1. with-redefs is not thread safe. As explained in the doc string, the changes will be visible in all threads. What's not obvious though is that the act of restoring the original values of the Vars will also be visible across all threads.
clojure.core/with-redefs
([bindings & body])
Macro
Added in 1.3
binding => var-symbol temp-value-expr
Temporarily redefines Vars while executing the body. The
temp-value-exprs will be evaluated and each resulting value will
replace in parallel the root value of its Var. After the body is
executed, the root values of all the Vars will be set back to their
old values. These temporary changes will be visible in all threads.
Useful for mocking out functions during testing.
(defn print-it []
(println "don't print this"))
(defn dont-print [x]
(with-redefs [print-it (constantly nil)]
(print-it)))
(defn -main []
(doall (pmap dont-print (range 1000)))
false)$ lein run
don't print this
don't print this
don't print this
don't print this
$ lein run
$ lein run
$ lein run
don't print this
don't print this
2. The test reporting in clojure.test is not thread safe. I don't think this is documented anywhere
(deftest test-counter-parallel
(doall (pmap (fn [x] (is true)) (range 1000))))
(deftest test-counter
(doall (map (fn [x] (is true)) (range 1000))))$ lein test :only pmap-bugs.core-test/test-counter-parallel
lein test pmap-bugs.core-test
Ran 1 tests containing 194 assertions. <== Should be 1000
0 failures, 0 errors.
$ lein test :only pmap-bugs.core-test/test-counter
lein test pmap-bugs.core-test
Ran 1 tests containing 1000 assertions.
0 failures, 0 errors.