Unfortunately, since the Batch wrapper was changed to have a generic error
type, when wrapping it in another Service, nothing constrains the error type,
so we have to specify it explicitly to avoid an inference hole. This is pretty
unergonomic -- from the compiler error message it's very unintuitive that the
right fix is to change `Batch::new` to `Batch::<_, _, SomeError>::new`.
The options are:
1. roll back the changes that make the error type generic, so that the error
type is a concrete type;
2. keep the error type generic but hardcode the error in the default
constructor and add an additional code path that allows overriding the
error.
However, there's a further issue with generic errors: the error type must be
Clone. This problem comes from the fact that there can be multiple Batch
handles that have to share access to errors generated by the inner Batch
worker, so there's not a way to work around this. However, almost all error
types aren't Clone, so there are fairly few error types that we would be
swapping in.
This suggests that in case (2) we would be maintaining extra code to allow
generic errors, but with restrictive enough generic bounds to make it
impractical to use generic error types. For this reason I think that (1) is a
better option.
This test doesn't compile in a way that reveals a problem with the design. The
verification service takes a `Request<'msg>` parameterized by the message
lifetime, and returns a future unconstrained by the message lifetime (it hashes
upfront to avoid requiring that `'msg` outlive `call`). But the `Batch`
middleware has the verification service working on its own task, so how can we
ensure that the message lives long enough to be read by the worker task?
The name "Buffer" is changed to "Batch" everywhere, and the worker task is rewritten.
Instead of having Worker implement Future directly, we have a consuming async run() function.
There's a lot of functional overlap between the batch design and tower-buffer's
existing internals, so we'll just vendor its source code and modify it.
If/when we upstream it, we can deduplicate common components.
Ramana Venkata
dependabot[bot]
Henry de Valence
Jane Lusby
Dimitris Apostolou
Jane Lusby