thread-local version of with-redefs

with-local-redefs.clj

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Commentary                                                            ;;
;;                                                                       ;;
;; The goal for writing this started with the idea to have tests run in  ;;
;; parallel using the leiningen plugin eftest                            ;;
;; https://github.com/weavejester/eftest.                                ;;
;;                                                                       ;;
;; With tests using with-redefs, it was not possible to run them in      ;;
;; parallel if they were changing the root binding of the same           ;;
;; vars. Here, we are binding the root of the var to one function that   ;;
;; respects per-thread rebindings, if any exist.                         ;;
;;                                                                       ;;
;; Known caveats:                                                        ;;
;; - This per-therad rebinding will only work with clojure concurrency   ;;
;;   primitives which copy per-thread bindings to newly spawned threads, ;;
;;   eg, using clojure futures. But will not work for, say a             ;;
;;   java.lang.Thread.                                                   ;;
;; - As of now this only supports functions being bound and not other    ;;
;;   vars which store values, say (def x 19) for example.                ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;



(def ^:dynamic local-redefinitions {})

(defn current->original-definition
  [v]
  (when (var? v)
    (get (meta v) ::original)))


(defn redefiniton-fn
  [a-var]
  (fn [& args]
    (let [current-f (get local-redefinitions
                         a-var
                         (current->original-definition a-var))]
      (apply current-f args))))


(defn dynamic-redefs
  [vars func]
  (let [un-redefs (remove #(::already-bound? (meta %)) vars)]
    (doseq [a-var un-redefs]
      (locking a-var
        (when-not (::already-bound? (meta a-var))
          (let [old-val (.getRawRoot ^clojure.lang.Var a-var)]
            (.bindRoot ^clojure.lang.Var a-var
                       (redefiniton-fn a-var))
            (alter-meta! a-var
                         (fn [m]
                           (assoc m
                                  ::already-bound? true
                                  ::original old-val))))))))
  (func))


(defn xs->map
  [xs]
  (reduce (fn [acc [k v]] (assoc acc `(var ~k) v))
          {}
          (partition 2 xs)))


(defmacro with-dynamic-redefs
  [bindings & body]
  ;; @TODO: Add support for non-functions
  (let [map-bindings (xs->map bindings)]
    `(let [old-rebindings# local-redefinitions]
       (binding [local-redefinitions (merge old-rebindings# ~map-bindings)]
         (dynamic-redefs ~(vec (keys map-bindings))
                         (fn [] ~@body))))))



(comment ;; for testing
  (defn funk [& args] {:original-args args})
  (dotimes [i 1000]
    (let [f1 (future (with-dynamic-redefs [funk (constantly -100)]
                       (Thread/sleep (rand-int 10))
                       {:100 (funk) :t (.getName (Thread/currentThread))}))

          f2 (future (with-dynamic-redefs [funk (constantly -200)]
                       (Thread/sleep (rand-int 1000))
                       {:200 (funk 9) :t (.getName (Thread/currentThread))}))
          f3 (future (do
                       (Thread/sleep (rand-int 1000))
                       {:orig (funk 9) :t (.getName (Thread/currentThread))}))]
      (when (or (not= (:100 @f1) -100)
                (not= (:200 @f2) -200)
                (not= (:orig @f3) {:original-args '(9)}))
        (println "FAIL")
        (prn @f1)
        (prn @f2)
        (println "----------------\n\n")))))

Hmm. Only case I can think of, currently, why it will revert to original definition is when it will use java native thread.
Or some custom future without doing binding-conveyor-fn

@mourjo

Other reason It might escape binding is due to laziness where body is lazy and body runs in different thread instead of thread which does binding or binding has been undone (see below). There is no solution to this problem either make code non lazy or use your solution (I am not sure if your solution avoids it though, at onset seems to me that it will faces same problem due to usage of local-redefinitions) .

For example

user=> (def ^:dynamic *x* 10)
#'user/*x*
user=> (def res (binding [*x* 20] (lazy-seq (repeat 10 *x*))))
#'user/res
user=> res
(10 10 10 10 10 10 10 10 10 10)

Lazy expressions wouldn't with my code either. For example:

(defn funk [& args] {:args args})

(with-dynamic-redefs [funk (constantly :redefined)]
  (map funk [1]))
=> ({:args (1)})

(with-dynamic-redefs [funk (constantly :redefined)]
  (doall (map funk [1])))
=> (:redefined)

Something else must be it, maybe some regression in my code. I'll get back to you on this in some time. I'd much rather prefer your solution.

@mourjo It is not regression. It is due to laziness https://cemerick.com/2009/11/03/be-mindful-of-clojures-binding/

@rdivyanshu I tried your approach but it doesn't seem to work if the Vars have already been compiled:

;; this is the source
(ns ptesttrial.core)

(defn funk [a]
  a)

;; this is the test
(ns ptesttrial.core-test
  (:require [clojure.test :refer :all]
            [ptesttrial.core :as pc]))


(deftest b-test
  (is (= 99 (pc/funk 99)))
  (.setDynamic #'pc/funk true)
  (binding [pc/funk (constantly -1)]
    (is (= -1 (pc/funk 99)))))

This test case fails:

expected: (= -1 (pc/funk 99))
  actual: (not (= -1 99))

Clojure docs say the following https://clojure.org/reference/vars#interning :

If a def expression does not find an interned entry in the current namespace for the symbol being def-ed, it creates one, otherwise it uses the existing Var. This find-or-create process is called interning. This means that, unless they have been unmap-ed, Var objects are stable references and need not be looked up every time.

I also tried using ns-unmap and it does not seem to work.

(ns ptesttrial.core-test
  (:require [clojure.test :refer :all]
            [ptesttrial.core :as pc]))


(deftest b-test
  (is (= 99 (pc/funk 99)))
  (.setDynamic #'pc/funk true)
  (ns-unmap 'ptesttrial.core 'funk)
  (binding [pc/funk (constantly -1)]
    (is (= -1 (pc/funk 99)))))

;; FAIL in (b-test) (core_test.clj:11)
;; expected: (= -1 (pc/funk 99))
;;   actual: (not (= -1 99))

This works fine.

(ns ptesttrial.core-test
  (:require [clojure.test :refer :all]
            [ptesttrial.core :as pc]))

(defmacro with-dynamic-redefs
  [bindings & body]  
  (let [vars (map (fn [x] `(var ~x)) (take-nth 2 bindings))]
    (doall (map #(.setDynamic (eval %)) vars))
    `(binding  ~bindings ~@body)))

(deftest b-test
  (is (= 99 (pc/funk 99)))
  (with-dynamic-redefs [pc/funk (constantly -1)]
    (is (= -1 (pc/funk 99)))))

Any reason for not doing this. Problem with your approach is setDynamic was not run during compilation so compiler emitted getRawRoot at call site (https://github.com/clojure/clojure/blob/b19b781b1f0f3f46aee5e951f415e0456a39cbcb/src/jvm/clojure/lang/Compiler.java#L5191). deftest put content of body in function which don't get invoked during compilation where as use of
defmacro make it run at compile.

@mourjo Just had this issue pop up on us. Thanks for sharing!

@rdivyanshu was trying to use your macro as a drop-in replacement to with-redefs, but think I've hit a problem.

The direct invocation usage works great:

(defn get-thing []
  :none)

;; fails, usually
(deftest with-redefs-not-thread-safe
  (let [results (pmap (fn [i]
                        (with-redefs [get-thing (constantly i)]
                          (get-thing)))
                      (range 100))]
    (is (= (range 100) results))))

(defmacro with-dynamic-redefs
  [bindings & body]
  (let [vars (map (fn [x] `(var ~x)) (take-nth 2 bindings))]
    (doall (map #(.setDynamic (eval %)) vars))
    `(binding  ~bindings ~@body)))

;; doesn't seem to fail?
(deftest with-dynamic-redefs-is-thread-safe
  (let [results (pmap (fn [i]
                        (with-dynamic-redefs [get-thing (constantly i)]
                          (get-thing)))
                      (range 100))]
    (is (= (range 100) results))))

But if I add indirection, it won't work until I re-evaluate the caller:

(defn get-thing []
  :none)

(defn get-indirect-thing []
  (get-thing))

;; fails, usually
(deftest with-redefs-not-thread-safe
  (let [results (pmap (fn [i]
                        (with-redefs [get-thing (constantly i)]
                          (get-indirect-thing)))
                      (range 100))]
    (is (= (range 100) results))))

(defmacro with-dynamic-redefs
  [bindings & body]
  (let [vars (map (fn [x] `(var ~x)) (take-nth 2 bindings))]
    (doall (map #(.setDynamic (eval %)) vars))
    `(binding  ~bindings ~@body)))

;; fails too
(deftest with-dynamic-redefs-is-thread-safe
  (let [results (pmap (fn [i]
                        (with-dynamic-redefs [get-thing (constantly i)]
                          (get-indirect-thing)))
                      (range 100))]
    (is (= (range 100) results))))

;; evaluating the caller again will make the test work
(defn get-indirect-thing []
  (get-thing))

;; now it doesn't fail
(deftest with-dynamic-redefs-is-thread-safe
  (let [results (pmap (fn [i]
                        (with-dynamic-redefs [get-thing (constantly i)]
                          (get-indirect-thing)))
                      (range 100))]
    (is (= (range 100) results))))

It looks like the same class of problem as before: callers that were defined before the macro runs get a non-dynamic version of the var.

For whoever finds this, the author actually made a lib that works with my example above: https://github.com/mourjo/dynamic-redef

@filipesilva It is happening due to there is no dynamic invocation of function get-thing. In get-indirect-thing you are invoking during definition. if you define get-indirect-thing according to below

(defn get-indirect-thing [f]
  (f))

And then call (get-indirect-thing get-thing) test passes. What you want in get-indirect-thing at run time is call current definition of get-thing but it is calling get-thing at time of definition. I am not sure if clojure provides such mechanism for functions with no arguments, it has been long since I wrote clojure code.

Hey thanks for getting back to me, wasn't expecting it at all :D

https://github.com/mourjo/dynamic-redef seems to handle the indirection case well so trying it out atm.

Thanks for this! Trying to understand this code - how are redefs reset when you go out of scope of the macro? looks like the var is permanently changed?

Yes the var is permanently changed. That is probably a good enough reason to only use this in tests. How it works is: the redefined var uses a dynamic (thread local) var to store local redefinitions.

As an aside: You might want to check out this repository (created based on this gist): https://github.com/mourjo/dynamic-redef

Ah okay. Would you accept a PR to have it remove itself once done, if I do that? (to the repo that is).
For my workflow permanently changing it means the changes leak out into my local dev env.

Absolutely, I would love your contribution!