Misuse of span.record in tracing middleware

[tombanksme]

Hello,
I was looking through the poem source code and noticed an issue in the tracing middleware.

The tracing docs say:

Note: The fields associated with a span are part of its Metadata. The Metadata describing a particular span is constructed statically when the span is created and cannot be extended later to add new fields. Therefore, you cannot record a value for a field that was not specified when the span was created:

Source

It appears that poem violates that rule in the tracing middleware.

I've not actually validated this is an issue; just something I ran into on another project.

I'm fairly new to Rust and the Tracing crate so I might be mistaken. I'm happy to raise a pull request for the fix.

[tombanksme]

Just had a chance to take a look and I don't think the PathPattern extraction for the tracing middleware works at all.

Using with(Tracing) creates a TracingEndpoint<Route> where TracingEndpoint::call is called first and then it passes the request onto Route::call. PathPattern is added to the request by Route::call so it doesn't exist when TracingEndpoint::call is called.

Does anyone have ideas of how we could fix this?

The only solution I can think of is to change the signature of Endpoint::call from fn call(&self, mut req: Request) to fn call(&self, &mut req: Request) so that we can read from the request after calling Route::call although I dread to think what other issues this change would cause.

[sunli829]

1. 'PathPattern' exists only with 'Route'.

poem/poem/src/route/router.rs

Line 339 in 0481e2e

let pattern = match matches.data.pattern.strip_suffix("/*--poem-rest") {

2. It just prepared span before, but actually use it here.

poem/poem/src/middleware/tracing_mw.rs

Line 109 in 0481e2e

.instrument(span)