url@ doesn't import tcp^

[LiberalArtist]

The documentation claims that url@ from net/url-unit imports tcp^, but this appears not to have been true since c69253c#diff-c932338aaf3ee8d02284a58f1e67c58a. This means you can't link against your own unit to replace the normal TCP interface, defeating most of the point of url@.

On the one hand, the fact that no one seems to have noticed this in eight years suggests that it hasn't been a problem for existing users of url@, and the docs do also claim that url@ "will likely be removed in the future" (though that seems very unusual for Racket). On the other hand, it doesn't seem like this was intended to be a breaking change, and the difference from the documented behavior is clear. More broadly, I don't see another equivalently-powerful way to interpose on the transport layer while re-using the HTTP implementation. (In particular, there isn't a http-client@.)

As an example, this program should raise an exception, but instead it prints the Racket homepage:

#lang racket

(require net/url-unit
         net/tcp-sig)

(define-unit custom-tcp@
  (import)
  (export tcp^)
  (define-syntax-rule (define-got-here name ...)
    (begin (define (name . args)
             (error 'name "got here"))
           ...))
  (define-got-here
    tcp-listen tcp-connect tcp-connect/enable-break
    tcp-accept tcp-accept/enable-break tcp-accept-ready?
    tcp-close tcp-listener? tcp-abandon-port tcp-addresses))
    
(define-values/invoke-unit/infer
  (link url@ custom-tcp@))

(displayln
 (call/input-url (string->url "https://racket-lang.org")
                 get-pure-port
                 port->string))