This allows more readable configuration files without additional
explanations in cases where a single module is used for multiple
purposes.
Also cleans up certain problems with modules that rely on block
names having certain semantics (e.g. endpoint modules).
Improper queue initialization by filling fields manually instead of
calling NewQueue resulted in Queue objects created in tests having
no workersStop channel (nil), making Queue.Close no-op. This caused
race-conditions because test code assumes that Queue.Close will wait
for queue goroutines to stop before returning.
Queue late-initialization logic is factored out into Queue.start
function. Now newTestQueueDir calls NewQueue, modifies fields of
returned object and calls Queue.start.
The name "context" is also used for several other entities. This is
simply confusing. We are getting rid of that fancy word and will leave
it only for context.Context.
Additionally, with the introduction of the new interfaces for most
stuff DeliveryContext struct is no longer used to pass arbitrary
data between modules, it now contains mostly message meta-data and
a few flags, though the latter is now discouraged because it may
lead to problems with concurrency.
Additionally, documentation for some fields in the structure was wrong.
The field DeliveryID is simply renamed to ID for clarity.
Since queue was too tightly coupled to the old interface. This commit
is basically the ground-up rewrite. I took this chance to add proper
tests and create a cleaner implementation in general.
Below is the list of small modifications to the overall queue design.
* Successful field is removed from PartialError since it turned out to
be redundant.
* testerLog utility is updated to be more useful with t.Parallel()
(used for queue log). Wrapping t.Log makes testing framework associate
log output with tests and output it in right order.
* Message header is now stored in a separate file. This is done instead
of implementing hacks for module.FileBuffer to skip header on each Open.
* Queue.Close now properly waits for workers to stop. It caused problems
with tests where I call Close while queue is attempting delivery.
* Previous failures are stored in QueueMetadata for future use in
bounce messages.
