Impl Default for CompressionBody when possible

[Ameobea]

This allows interop with tonic::transport::Server and tonic::transport::server::Router::serve_with_shutdown which require that the responses implement Default.

EDIT: It turns out that this Default requirement was actually coming from CorsMiddleware from this crate.

I was able to work around this by switching the order of the middleware to put CompressionLayer before CorsLayer. It's up to you all if you still want to merge this, but yeah.

Motivation

When trying to use CompressionLayer in a tonic::transport::Server, I was unable to due to missing Default implementation on the CompressionBody.

error[E0277]: the trait bound `CompressionBody<UnsyncBoxBody<bytes::Bytes, tonic::Status>>: std::default::Default` is not satisfied
   --> control_plane/src/server.rs:158:10
    |
158 |         .serve_with_shutdown(addr, shutdown_signal())
    |          ^^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` is not implemented for `CompressionBody<UnsyncBoxBody<bytes::Bytes, tonic::Status>>`
    |
    = help: the trait `Service<http::Request<hyper::Body>>` is implemented for `foundation::middleware::request_id::RequestIdMiddleware<S>`
    = note: required for `tower_http::cors::Cors<Compression<Routes, OnlyCompressGrpcWebPredicate>>` to implement `Service<http::Request<hyper::Body>>`
    = note: 2 redundant requirements hidden
    = note: required for `foundation::middleware::request_id::RequestIdMiddleware<foundation::middleware::logging::LoggingMiddleware<tower_http::propagate_header::PropagateHeader<tower_http::cors::Cors<Compression<Routes, OnlyCompressGrpcWebPredicate>>>>>` to implement `Service<http::Request<hyper::Body>>`

Solution

This implements a trivial Default for CompressionBody when the inner Body also implements Default.

[davidpdrsn]

I think this is probably fine but tower-http's compression wont do anything for tonic/gRPC since gRPC has its own compression scheme, which tonic does support.

[Ameobea]

We have a special situation since we're using grpc-web to serve a gRPC server for use by browser clients.

grpc-web doesn't support the native gRPC compression, and expects you to compress at the HTTP level. So we're using a custom tower_http::compression::Predicate to force compression in the case that content_type.starts_with("application/grpc-web").