Fix MT project-cache file access compatibility - #14871
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 85b185e7-08c7-4a95-aa04-d674ec05a321
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🔵 Needs a closer look
It changes wire protocol/versioning and TaskHost/engine execution behavior in a broad, compatibility-sensitive area that warrants final human review.
Pull request overview
This PR improves compatibility and correctness of file-access reporting when tasks execute in out-of-proc TaskHosts, especially under experimental multi-threaded (-mt) builds, while preserving backward-compatible TaskHost packet behavior.
Changes:
- Fixes
FileAccessDataround-tripping by avoiding struct boxing during TaskHost completion packet deserialization. - Propagates and gates
ReportFileAccessesacross TaskHost configuration + replay, and rejects-mtcombined with-reportFileAccesses(CLI +BuildManager). - Updates protocol versioning, resources/localization, unit tests, and multithreading spec documentation accordingly.
File summaries
| File | Description |
|---|---|
| src/Shared/TaskHostTaskComplete.cs | Fix struct translation to preserve FileAccessData through deserialization. |
| src/Shared/TaskHostConfiguration.cs | Add versioned ReportFileAccesses flag to TaskHost configuration packet. |
| src/MSBuild/XMake.cs | Reject incompatible -mt + -reportFileAccesses at command-line parsing. |
| src/MSBuild/TaskExecutionContext.cs | Add per-task file-access collection gated by configuration. |
| src/MSBuild/OutOfProcTaskHostNode.cs | Collect file accesses per task execution context and serialize per-task results. |
| src/MSBuild/Resources/Strings.resx | Add MSB1073 message for incompatible switch combination. |
| src/MSBuild/Resources/xlf/Strings.zh-Hant.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.zh-Hans.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.tr.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.ru.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.pt-BR.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.pl.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.ko.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.ja.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.it.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.fr.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.es.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.de.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild/Resources/xlf/Strings.cs.xlf | Localized resource entry for MSB1073 (new string). |
| src/MSBuild.UnitTests/XMake_Tests.cs | Add CLI and TaskHost behavior regression coverage (incompatibility + file access transport). |
| src/Framework/BackEnd/NodePacketTypeExtensions.cs | Bump packet version and define min version for new TaskHostConfiguration field. |
| src/Build/Instance/TaskFactories/TaskHostTask.cs | Propagate ReportFileAccesses into TaskHost config; gate replay on build parameters. |
| src/Build/BackEnd/BuildManager/BuildParameters.cs | Document incompatibility between MultiThreaded and ReportFileAccesses. |
| src/Build/BackEnd/BuildManager/BuildManager.cs | Reject incompatible combination for programmatic callers at BeginBuild. |
| src/Build/Resources/Strings.resx | Add engine-level error string for incompatible configuration. |
| src/Build/Resources/xlf/Strings.zh-Hant.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.zh-Hans.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.tr.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.ru.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.pt-BR.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.pl.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.ko.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.ja.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.it.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.fr.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.es.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.de.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build/Resources/xlf/Strings.cs.xlf | Localized resource entry for engine-level incompatible configuration (new string). |
| src/Build.UnitTests/BackEnd/TaskHostTaskComplete_Tests.cs | Add regression test ensuring FileAccessData round-trips correctly. |
| src/Build.UnitTests/BackEnd/TaskHostFactory_Tests.cs | Add regression tests for TaskHost gating + replay logic. |
| src/Build.UnitTests/BackEnd/TaskHostConfiguration_Tests.cs | Add regression tests for protocol-version-gated ReportFileAccesses. |
| src/Build.UnitTests/BackEnd/BuildManager_Tests.cs | Add regression test for programmatic incompatibility rejection pre-build. |
| documentation/specs/multithreading/multithreaded-msbuild.md | Document /reportfileaccesses incompatibility with current MT implementation. |
Review details
- Files reviewed: 43/43 changed files
- Comments generated: 1
- Review effort level: Lite
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dfederm
approved these changes
Sep 8, 2026
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Fixes #14824
Fixes #14825
Fixes #14826
Summary
FileAccessDatavalues across TaskHost deserialization without changing the existing payload format.ReportFileAccessesto TaskHost with packet-version compatibility, gate collection and replay, and keep reported accesses isolated per task execution context.-mttogether with-reportFileAccessesusing actionableMSB1073; programmaticBuildManagercallers receive the equivalent validation.Compatibility
Project-cache lookup/materialization remains compatible with MT. Only Detours-based cache population is rejected because concurrent projects in one process cannot be attributed correctly. CLR2, CLR4, and negotiated .NET TaskHost packet formats retain compatible behavior.
Validation