race condition in upmap causes null pointer exception

[shaunlebron]

Using upmap sometimes results in this exception:

java.lang.NullPointerException: Cannot invoke "java.util.concurrent.Future.get()" because "fut" is null
core.clj:2317	clojure.core / deref_future
core.clj:2337	clojure.core / deref
impl.clj:58	com.climate.claypoole.impl / deref_fixing_exceptions
impl.clj:58	com.climate.claypoole.impl / deref_fixing_exceptions

It appears that pmap-core zips the ordered tasks with an unordered queue of derefable results of those tasks, and sometimes the second value (returned by a blocking take from the unordered queue) returns nil, causing deref-fixing-exceptions to deref that nil value, throwing the above exception.

So, something in the implementation of the blocking take from the unordered queue is causing it to return nil. I can only guess this is a race condition because it seemed to happen randomly. Anecdotally, I saw it happening more in 1.1.4 than in 1.2.2.

I don’t have steps to reproduce, but we fixed the issue in our system by just switching from upmap to pmap, which avoids what I think is the problematic zip operation above.