port-write-handler is not reset after a run

[sorawee]

Paste the following code in a buffer and run it.

#lang racket/base

(let ([write-handler (port-write-handler (current-output-port))])
  (port-write-handler (current-output-port)
                      (λ (val port)
                        (write-handler "a" port))))

(write 1)

This prints "a" as expected. Now, paste the following code and then run it.

#lang racket/base

(write 1)

It still prints "a"!

---

``` ((alist-get 'racket-mode package-alist) #s(package-desc racket-mode (20190612 1928) "Major mode for Racket language." ((emacs (24 3)) (faceup (0 0 2))) nil nil "/Users/sorawee/.emacs.d/elpa/26.2/develop/racket-mode-20190612.1928" ((:url . "https://www.racket-mode.com/") (:maintainer "Greg Hendershott") (:authors ("Greg Hendershott"))) nil)) ((emacs-version "26.2") (emacs-uptime "5 days, 6 hours, 44 minutes, 24 seconds") (system-type darwin) (major-mode racket-mode) (racket--el-source-dir "/Users/sorawee/.emacs.d/elpa/26.2/develop/racket-mode-20190612.1928/") (racket--rkt-source-dir "/Users/sorawee/.emacs.d/elpa/26.2/develop/racket-mode-20190612.1928/racket/") (racket-program "racket") (racket-command-port 55555) (racket-command-startup nil) (racket-command-timeout 10) (racket-memory-limit 2048) (racket-error-context medium) (racket-retry-as-skeleton t) (racket-error-context medium) (racket-history-filter-regexp "\\`\\s *\\S ?\\S ?\\s *\\'") (racket-images-inline t) (racket-images-keep-last 100) (racket-use-repl-submit-predicate nil) (racket-images-system-viewer "display") (racket-pretty-print t) (racket-indent-curly-as-sequence t) (racket-indent-sequence-depth 0) (racket-pretty-lambda nil) (racket-smart-open-bracket-enable nil) (racket-module-forms "\\s(\\(?:module[*+]?\\|library\\)") (racket-logger-config ((cm-accomplice . warning) (GC . info) (module-prefetch . warning) (optimizer . info) (racket/contract . error) (sequence-specialization . info) (* . fatal))))) ```

[greghendershott]

Each racket-run gets a fresh custodian including a fresh dedicated thread for the REPL. As a result, parameters are reset. Therefore you expect port-write-handler to be reset. But it is not.

Is that correct?

I'm not very familiar with it, but I notice that the documentation for port-write-handler and friends doesn't actually use the word "parameter". Is it possible that it's a function that has a parameter-like signature, but is not actually a parameter? Many "handlers" in Racket are parameters; maybe this isn't?

Maybe you could research this (e.g. ask on Slack or mailing list) and report back?

[greghendershott]

For that recipe Dr Racket prints 1 not "a".

I don't know whether Dr Racket explicitly resets this handler. Instead it might be recreating the port (a special port for its GUI REPL). Whereas Racket Mode is reusing stdout a.k.a. current-output-port here -- so if you stick some handler on that port, it remains in effect because it's still the same port.

[greghendershott]

Like some other bugs you've filed recently, this seems to be an interesting edge case that seemingly no one else has encountered -- or at least reported -- in five years. As a result, although I guess I appreciate knowing about it, I'm not sure what action to take beyond the time already spent discussing this, because I'm not sure what is the cost:benefit ratio here.

[greghendershott]

Well this stuck in my brain overnight. I pushed a commit to address this.

I'm still not sure this is a great use of time, and, it's possible fixing this will cause some other, worse bug. But I pushed a commit to a topic branch. Maybe I will slow-roll actually merging it to master.