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Piping seque into seque can deadlock

Description

I'm not sure if this is a supported scenario, but the following deadlocks in Clojure 1.3:

(let [xs (seque (range 150000)) ys (seque (filter odd? xs))] (apply + ys))

Cause: As I understand it, the problem is that ys' fill takes place on an agent thread, so when it calls xs' drain, the (send-off agt fill) does not immediately trigger xs' fill, but is instead put on the nested list to be performed when ys' agent returns. Unfortunately, ys' fill will eventually block trying to take from xs, and so it never returns and the pending send-offs are never sent.

Approach: Use (release-pending-sends) in seque's drain function to avoid the deadlock when a seque is fed into another seque.

Patch: clj-823-v1.patch

Screened by: Alex Miller

Environment

Windows 7; JVM 1.6; Clojure 1.3 beta 1

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1
  • 22 Aug 2014, 12:13 AM

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Andy Fingerhut August 22, 2014 at 12:13 AM

clj-823-v1.patch uses (release-pending-sends) in seque's drain function in an attempt to avoid the deadlock when a seque is fed into another seque, as suggested by Stuart Halloway. It adds Peter Monks's small quick test case demonstrating the deadlock, which fails (i.e. hangs until killed) without the change and passes with it.

Stuart Halloway March 30, 2013 at 3:16 PM

release-pending-sends?

import January 7, 2013 at 9:43 PM

Comment made by: pmonks

Reproduced on 1.4.0 and 1.5.0-RC1 as well, albeit with this example:

(seque 3 (seque 3 (range 10)))

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Created August 3, 2011 at 7:28 PM
Updated August 29, 2014 at 6:00 PM
Resolved August 29, 2014 at 6:00 PM