Tools.nrepl middleware-dependency sorting issues

[Gary Trakhman]

... inspired this workaround, is there a better way?

Essentially, :expects and :requires are breaking the sort order when they shouldn't, causing certain middlewares to just be in totally the wrong place.  I couldn't find the pattern after staring at for a bit.  Since leiningen calls linearize-middleware-stack internally, there's no more sensible way to work around this and compose with whatever other middlewares people might be using.

I might be able to scrap together a minimal test case..  what do you think?

(ns cider.nrepl.middleware.default

  "Default handler for convenience, works around a flawed dependency mechanism for now"

  (:require [clojure.tools.nrepl.middleware :as mw :refer [set-descriptor!]]

            [clojure.tools.nrepl.middleware.pr-values :refer [pr-values]]

            [clojure.tools.nrepl.middleware.session :refer [session]]

            [clojure.tools.nrepl.misc :refer [response-for]]

            [clojure.tools.nrepl.transport :as t]

            [cider.nrepl.middleware

             [info :as info]

             [complete :as complete]

             [stacktrace :as stacktrace]

             [classpath :as classpath]

             [inspect :as inspect]

             [trace :as trace]

             [exceptions :as exceptions]]

            [cider.nrepl.middleware.util.cljs :as cljs]))

(defmacro defcomposed-handler

  "Generates a handler that subsumes all the other handlers, takes a name,

a sequence of middleware symbols, and a descriptor-fn to transform the derived descriptor."

  [name mws descriptor-fn]

  (let [vars (map resolve mws)

        descriptors (map (comp ::mw/descriptor meta) vars)

        handles (into {} (mapcat :handles descriptors))]

    `(do (defn ~name

           [handler#]

           (-> handler# ~@vars))

         (set-descriptor!

          #'wrap-default

          (~descriptor-fn

           {:handles ~handles})))))

(defn conj-set

  [s val]

  (set (conj s val)))

(defcomposed-handler wrap-default

  [info/wrap-info

   complete/wrap-complete

   stacktrace/wrap-stacktrace

   classpath/wrap-classpath

   inspect/wrap-inspect

   trace/wrap-trace

   exceptions/wrap-exceptions]

  (comp

   #(update-in % [:requires] conj-set #'session)

   #(update-in % [:expects] conj-set #'pr-values)

   cljs/maybe-piggieback))

[Gary Trakhman]

The conflict is introduced by trying constrain all the cider-middlewares to live between pr-values and session, with wrap-exceptions needing to be the first along the chain.

This mostly looks good, except for wrap-exceptions/session:

(wrap-describe

 interruptible-eval

 pr-values

 wrap-load-file

 add-stdin

 wrap-exceptions

 wrap-stacktrace

 session

 wrap-complete

 wrap-inspect

 wrap-classpath

 wrap-info

 wrap-trace)

If I add the constraints adding info,complete,and pr-values to the :expects of wrap-exceptions, I get something like:

(wrap-describe

 wrap-complete

 wrap-info

 wrap-exceptions

 interruptible-eval

 pr-values

 wrap-load-file

 add-stdin

 wrap-stacktrace

 session

 wrap-inspect

 wrap-classpath

 wrap-trace)

Which means nothing hits pr-values, which means everything breaks :-).

[Jony Hudson]

I'm not sure it's relevant, but one thing that caught me out is that the "interruptible-eval" middleware has a hard dependency on the "pr-values" middleware (i.e. it depends on #'clojure.tools.nrepl.middleware.pr-values/pr-values). I found this caused trouble when I was trying to insert my own pr'ing middleware.

Jony