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[CLJ-1077] thread-bound? returns true (implying set! should succeed) even for non-binding thread Created: 26/Sep/12  Updated: 30/Jul/15

Status: Open
Project: Clojure
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Enhancement Priority: Major
Reporter: Paul Stadig Assignee: Unassigned
Resolution: Unresolved Votes: 3
Labels: None

Attachments: File thread-bound.diff    
Patch: Code
Approval: Triaged


thread-bound? returns true for a non-binding thread, this result (according to the docstring) implies that set! should succeed. However, thread-bound? does not check that any binding that might exist was created by the current thread, and calling set! fails with an exception when it is called from a non-binding thread, even though thread-bound? returns true.

thread-bound? should return false if there is a binding, and that binding was not established by the current thread.

Here is an example REPL session where a thread establishes a binding, those bindings are conveyed to a second thread, the second thread checks thread-bound? to see if it can set the binding, thread-bound? returns true indicating that the binding can be set, the second thread tries to set the binding, and the second thread gets an IllegalStateException:

    Clojure 1.5.1
    user=> (def ^:dynamic *set-me* nil)
    user=> (defn try-to-set [] (binding [*set-me* 1] (doall (pcalls #(if (thread-bound? #'*set-me*) (set! *set-me* (inc *set-me*)))))))
    user=> (try-to-set)
    IllegalStateException Can't set!: *set-me* from non-binding thread  clojure.lang.Var.set (Var.java:230)

Comment by Paul Stadig [ 01/Oct/12 10:07 AM ]

I have attached a patch that changes clojure.lang.Var and clojure.core/thread-bound? to only return true if a Var is set!-able.

Comment by Alex Miller [ 19/Aug/13 12:16 PM ]

REPL example?

Comment by Joe Gallo [ 20/Aug/13 7:55 AM ]

Sure thing, Alex – here's a repl example I just ran this morning.

; nREPL 0.1.7
user> (def ^:dynamic *set-me* nil)
user> (defn try-to-set [] (binding [*set-me* 1] (doall (pcalls #(if (thread-bound? #'*set-me*) (set! *set-me* (inc *set-me*)))))))
user> (try-to-set)
IllegalStateException Can't set!: *set-me* from non-binding thread  clojure.lang.Var.set (Var.java:230)
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