(\
-                    instancename = 'FOLDER_' + self.remoterepos.getname(),
-                    target = syncfolder,
-                    name = "Folder %s [acc: %s]" % (remotefolder, self),
-                    args = (self, remotefolder, quick))
-                thread.start()
-                folderthreads.append(thread)
+                if self.config.get('general', 'single-thread') == 'False':
+                    thread = InstanceLimitedThread(\
+                        instancename = 'FOLDER_' + self.remoterepos.getname(),
+                        target = syncfolder,
+                        name = "Folder %s [acc: %s]" % (remotefolder, self),
+                        args = (self, remotefolder, quick))
+                    thread.start()
+                    folderthreads.append(thread)
+                else:
+                    syncfolder(self, remotefolder, quick)
             # wait for all threads to finish
             for thr in folderthreads:
                 thr.join()
@@ -372,8 +375,7 @@ class SyncableAccount(Account):
             self.ui.error(e, exc_info()[2], msg = "Calling hook")
 
 def syncfolder(account, remotefolder, quick):
-    """This function is called as target for the
-    InstanceLimitedThread invokation in SyncableAccount.
+    """Synchronizes given remote folder for the specified account.
 
     Filtered folders on the remote side will not invoke this function."""
     remoterepos = account.remoterepos

offlineimap/folder/Base.py

@@ -403,7 +403,7 @@ class BaseFolder(object):
                 break
             self.ui.copyingmessage(uid, num+1, num_to_copy, self, dstfolder)
             # exceptions are caught in copymessageto()
-            if self.suggeststhreads():
+            if self.suggeststhreads() and self.config.get('general', 'single-thread') == 'False':
                 self.waitforthread()
                 thread = threadutil.InstanceLimitedThread(\
                     self.getcopyinstancelimit(),

offlineimap/init.py

@@ -251,6 +251,9 @@ class OfflineImap:
                 if type.lower() == 'imap':
                     imaplib.Debug = 5
 
+        # XXX: can we avoid introducing fake configuration item?
+        config.set_if_not_exists('general', 'single-thread', 'True' if options.singlethreading else 'False')
+
         if options.runonce:
             # FIXME: maybe need a better
             for section in accounts.getaccountlist(config):

offlineimap/threadutil.py

@@ -39,6 +39,8 @@ def semaphorereset(semaphore, originalstate):
         semaphore.release()
 
 class threadlist:
+    """Store the list of all threads in the software so it can be used to find out
+    what's running and what's not."""
     def __init__(self):
         self.lock = Lock()
         self.list = []