Mention issue with more than one thread and reduce workers

Due to how it appears the filesystem transport in kombu is
not thread-safe we will work around this by not having more
than one worker active at the same time in this example.

Oddly it appears the memory transport is unaffected (but
from looking at the code it doesn't look safe either), this
may just be due to how the python memory model works though.

Part of blueprint more-examples

Change-Id: Idaf04fb1a6a622af292511bbcf25329c9a5aab53

taskflow/examples/wbe_simple_linear.py

@@ -53,7 +53,12 @@ USE_FILESYSTEM = False
 BASE_SHARED_CONF = {
     'exchange': 'taskflow',
 }
-WORKERS = 2
+
+# Until https://github.com/celery/kombu/issues/398 is resolved it is not
+# recommended to run many worker threads in this example due to the types
+# of errors mentioned in that issue.
+MEMORY_WORKERS = 2
+FILE_WORKERS = 1
 WORKER_CONF = {
     # These are the tasks the worker can execute, they *must* be importable,
     # typically this list is used to restrict what workers may execute to
@@ -90,6 +95,7 @@ if __name__ == "__main__":
 
     tmp_path = None
     if USE_FILESYSTEM:
+        worker_count = FILE_WORKERS
         tmp_path = tempfile.mkdtemp(prefix='wbe-example-')
         shared_conf.update({
             'transport': 'filesystem',
@@ -100,6 +106,7 @@ if __name__ == "__main__":
             },
         })
     else:
+        worker_count = MEMORY_WORKERS
         shared_conf.update({
             'transport': 'memory',
             'transport_options': {
@@ -115,8 +122,8 @@ if __name__ == "__main__":
 
     try:
         # Create a set of workers to simulate actual remote workers.
-        print('Running %s workers.' % (WORKERS))
-        for i in range(0, WORKERS):
+        print('Running %s workers.' % (worker_count))
+        for i in range(0, worker_count):
             worker_conf['topic'] = 'worker-%s' % (i + 1)
             worker_topics.append(worker_conf['topic'])
             w = worker.Worker(**worker_conf)