RedisBroker: Leave existing max_concurrency values alone

- Leave already-set concurrency values alone, always.
- Fix some unit tests that were throwing `InvalidJobSignatureError` errors
- Fix incompatibilties with tox >= 4.0

Fixes: Issue #27

spinach/brokers/redis_scripts/set_concurrency_keys.lua

@@ -19,11 +19,10 @@ for i=3, #ARGV do
     if max_concurrency ~= -1 then
         new_task_names[task["name"]] = true
 
-        -- Override max_concurrency whatever it is already set to, if
-        -- anything.
-        redis.call('hset', max_concurrency_key, task["name"], max_concurrency)
-        -- Check to see if current_concurrency exists before initialising
-        -- it.
+        -- Check to see if concurrency keys exist before initialising them.
+        if redis.call('hexists', current_concurrency_key, task["name"]) == 0 then
+            redis.call('hset', max_concurrency_key, task["name"], max_concurrency)
+        end
         if redis.call('hexists', current_concurrency_key, task["name"]) == 0 then
             redis.call('hset', current_concurrency_key, task["name"], 0)
         end

tests/test_engine.py

@@ -56,14 +56,14 @@ def test_schedule_at(patch_now):
     s = Engine(broker, namespace='tests')
     s.attach_tasks(tasks)
 
-    job = s.schedule_at('bar_task', now, three=True)
+    job = s.schedule_at('bar_task', now)
 
     bar_job = broker.enqueue_jobs.call_args[0][0][0]
     assert bar_job == job
     assert bar_job.task_name == 'bar_task'
     assert bar_job.at == now
     assert bar_job.task_args == ()
-    assert bar_job.task_kwargs == {'three': True}
+    assert bar_job.task_kwargs == {}
 
 
 def test_schedule(patch_now):
@@ -101,7 +101,7 @@ def test_schedule_batch(patch_now):
 
     batch = Batch()
     batch.schedule('foo_task', 1, 2)
-    batch.schedule_at('bar_task', now, three=True)
+    batch.schedule_at('bar_task', now)
     jobs = s.schedule_batch(batch)
 
     broker.enqueue_jobs.assert_called_once_with([ANY, ANY])
@@ -118,7 +118,7 @@ def test_schedule_batch(patch_now):
     assert bar_job.task_name == 'bar_task'
     assert bar_job.at == now
     assert bar_job.task_args == ()
-    assert bar_job.task_kwargs == {'three': True}
+    assert bar_job.task_kwargs == {}
 
 
 def test_execute(spin):

tox.ini

@@ -7,7 +7,7 @@ envdir =
     py3: {toxworkdir}/py3
     pep8: {toxworkdir}/py3
 usedevelop = True
-whitelist_externals =
+allowlist_externals =
     docker-compose
 deps =
     pytest