BUG: Negative server time and C# remainder results in negative index

[CoolCatMona]

The following line

_loadedIndex = (int)(Networking.GetServerTimeInMilliseconds() / 1000f / slideDurationSeconds) % imageUrls.Length;

Uses Networking.GetServerTimeInMilliseconds() as well as the remainder operator

This can create an edge case where _loadedIndex is a negative value, causing an out of bounds exception

• Networking.GetServerTimeInMilliseconds() will return a signed integer (-2,147,483,648 to 2,147,483,647), however the photon call it depends on will return a value between 0 and 4,294,967.295, when the server timestamp is received, it has a chance of wrapping around to a negative number.
• The remainder operator in C# is different than moduolo and has a tough time dealing with negative numbers, so something like -5 % 4 in C# will return -1, while -5 % 4 when using % as modulo should return 3

Attached is a stacktrace with some personal information removed and some extra logging I added when debugging

2023.03.18 18:56:21 Error      -  [UdonBehaviour] An exception occurred during Udon execution, this UdonBehaviour will be halted.
VRC.Udon.VM.UdonVMException: The VM encountered an error!
Exception Message:
  An exception occurred during EXTERN to 'SystemObjectArray.__Get__SystemInt32__SystemObject'.
      Parameter Addresses: 0x00000009, 0x00000006, 0x00000029
  
  Index was outside the bounds of the array.
----------------------
Program Counter was at: 668
----------------------
Stack Dump:
  0: 0x0000000A
  1: 0x0000000D
----------------------
Heap Dump:
  0x00000000: -7774693031578337865
  0x00000001: SlideshowFrame
  0x00000002: 4294967295
  0x00000003: VRC.SDKBase.VRCUrl[]
  0x00000004: Weather Displays Screens (UnityEngine.MeshRenderer)
  0x00000005: 15
  0x00000006: -4
  0x00000007: VRC.SDK3.Image.VRCImageDownloader
  0x00000008: Weather Displays Screens (VRC.Udon.UdonBehaviour)
  0x00000009: UnityEngine.Texture2D[]
  0x0000000A: 4294967295
  0x0000000B: Weather Displays Screens (VRC.Udon.UdonBehaviour)
  0x0000000C: 108
  0x0000000D: 152
  0x0000000E: Weather Displays Screens (VRC.Udon.UdonBehaviour)
  0x0000000F: LoadNextRecursive
  0x00000010: Update
  0x00000011: 1000
  0x00000012: Loading index {0}
  0x00000013: imageUrls.Length {0}
  0x00000014: _downloadedTextures.Length {0}
  0x00000015: null
  0x00000016: True
  0x00000017: VRC.SDK3.Image.VRCImageDownloader+ImageDownloader
  0x00000018: Image loaded: {0} bytes.
  0x00000019: null
  0x0000001A: Image not loaded: {0}: {1}.
  0x0000001B: 5
  0x0000001C: -878843116
  0x0000001D: -8.788431E+08
  0x0000001E: -878843.1
  0x0000001F: -58589.54
  0x00000020: -58589
  0x00000021: -58589.54296875
  0x00000022: -58589
  0x00000023: 5
  0x00000024: Loading index -4
  0x00000025: 5
  0x00000026: imageUrls.Length 5
  0x00000027: 5
  0x00000028: _downloadedTextures.Length 5
  0x00000029: null
  0x0000002A: null
  0x0000002B: null
  0x0000002C: False
  0x0000002D: Weather Screens LED (Instance) (UnityEngine.Material)
  0x0000002E: null
  0x0000002F: VRC.SDK3.Image.TextureInfo
  0x00000030: [removed]
  0x00000031: VRC.SDK3.Image.VRCImageDownloader+ImageDownloader
  0x00000032: 4194303
  0x00000033: Image loaded: 4194303 bytes.
  0x00000034:  (UnityEngine.Texture2D)
  0x00000035: Unknown
  0x00000036: null
  0x00000037: null
  0x00000038: null
  0x00000039: SystemArray.__get_Length__SystemInt32
  0x0000003A: UnityEngineTexture2DArray.__ctor__SystemInt32__UnityEngineTexture2DArray
  0x0000003B: VRCSDK3ImageVRCImageDownloader.__ctor____VRCSDK3ImageVRCImageDownloader
  0x0000003C: VRCUdonCommonInterfacesIUdonEventReceiver.__SendCustomEventDelayedSeconds__SystemString_SystemSingle_VRCUdonCommonEnumsEventTiming__SystemVoid
  0x0000003D: VRCSDKBaseNetworking.__GetServerTimeInMilliseconds__SystemInt32
  0x0000003E: SystemConvert.__ToSingle__SystemInt32__SystemSingle
  0x0000003F: SystemSingle.__op_Division__SystemSingle_SystemSingle__SystemSingle
  0x00000040: SystemConvert.__ToDouble__SystemSingle__SystemDouble
  0x00000041: SystemMath.__Truncate__SystemDouble__SystemDouble
  0x00000042: SystemConvert.__ToInt32__SystemDouble__SystemInt32
  0x00000043: SystemInt32.__op_Remainder__SystemInt32_SystemInt32__SystemInt32
  0x00000044: SystemString.__Format__SystemString_SystemObject__SystemString
  0x00000045: UnityEngineDebug.__Log__SystemObject__SystemVoid
  0x00000046: SystemObjectArray.__Get__SystemInt32__SystemObject
  0x00000047: UnityEngineObject.__op_Inequality__UnityEngineObject_UnityEngineObject__SystemBoolean
  0x00000048: UnityEngineRenderer.__get_sharedMaterial__UnityEngineMaterial
  0x00000049: UnityEngineMaterial.__set_mainTexture__UnityEngineTexture__SystemVoid
  0x0000004A: VRCSDK3ImageTextureInfo.__ctor____VRCSDK3ImageTextureInfo
  0x0000004B: VRCSDK3ImageTextureInfo.__set_GenerateMipMaps__SystemBoolean
  0x0000004C: VRCSDKBaseVRCUrlArray.__Get__SystemInt32__VRCSDKBaseVRCUrl
  0x0000004D: UnityEngineRenderer.__get_material__UnityEngineMaterial
  0x0000004E: VRCSDK3ImageVRCImageDownloader.__DownloadImage__VRCSDKBaseVRCUrl_UnityEngineMaterial_VRCUdonCommonInterfacesIUdonEventReceiver_VRCSDK3ImageTextureInfo__VRCSDK3ImageIVRCImageDownload
  0x0000004F: VRCSDK3ImageIVRCImageDownload.__get_SizeInMemoryBytes__SystemInt32
  0x00000050: VRCSDK3ImageIVRCImageDownload.__get_Result__UnityEngineTexture2D
  0x00000051: SystemObjectArray.__Set__SystemInt32_SystemObject__SystemVoid
  0x00000052: VRCSDK3ImageIVRCImageDownload.__get_Error__VRCSDK3ImageVRCImageDownloadError
  0x00000053: SystemObject.__ToString__SystemString
  0x00000054: VRCSDK3ImageIVRCImageDownload.__get_ErrorMessage__SystemString
  0x00000055: SystemString.__Format__SystemString_SystemObject_SystemObject__SystemString
  0x00000056: VRCSDK3ImageVRCImageDownloader.__Dispose__SystemVoid
----------------------
Inner Exception:
 ---> VRC.Udon.VM.UdonVMException: An exception occurred during EXTERN to 'SystemObjectArray.__Get__SystemInt32__SystemObject'.
    Parameter Addresses: 0x00000009, 0x00000006, 0x00000029
 ---> System.IndexOutOfRangeException: Index was outside the bounds of the array.
  at VRC.Udon.Wrapper.Modules.ExternSystemObjectArray.__Get__SystemInt32__SystemObject (VRC.Udon.Common.Interfaces.IUdonHeap heap, System.Span`1[T] parameterAddresses) [0x00000] in <00000000000000000000000000000000>:0 
  at ÏÍÌÌÎÌÏÎÎÏÍÏÏÏÍÏÌÍÏÎÍÍÍ.Invoke (System.Object ÌÍÏÌÏÍÍÌÌÌÏÎÍÎÏÍÏÍÌÍÏÏÌ, ÏÌÍÍÎÏÌÎÏÍÌÌÍÏÍÎÍÍÌÏÎÍÏ ÌÍÏÌÏÏÏÎÌÎÏÌÌÏÌÌÏÏÍÍÎÌÎ) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.VM.UdonVM.Interpret () [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonBehaviour.RunProgram (System.UInt32 entryPoint) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonBehaviour.RunProgram (System.String eventName) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.ClientBindings.UdonEventScheduler.RunScheduledEvents (VRC.Udon.Common.Enums.EventTiming eventTiming) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonManager.Update () [0x00000] in <00000000000000000000000000000000>:0 
   --- End of inner exception stack trace ---
  at VRC.Udon.VM.UdonVM.Interpret () [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonBehaviour.RunProgram (System.UInt32 entryPoint) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonBehaviour.RunProgram (System.String eventName) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.ClientBindings.UdonEventScheduler.RunScheduledEvents (VRC.Udon.Common.Enums.EventTiming eventTiming) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonManager.Update () [0x00000] in <00000000000000000000000000000000>:0 
   --- End of inner exception stack trace ---
  at VRC.Udon.VM.UdonVM.Interpret () [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonBehaviour.RunProgram (System.UInt32 entryPoint) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonBehaviour.RunProgram (System.String eventName) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.ClientBindings.UdonEventScheduler.RunScheduledEvents (VRC.Udon.Common.Enums.EventTiming eventTiming) [0x00000] in <00000000000000000000000000000000>:0 
  at VRC.Udon.UdonManager.Update () [0x00000] in <00000000000000000000000000000000>:0 

This issue will occur occasionally given enough time

As a potential fix one could try to implement a modulo function that can handle negative numbers

    public int Mod(int a, int b)
    {
        return ((a % b) + b) % b;
    }

I'm not immediately sure how to handle Networking.GetServerTimeInMilliseconds() returning negative values occasionally

[Thelvaen]

I just ran into the same issue, and I just went with

Math.Abs(Networking.GetServerTimeInMilliseconds()) to bypass the issue, going "down" instead of "up" when the server time are negative is not really an issue for a fancy Slide Show :)