Do it without a new flag

This is safer against other conditions where _deque ends up with items when we
start a scan - for example, if we chunk up deque processing.

src/allmydata/frontends/magic_folder.py

@@ -950,10 +950,6 @@ class QueueMixin(HookMixin):
         """
         action = PROCESSING_LOOP(**self._log_fields)
 
-        # Keep track of the fact that we're just starting up and we can and
-        # should skip a collective scan for one iteration.
-        self._startup_iteration = True
-
         # Note that we don't put the processing iterations into the logging
         # action because we expect this loop to run for the whole lifetime of
         # the process.  The tooling for dealing with incomplete action trees
@@ -977,19 +973,17 @@ class QueueMixin(HookMixin):
         with action.context():
             d = DeferredContext(defer.Deferred())
 
-            if self._startup_iteration:
-                # During startup we scanned the collective for items to
-                # download.  We do not need to perform another scan before
-                # processing our work queue.  More importantly, the logic for
-                # determining which items to download is *not correct* in the
-                # case where two scans are performed with no intermediate
-                # emptying of the work queue.  Therefore, skip the scan in the
-                # first processing iteration.  Either there will be work in
-                # the queue from the initial scan or not.  Either way, when we
-                # get here again on the next iteration, we'll go the other way
-                # and perform a scan.
-                self._startup_iteration = False
-            else:
+            # During startup we scanned the collective for items to
+            # download.  We do not need to perform another scan before
+            # processing our work queue.  More importantly, the logic for
+            # determining which items to download is *not correct* in the
+            # case where two scans are performed with no intermediate
+            # emptying of the work queue.  Therefore, skip the scan in the
+            # first processing iteration.  Either there will be work in
+            # the queue from the initial scan or not.  Either way, when we
+            # get here again on the next iteration, we'll go the other way
+            # and perform a scan.
+            if not self._deque:
                 # adds items to our deque
                 d.addCallback(lambda ignored: self._perform_scan())