NullReferenceException and SIGSEGV

[marc-antoine-girard]

I'm not sure if you can make any sense of these errors and if they are caused by Nova or something else, but I figured I would share it and potentially, you might find something.

We've been having a lot of SIGSEGV recently, which are usually related to multithreading, and Nova is the only code we have that uses Burst / Jobs.

First error is:
System.NullReferenceException Nova.CoreBlock in Nova.InternalNamespace_0.InternalNamespace_2.InternalType_141.get_InternalProperty_202

Stacktrace:

System.NullReferenceException: null
  ?
  File "C:\project\Assets\Nova\Scripts\Public\Components\CoreBlock.cs", line 129, col 60, in Nova.InternalNamespace_0.InternalNamespace_2.InternalType_141.get_InternalProperty_202
  File "C:\project\Assets\Nova\Scripts\Internal\InternalScript_118.cs", line 110, col 13, in InternalMethod_498
  File "C:\project\Assets\Nova\Scripts\Internal\InternalScript_118.cs", line 320, col 21, in InternalMethod_2056
  File "C:\project\Assets\Nova\Scripts\Public\DataTypes\Interaction.cs", line 707, col 13, in InternalMethod_3007
  File "C:\project\Assets\Nova\Scripts\Public\DataTypes\Interaction.cs", line 591, col 13, in Point
  File "C:\project\Assets\Scripts\Runtime\Behaviours\UIRaycaster.cs", line 143, col 13, in Interact

As for the SIGSEGV:
Unity.Collections.LowLevel.Unsafe.UnsafeList1.Add`

Stacktrace:

OS Version: Android 12 (SQ3A.220605.009.A1)
Report Version: 104

Exception Type: Unknown (SIGSEGV)

Application Specific Information:
Segfault

Thread 0 Crashed:
lib_burst_generated     +0x063772   Unity.Collections.LowLevel.Unsafe.UnsafeList`1<Nova.InternalNamespace_0.InternalNamespace_10.InternalType_417>.Add(UnsafeList.cs:500)
lib_burst_generated     +0x063772   Nova.InternalNamespace_0.InternalNamespace_4.InternalType_164`1<Nova.InternalNamespace_0.InternalNamespace_10.InternalType_417>.InternalMethod_785(InternalScript_22.cs:29)
lib_burst_generated     +0x063772   Nova.InternalNamespace_0.InternalNamespace_10.InternalType_410.InternalMethod_1882(InternalScript_252.cs:725)
lib_burst_generated     +0x063772   Nova.InternalNamespace_0.InternalNamespace_10.InternalType_410.InternalMethod_1651(InternalScript_252.cs:385)
lib_burst_generated     +0x063772   Nova.InternalNamespace_0.InternalNamespace_10.InternalType_410.Execute(InternalScript_252.cs:153)
lib_burst_generated     +0x063772   Nova.InternalNamespace_0.InternalNamespace_5.InternalNamespace_6.InternalType_202.InternalType_203`1<Nova.InternalNamespace_0.InternalNamespace_10.InternalType_410>.Execute(ref Nova.InternalNamespace_0.InternalNamespace_10.InternalType_410 jobData, Syst...(InternalScript_280.cs:144)
lib_burst_generated     +0x0622ec   2c65704a8545e70a9c438a1d0864fa6f
libunity                +0x841a52   ExecuteJob(ManagedJobData*, void (*)(void*, void*, void*, void*, int), int, unsigned char*)
libunity                +0x841d6e   ExecuteJobCopyData(ManagedJobData*, void (*)(void*, void*, void*, void*, int), int)
libunity                +0x841148   ForwardJobForEachToManagedDefer(ManagedJobData*, unsigned int)
libunity                +0x83f71c   ujob_execute_job(ujob_control_t*, ujob_lane_t*, ujob_job_t*, ujob_handle_t, int)
libunity                +0x83f9f4   lane_guts(ujob_control_t*, ujob_lane_t*, int, int)
libunity                +0x83f3dc   ujob_wait_for(ujob_control_t*, ujob_handle_t, int)
libunity                +0x81ff82   JobQueue::WaitForJobGroupID(JobGroupID, JobQueue::JobQueueWorkStealMode)
libunity                +0x817bfe   CompleteFenceInternal(JobFence&, WorkStealMode)
libunity                +0x39ed96   JobHandle_CUSTOM_ScheduleBatchedJobsAndComplete(JobFence&)
<unknown>               0xb2f428ea  <unknown>

Might be a timing issue, it seems to happen when a scene is unloaded in most cases. I can provide additionnal information if you think this is related to Nova, or worth investigating.

[dkonik]

Hi, thanks for posting the call stacks. We will likely follow up with additional questions as we work on trying to reproduce both of these issues, but a couple of follow up questions:

1. The NullReferenceException you posted seems to be input related, could you provide some more info about what scenarios this occurs in? I.e., how do you go about reproducing it (e.g. does it happen when you click, drag, or scroll), is it a consistent repro, etc.
2. For the SIGSEGV issue, it looks like the crash you posted happened on an android device. Are you only able to reproduce this on android devices, or do you also hit this issue in editor?
3. Could you provide the versions in your project of the following:

• Unity
• Nova
• Collections package
• Burst package
• Jobs package

Also, any other pertinent information would of course be useful!

[marc-antoine-girard]

First, I would just like to point out that we might have found the cause of our SIGSEGV, which was Unity related, and that these issues might've simply happened because of bad timing during the crash. I will still answer your questions in case it happens again, but I wouldn't worry to much about it if no one has these issues.

1. The Exception happened when we were switching scene. The call originated from an Interaction.Point() in a class very close to what you have in your XR Sample repo to handle inputs. The Point is called every frame in Update regardless of if there is inputs. I'm fairly certain I wasn't pressing on anything so either I was pointing on a UI or I wasn't. I've tried to reproduce it, but never could, until I realized that both errors happened when I was trying to fix the SIGSEGV and updated Unity to see if it was a Unity thing. I reverted my changes when I saw that it created more bugs, but the error happened in 2022.2.19f1, while it never happened in our current setup.
2. Both issues never happened in the editor, only on Android, technically Meta Quest 2.

• Unity: 2022.2.19f1
• Nova: 1.3.0
• Collection: 1.2.4
• Burst: 1.8.4
• Jobs: N/A, (I guess it's now integrated in Unity, but if not, I can't find it in my project or manifest.)
• URP: 14.0.7
• Oculus XR Plugin: 3.3.0

Note that it never happened in our current setup, so I'm adding it here (things that changed in bold):

• Unity: 2022.2.12f1
• Nova: 1.3.0
• Collection: 1.2.4
• Burst: 1.8.3
• URP: 14.0.6
• Oculus XR Plugin: 3.2.3

4. We change scenes in Additive. The player and its UI pointers are in a persistent scene and the scene containing the UI is unloaded using UnloadSceneAsync