IndexOutOfRangeException in ILCompiler.LazyGenericsSupport.GraphBuilder.WalkMethod

[am11]

Build Information

Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1242593&view=logs&j=7dbf7d19-364a-5b42-ef94-ed3a1f9dc4c8
Build error leg or test failing: Build / linux_musl-x64 Debug Libraries_CheckedCoreCLR / Build product
Pull request: #122833

Error Message

  XUnitLogChecker -> /__w/1/s/artifacts/tests/coreclr/linux.x64.Release/Common/XUnitLogChecker/XUnitLogChecker/XUnitLogChecker.dll
  Generating native code
##[error]EXEC(0,0): error : (NETCORE_ENGINEERING_TELEMETRY=Build) One or more errors occurred. (Code generation failed for method '[S.P.CoreLib]System.AppContext.OnUnhandledException(object)')
EXEC : error : One or more errors occurred. (Code generation failed for method '[S.P.CoreLib]System.AppContext.OnUnhandledException(object)') [/__w/1/s/src/tests/Common/XUnitLogChecker/XUnitLogChecker.csproj]
  System.AggregateException: One or more errors occurred. (Code generation failed for method '[S.P.CoreLib]System.AppContext.OnUnhandledException(object)')
   ---> ILCompiler.CodeGenerationFailedException: Code generation failed for method '[S.P.CoreLib]System.AppContext.OnUnhandledException(object)'
   ---> System.IndexOutOfRangeException: Index was outside the bounds of the array.
     at ILCompiler.LazyGenericsSupport.GraphBuilder.WalkMethod(EcmaMethod) + 0xc5f
     at ILCompiler.LazyGenericsSupport.GraphBuilder..ctor(EcmaModule) + 0x637
     at ILCompiler.LazyGenericsSupport.CycleInfoHashtable.CreateValueFromKey(EcmaModule) + 0x3c
     at Internal.TypeSystem.LockFreeReaderHashtable`2.CreateValueAndEnsureValueIsInTable(TKey) + 0x14
     at ILCompiler.LazyGenericsSupport.GenericCycleDetector.FormsCycle(TypeSystemEntity, LazyGenericsSupport.ModuleCycleInfo&) + 0xaf
     at ILCompiler.LazyGenericsSupport.GenericCycleDetector.DetectCycle(TypeSystemEntity, TypeSystemEntity) + 0x14f
     at Internal.IL.ILImporter.GetMethodEntrypoint(MethodDesc) + 0x62
     at Internal.IL.ILImporter.ImportCall(ILOpcode, Int32) + 0xe85
     at Internal.IL.ILImporter.ImportBasicBlock(ILImporter.BasicBlock) + 0x1583
     at Internal.IL.ILImporter.ImportBasicBlocks() + 0x59
     at Internal.IL.ILImporter.Import() + 0x6cb
     at ILCompiler.ILScanner.CompileSingleMethod(ScannedMethodNode) + 0x53
     --- End of inner exception stack trace ---
     at ILCompiler.ILScanner.CompileSingleMethod(ScannedMethodNode) + 0x104
     at System.Threading.Tasks.Parallel.<>c__DisplayClass19_0`2.<ForWorker>b__1(RangeWorker&, Int64, Boolean&) + 0x362
  --- End of stack trace from previous location ---
     at System.Threading.Tasks.Parallel.<>c__DisplayClass19_0`2.<ForWorker>b__1(RangeWorker&, Int64, Boolean&) + 0x51a
     at System.Threading.Tasks.TaskReplicator.Replica.Execute() + 0x65
     --- End of inner exception stack trace ---
     at System.Threading.Tasks.TaskReplicator.Run[TState](TaskReplicator.ReplicatableUserAction`1, ParallelOptions, Boolean) + 0x155
     at System.Threading.Tasks.Parallel.ForWorker[TLocal,TInt](TInt, TInt, ParallelOptions, Action`1, Action`2, Func`4, Func`1, Action`1) + 0x2d4
  --- End of stack trace from previous location ---
     at System.Threading.Tasks.Parallel.ThrowSingleCancellationExceptionOrOtherException(ICollection, CancellationToken, Exception) + 0x31
     at System.Threading.Tasks.Parallel.ForWorker[TLocal,TInt](TInt, TInt, ParallelOptions, Action`1, Action`2, Func`4, Func`1, Action`1) + 0x403
     at ILCompiler.ILScanner.CompileMultiThreaded(List`1) + 0x22e
     at ILCompiler.ILScanner.ComputeDependencyNodeDependencies(List`1) + 0x16f
     at ILCompiler.DependencyAnalysisFramework.DependencyAnalyzer`2.ComputeMarkedNodes() + 0x9f
     at ILCompiler.ILScanner.ILCompiler.IILScanner.Scan() + 0x55f
     at ILCompiler.Program.<Run>g__RunScanner|4_0(Program.<>c__DisplayClass4_0&) + 0x19c
     at ILCompiler.Program.Run() + 0x2c3d
     at ILCompiler.ILCompilerRootCommand.<>c__DisplayClass261_0.<.ctor>b__0(ParseResult) + 0x31a
##[error].packages/microsoft.dotnet.ilcompiler/10.0.0/build/Microsoft.NETCore.Native.targets(330,5): error MSB3073: (NETCORE_ENGINEERING_TELEMETRY=Build) The command ""/__w/1/s/.packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/10.0.0/tools/ilc" @"/__w/1/s/artifacts/tests/coreclr/obj/linux.x64.Release/Managed/Common/XUnitLogChecker/XUnitLogChecker/native/XUnitLogChecker.ilc.rsp"" exited with code 1.
/__w/1/s/.packages/microsoft.dotnet.ilcompiler/10.0.0/build/Microsoft.NETCore.Native.targets(330,5): error MSB3073: The command ""/__w/1/s/.packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/10.0.0/tools/ilc" @"/__w/1/s/artifacts/tests/coreclr/obj/linux.x64.Release/Managed/Common/XUnitLogChecker/XUnitLogChecker/native/XUnitLogChecker.ilc.rsp"" exited with code 1. [/__w/1/s/src/tests/Common/XUnitLogChecker/XUnitLogChecker.csproj]

Build FAILED.

{
  "ErrorMessage": ["CodeGenerationFailedException", "IndexOutOfRangeException", "LazyGenericsSupport"],
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1242593
Error message validated: [ILCompiler.CodeGenerationFailedException: Code generation failed for method '[S.P.CoreLib]System.AppContext.OnUnhandledException(object)']
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 1/4/2026 1:34:38 AM UTC

Report

Build	Definition	Step Name	Console log	Pull Request
1242593	dotnet/runtime	Build product	Log	#122833

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	1	1

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1242593
Error message validated: [CodeGenerationFailedException IndexOutOfRangeException LazyGenericsSupport]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 1/5/2026 5:54:08 AM UTC

Report

Build	Definition	Step Name	Console log	Pull Request
1242593	dotnet/runtime	Build product	Log	#122833

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	1	1

Report

Build	Definition	Step Name	Console log	Pull Request
1242593	dotnet/runtime	Build product	Log	#122833

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
0	1	1

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[am11]

What's happening:

• methodContext is an array of generic parameters for the current method being scanned
• Index is a number from the IL metadata saying "use the n^th method generic parameter"
• Crash: methodContext[Index] where Index >= methodContext.Length

Why would IL have wrong index:

1. Corrupt/malformed metadata
2. Wrong type was retrieved from cache (see diff below)
3. The contexts are being miaxed up between different methods during parallel scanning

Since it's 1 in 1000+ type of race, could be 3.?

diff for 2:

diff --git a/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaMethod.cs b/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaMethod.cs
index a4b02943f9e..5612e580d76 100644
--- a/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaMethod.cs
+++ b/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaMethod.cs
@@ -434,7 +434,7 @@ private void ComputeGenericParameters()
             }
             else
             {
-                _genericParameters = TypeDesc.EmptyTypes;
+                Interlocked.CompareExchange(ref _genericParameters, TypeDesc.EmptyTypes, null);
             }
         }
 
diff --git a/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaType.cs b/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaType.cs
index af495446823..fd4b7061cea 100644
--- a/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaType.cs
+++ b/src/coreclr/tools/Common/TypeSystem/Ecma/EcmaType.cs
@@ -95,7 +95,7 @@ private void ComputeGenericParameters()
             }
             else
             {
-                _genericParameters = EmptyTypes;
+                Interlocked.CompareExchange(ref _genericParameters, EmptyTypes, null);
             }
         }