The problem I'm trying to solve is how to stub a variable (e.g. a function
from a third-party lib) in tests so that the stubbing occurs only in the
current thread with other threads continuing using var's root value. It's
important because unit tests may be run in parallel. Without thread-local
binding two threads stubbing the same function will lead to race conditions:
(binding [somelib/foo foo-stub] ; throws java.lang.IllegalStateException:
Can't dynamically bind non-dynamic var
; invoke tested code which depends on foo
; assert stuff
)
I've tried to use *.setDynamic* (as described in blog post [1]) but it
doesn't work without direct *deref*-ing of the var:
(def static-var 123)
(defn quz[]
(.setDynamic #'static-var true)
(binding [static-var 1000]
(println "static-var =" static-var "deref =" @#'static-var)))
(quz) ; => static-var = 123 deref = 1000
This approach seems to be used in a recent *bolth* lib [2]. And The strange
thing is that in REPL running this code line by line works:
user=> (def static-var 123)
> #'user/static-var
> user=> (.setDynamic #'static-var true)
> #'user/static-var
> user=> (binding [static-var 1000] (println "static-var =" static-var))
> static-var = 1000
Looking at Var class implementation I couldn't figure out why .*setDynamic*
call wouldn't work. My guess is that compiler somehow caches initial static
Var value for performance reasons?..
So the questions are:
1) Is it a bug that *.setDynamic* + *binding* don't work?
2) Is there any other way to temporally rebind static variable
thread-locally? Considering I can't add *^:dynamic* into third-party lib
and don't want to write a wrapper or use dependency injection in order to
explicitly substitute the dependency in my unit tests.
3) Is there a Clojure parallel test runner which runs tests in new
processes instead of threads? This would eliminate any race conditions.
Python's *nose* test runner works this way [3].
4) Maybe crazy: does Clojure allow dynamically rebinding the symbol to a
new Var instance so that I could set *'static-var* to point at *(.setDynamic
(Var/create)*?
5) Even crazier idea: can I change the nature of the var so that it behaves
like an InheritedThreadLocal instead of ThreadLocal, but without forcing a
user to *deref* it (as it was described in [4])?
Links:
[1] http://blog.zolotko.me/2012/06/making-variable-dynamic-in-clojure.html
[2]
https://github.com/yeller/bolth/blob/323532683e3f66ae11566db5423c1e927e51818e/src/bolth/runner.clj#L99
[3]
http://nose.readthedocs.org/en/latest/doc_tests/test_multiprocess/multiprocess.html
[4] https://aphyr.com/posts/240-configuration-and-scope - see
"Thread-inheritable dynamic vars in Clojure"
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