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[CLJ-1175] NPE in clojure.lang.Delay/deref Created: 06/Mar/13  Updated: 14/Aug/13  Resolved: 14/Aug/13

Status: Closed
Project: Clojure
Component/s: None
Affects Version/s: None
Fix Version/s: Release 1.6

Type: Defect Priority: Minor
Reporter: Alan Malloy Assignee: Stuart Halloway
Resolution: Completed Votes: 3
Labels: None

Attachments: Text File delayed-exceptions.patch    
Patch: Code and Test
Approval: Ok


Delays get into a corrupt state if an exception is thrown, allowing deref to behave differently on subsequent calls.

This can manifest in multiple ways, depending on the expression being delayed:

;; calling a local as a function causes an NPE inside clojure.lang.Delay
(let [f #(/ 1 0) d (delay (f))]
  [(try (force d) 
        (catch Exception e e))
   (try (force d)
        (catch Exception e e))])
[#<ArithmeticException java.lang.ArithmeticException: Divide by zero> 
 #<NullPointerException java.lang.NullPointerException>]
;; if nil is a valid value, this can cause subsequent forces to "succeed"
;; even though the first fails as it should
(let [x (java.util.Date.) d (delay (seq x))]
  [(try (force d) 
        (catch Exception e e))
   (try (force d)
        (catch Exception e e))])
[#<IllegalArgumentException java.lang.IllegalArgumentException: Don't know how to create ISeq from: java.util.Date> 

Cause: clojure.core/delay creates a ^:once function, promising to call it only once, so that the compiler can do locals-clearing on its lexical closure. However, when an exception is thrown the Delay object is left thinking it has never called the function, so when it is forced again it calls the function again, breaking its promise to the compiler and causing the function to run after its locals have been cleared.

Solution: Make Delay remember the first exception that was thrown, and rethrow it on subsequent force attempts. This makes sense, in a way: the "result" of the expression was this exception e being thrown, and so this should happen every time.

Patch: delayed-exceptions.patch

Comment by Alex Miller [ 08/Aug/13 2:07 AM ]

Cleaned up description.

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