Collection was modified in SdkResolverService.ResolveSdkUsingResolversWithPatternsFirst

[KirillOsenkov]

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at bool System.Collections.Generic.List<T>+Enumerator.MoveNextRare()
   at SdkResult Microsoft.Build.BackEnd.SdkResolution.SdkResolverService.ResolveSdkUsingResolversWithPatternsFirst(int submissionId, SdkReference sdk, LoggingContext loggingContext, ElementLocation sdkReferenceLocation, string solutionPath, string projectPath, bool interactive, bool isRunningInVisualStudio, bool failOnUnresolvedSdk)
   at SdkResult Microsoft.Build.BackEnd.SdkResolution.CachingSdkResolverService.ResolveSdk(int submissionId, SdkReference sdk, LoggingContext loggingContext, ElementLocation sdkReferenceLocation, string solutionPath, string projectPath, bool interactive, bool isRunningInVisualStudio, bool failOnUnresolvedSdk)
   at void Microsoft.Build.Evaluation.Evaluator<P, I, M, D>.ExpandAndLoadImportsFromUnescapedImportExpressionConditioned(string directoryOfImportingFile, ProjectImportElement importElement, out List<ProjectRootElement> projects, out SdkResult sdkResult)
   at List<ProjectRootElement> Microsoft.Build.Evaluation.Evaluator<P, I, M, D>.ExpandAndLoadImports(string directoryOfImportingFile, ProjectImportElement importElement, out SdkResult sdkResult)
   at void Microsoft.Build.Evaluation.Evaluator<P, I, M, D>.EvaluateImportElement(string directoryOfImportingFile, ProjectImportElement importElement)
   at void Microsoft.Build.Evaluation.Evaluator<P, I, M, D>.PerformDepthFirstPass(ProjectRootElement currentProjectOrImport)
   at void Microsoft.Build.Evaluation.Evaluator<P, I, M, D>.Evaluate() x 2
   at void Microsoft.Build.Evaluation.Project+ProjectImpl.Reevaluate(ILoggingService loggingServiceForEvaluation, ProjectLoadSettings loadSettings, EvaluationContext evaluationContext)
   at void Microsoft.Build.Evaluation.Project+ProjectImpl.ReevaluateIfNecessary(ILoggingService loggingServiceForEvaluation, ProjectLoadSettings loadSettings, EvaluationContext evaluationContext) x 2
   at void Microsoft.Build.Evaluation.Project+ProjectImpl.Initialize(IDictionary<string, string> globalProperties, string toolsVersion, string subToolsetVersion, ProjectLoadSettings loadSettings, EvaluationContext evaluationContext, bool interactive)
   at new Microsoft.Build.Evaluation.Project(string projectFile, IDictionary<string, string> globalProperties, string toolsVersion, string subToolsetVersion, ProjectCollection projectCollection, ProjectLoadSettings loadSettings, EvaluationContext evaluationContext, IDirectoryCacheFactory directoryCacheFactory, bool interactive)
   at Project Microsoft.Build.Evaluation.Project.FromFile(string file, ProjectOptions options)

just randomly saw this during project evaluation

[KirillOsenkov]

Does this need a ToArray()?

msbuild/src/Build/BackEnd/Components/SdkResolution/SdkResolverService.cs

Line 179 in bd5cf8a

foreach (SdkResolverManifest manifest in _specificResolversManifestsRegistry)

Looks like there's some weird recursion going on, need to ensure we protect against reentrancy

[KirillOsenkov]

@AR-May

[KirillOsenkov]

or could it be because of parallelism, when multiple project evaluations on multiple threads call into the SDK resolution?