Nine end-to-end tests for the proxy feature: 6 network tests exercising
every proxy code path through a real hyper forward proxy (TestProxy) and
a mockito origin, plus 3 CLI tests verifying flag parsing and error handling.
TestProxy binds to 127.0.0.1:0, forwards in absolute-form, counts requests
via an atomic so we can assert traffic actually traversed the proxy.
Key issues resolved during implementation:
- ENV_MUTEX serializes all tests that mutate HTTPS_PROXY/HTTP_PROXY in both
the unit test module and the integration suite. Without it, parallel test
execution within a single binary produces nondeterministic failures.
- reqwest's blocking::Client owns an internal tokio Runtime. Dropping it
inside a #[tokio::test] async fn panics on tokio >= 1.49. All reqwest
work runs inside spawn_blocking so the Client drops on a thread-pool
thread where that's permitted.
- service_fn's closure can't carry a return-type annotation, and async
blocks don't support one either. The handler is extracted to a named
async fn (proxy_handler) so the compiler can see the concrete
Result<Response, Infallible> and satisfy serve_connection's Error bound.
