[Bug] Package-level urlParamFn overwritten on each NewAppWith() call — data race with multiple App instances

[ftery0]

Problem

urlParamFn is a package-level variable in core/app.go that gets overwritten every time NewAppWith() is called:

// core/app.go
var urlParamFn = func(r *http.Request, key string) string {
    return chi.URLParam(r, key)
}

func NewAppWith(adapter RouterAdapter) *App {
    ...
    urlParamFn = adapter.URLParam // overwrites the package-level var
    ...
}

func URLParam(r *http.Request, key string) string {
    return urlParamFn(r, key) // reads the package-level var
}

If two App instances are created with different adapters, the second call to NewAppWith() silently overwrites the urlParamFn used by the first instance. Parallel tests that each create their own App will trigger a data race detectable with go test -race.

Impact

• Running go test -race ./... on a test suite with multiple App instances reports a data race on urlParamFn
• The first app's URLParam silently delegates to the wrong adapter after the second app is created

Proposed Fix

Store urlParamFn on the App struct instead of as a package-level variable, and make URLParam a method rather than a free function — or pass the resolver through the request context.

Environment

• File: core/app.go:41, 64