Bug 32685 - NRE in ServicePoint.CheckAvaliableForRecycling
Summary: NRE in ServicePoint.CheckAvaliableForRecycling
Alias: None
Product: Class Libraries
Classification: Mono
Component: System ()
Version: master
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Marek Safar
: 32689 34745 35226 ()
Depends on:
Reported: 2015-07-31 18:12 UTC by Tom Philpot
Modified: 2016-03-14 21:46 UTC (History)
13 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Tom Philpot 2015-07-31 18:12:10 UTC
We are seeing the following stack trace in a Xamarin.iOS app in Raygun.

NullReferenceException: Object reference not set to an instance of an object
 System.Net.ServicePoint.CheckAvailableForRecycling(System.DateTime& outIdleSince) in <filename unknown>:0
 System.Net.ServicePoint.IdleTimerCallback(System.Object obj) in <filename unknown>:0
 System.Threading.Timer+Scheduler.TimerCB(System.Object o) in <filename unknown>:0

We are using Xamarin.iOS and have seen the error reported on iOS 8 and 9.

I believe the error has to do with this commit: https://github.com/mono/mono/commit/a769e74c5320f2cfa85efe80f2e1c1d8470c271b

Specifically, https://github.com/mono/mono/blob/a769e74c5320f2cfa85efe80f2e1c1d8470c271b/mcs/class/System/System.Net/ServicePoint.cs#L317 might need a check for groups != null.

Possible repro:
1. Thread 1 calls CheckAvailableForRecycling
2. Thread 2 calls CheckAvailableForRecycling
3. Thread 1 locks (this) and builds groupList
4. Thread 2 locks (this) and builds groupList, while Thread 1 is iterating groupList (lines 304-310)
5. Thread 1 locks (this) on line 312, while Thread 2 is iterating groupList.
6. While holding lock, Thread 1 sets groups to null.
7. Thread 1 exits lock
8. Thread 2 locks (this) on line 312 and groups is null. => NRE
Comment 1 Tom Philpot 2015-07-31 18:43:22 UTC
See https://github.com/mono/mono/pull/1955
Comment 2 Marek Safar 2015-08-01 02:55:31 UTC
Fixed in mono master and Mono 4.2
Comment 3 Marek Safar 2015-08-02 07:26:23 UTC
*** Bug 32689 has been marked as a duplicate of this bug. ***
Comment 4 Marek Safar 2016-01-14 14:50:27 UTC
*** Bug 34745 has been marked as a duplicate of this bug. ***
Comment 5 Marek Safar 2016-01-14 14:50:36 UTC
*** Bug 35226 has been marked as a duplicate of this bug. ***
Comment 6 Marek Safar 2016-01-14 14:51:40 UTC
Only Xamarin.iOS 9.2 or newer versions have the fix included
Comment 7 Lucian Torje 2016-02-11 09:41:24 UTC
Not fixed: Xcode 7.2, mtouch (cycle6-xi: eb4c1ef), /Library/Frameworks/Mono.framework/Versions/4.2.1/lib/mono/4.5