Report actual launch path in MSB4216 for Runtime="NET" task host by ViktorHofer · Pull Request #13889 · dotnet/msbuild

ViktorHofer · 2026-05-28T09:52:21Z

Summary

When a Runtime="NET" task host launch fails, the MSB4216 error message
always pointed at <sdk>\MSBuild.exe — the .NET task host app host — even
when the launcher fell back to dotnet.exe MSBuild.dll because no app host
was present in the SDK.

SDK 10.0.x ships MSBuild.dll only; the app host first appears in 10.0.300.
On those SDKs users see an MSB4216 referencing a file that never existed
(see #13879 for a concrete repro on Windows-ARM with SDK 10.0.108).

Change

Centralize the apphost-vs-fallback decision in a new
NodeProviderOutOfProcTaskHost.ResolveNetTaskHostLaunchPath helper that
returns (LaunchPath, UseAppHost). Use it from both the launcher
(ResolveAppHostOrFallback) and the error path
(TaskHostTask.LogErrorUnableToCreateTaskHost) so the reported path
always matches the path that was actually launched.

This is a logging-only change; launch behavior is unchanged.

Notes

This is a partial fix for #13879. The underlying handshake bug
(IsAllowedBitnessMismatch missing Arm64 tolerance) remains and will be
fixed separately.

When a .NET task host launch fails, LogErrorUnableToCreateTaskHost always formatted the path as MSBuild.exe (the apphost) regardless of whether ResolveAppHostOrFallback actually launched the apphost or fell back to dotnet.exe MSBuild.dll. SDK 10.0.x ships MSBuild.dll only — the apphost first ships in 10.0.300 — so users on those SDKs see an MSB4216 error pointing at an MSBuild.exe that never existed. Centralize the apphost-vs-fallback decision in a new NodeProviderOutOfProcTaskHost.ResolveNetTaskHostLaunchPath helper and use it from both the launcher (ResolveAppHostOrFallback) and the error path (TaskHostTask.LogErrorUnableToCreateTaskHost) so the reported path always matches the path that was actually launched. This is a logging-only change; launch behavior is unchanged. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

This PR updates MSBuild’s out-of-proc .NET task host handling so MSB4216 reports the actual launch target path for Runtime="NET" task hosts (app host vs dotnet-hosted MSBuild.dll), aligning user-facing diagnostics with the launcher’s resolution logic.

Changes:

Centralizes apphost-vs-fallback selection in NodeProviderOutOfProcTaskHost.ResolveNetTaskHostLaunchPath.
Uses the shared resolver in both the launcher (ResolveAppHostOrFallback) and MSB4216 error construction (TaskHostTask.LogErrorUnableToCreateTaskHost).
Updates trace output and fallback command line to consistently use the resolved launch path.

Show a summary per file

File	Description
src/Build/Instance/TaskFactories/TaskHostTask.cs	Uses the shared resolver to report the resolved NET task host launch path in MSB4216 instead of always pointing at the app host.
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs	Introduces `ResolveNetTaskHostLaunchPath` and updates launcher logic to use the resolved app host or `MSBuild.dll` fallback consistently.

Copilot's findings

Files reviewed: 2/2 changed files
Comments generated: 3

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

github-actions

Note

🔒 Integrity filter blocked 2 items

The following items were blocked because they don't meet the GitHub integrity level.

#13889 pull_request_read: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
#13889 pull_request_read: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".

To allow these resources, lower min-integrity in your GitHub frontmatter:

tools:
  github:
    min-integrity: approved  # merged | approved | unapproved | none

Generated by Expert Code Review (on open) for issue #13889 · ● 20.3M

Cover the app-host-present and app-host-missing branches of the helper, ensuring the MSB4216 error path stays aligned with the launcher logic. Addresses review feedback on dotnet#13889. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

ViktorHofer · 2026-05-28T18:53:27Z

@rainersigwald @JanProvaznik would appreciate your review on this one. Straightforward minimal change to log the actual invocation instead of always the apphost.

## Summary `Runtime="NET"` task host launches fail with `MSB4216` whenever the worker node and TaskHost node architectures don't line up the way the original implementation expected. Tracked as [#13879](#13879) (Bug A). > Terminology: the **worker node** is the MSBuild process that launches the task host (the "parent"); the **TaskHost node** is the launched `Runtime="NET"` task host process (the "child"). This wording was adopted following review feedback. Two coordinated handshake changes — one on each side — close every reasonable worker/TaskHost arch combination: ### 1. Worker node side — `CommunicationsUtilities.GetHandshakeOptions` For a NET task host the launched TaskHost node's architecture is whatever the .NET SDK shipped, not the worker node's. Emitting the worker node's arch bit produces a wire-level mismatch with already-shipped SDK TaskHost nodes whose arch differs. Suppress the `X64`/`Arm64` bit when invoked by the worker node for a NET task host (detected by `Runtime="net"` in the explicit `TaskHostParameters`). The TaskHost-node path (`TaskHostParameters.Empty`) is unaffected, so already-deployed worker nodes that still emit an arch bit continue to match. ### 2. TaskHost node side — `NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch` The .NET task host side relaxation that lets a worker node without an arch bit on the wire connect to an SDK TaskHost node. Previously only tolerated `expected = X64`, so any arm64 SDK TaskHost node rejected the connection. Now also tolerates `expected = Arm64`. True cross-arch mismatches (worker `X64` ↔ TaskHost `Arm64`) remain rejected. Method is promoted to `internal static` so the test project can exercise the tolerance matrix directly. It was a stateless pure predicate already. ## Effect across worker node / SDK combinations `current` = behavior with this PR applied (worker-node change in VS MSBuild, TaskHost-node change in a future SDK). For each combo, "current behavior" notes whether the SDK side needs to update. | Worker node | TaskHost node (SDK MSBuild.dll) | Wire arch bit | Old behavior | Current behavior | |---|---|---|---|---| | x86 .NET Fx VS | x64 SDK | none → x64 | ✅ (existing `expectedIsX64` tolerance) | ✅ | | x86 .NET Fx VS | arm64 SDK | none → arm64 | ❌ MSB4216 (Bug A) — **the user's binlog** | ✅ once SDK ships with TaskHost-side `expectedIsArm64` tolerance | | amd64 .NET Fx VS | x64 SDK | x64 → x64 | ✅ (matches exactly) | ✅ (after worker fix: no bit on wire; already-shipped SDK tolerates none → x64) | | amd64 .NET Fx VS | arm64 SDK | x64 → arm64 | ❌ MSB4216 | ✅ once VS picks up worker-side suppression; SDK must also have TaskHost-side Arm64 tolerance | | arm64 .NET Fx VS | x64 SDK | arm64 → x64 | ❌ MSB4216 | ✅ once VS picks up worker-side suppression (works against already-shipped SDKs) | | arm64 .NET Fx VS | arm64 SDK | arm64 → arm64 | ✅ (matches exactly) | ✅ | | old worker node (pre-this-PR) | new SDK TaskHost node | x64/arm64 → matching | unchanged | unchanged — still works if arches match, still hits Bug A if they don't (worker has no fix) | ## Where each fix takes effect | Failing combo | Fixed by | Ships in | |---|---|---| | x86 .NET Fx VS → win-arm64 SDK | TaskHost-side Arm64 tolerance | a new .NET SDK build that picks up this PR | | amd64 .NET Fx VS → win-arm64 SDK | worker-side arch suppression + TaskHost-side Arm64 tolerance | next VS insertion of MSBuild (worker side) plus a new SDK build (TaskHost side) | | arm64 .NET Fx VS → win-x64 SDK | worker-side arch suppression (existing `expectedIsX64` tolerance already covers the TaskHost node) | next VS insertion of MSBuild only — no SDK update needed | The reported binlog (Roslyn build on Windows-ARM, SDK 10.0.108, x86 VS 18 MSBuild worker node) is the first row — needs the SDK side to pick this up. ## Tests `src/Build.UnitTests/BackEnd/NodeEndpointOutOfProcBase_Tests.cs` (6 cases): - `NoArchBitWorkerNode_X64TaskHost_IsTolerated` - `NoArchBitWorkerNode_Arm64TaskHost_IsTolerated` - `X64WorkerNode_X64TaskHost_NotConsideredMismatch` - `X64WorkerNode_Arm64TaskHost_NotTolerated` - `Arm64WorkerNode_X64TaskHost_NotTolerated` - `NoArchBitWorkerNode_NoArchBitTaskHost_NotTolerated` (guards against a future simplification to `return receivedIsX86;`) `src/Build.UnitTests/BackEnd/CommunicationsUtilities_Tests.cs` (5 cases): - `GetHandshakeOptions_NetTaskHostWorkerNode_SuppressesArchBit` × { x64, arm64, x86 } - `GetHandshakeOptions_NonNetTaskHostWorkerNode_KeepsX64ArchBit` - `GetHandshakeOptions_NonNetTaskHostWorkerNode_KeepsArm64ArchBit` - `GetHandshakeOptions_NetTaskHostNode_KeepsArchBit` All 11 pass on net10.0. ## References - Closes #13879 (Bug A). - Companion logging fix: #13889. --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings May 28, 2026 09:52

Copilot started reviewing on behalf of ViktorHofer May 28, 2026 09:52 View session

ViktorHofer requested review from JanProvaznik and rainersigwald May 28, 2026 09:53

Copilot AI reviewed May 28, 2026

View reviewed changes

Comment thread src/Build/Instance/TaskFactories/TaskHostTask.cs

Comment thread src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs Outdated

Comment thread src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs

ViktorHofer mentioned this pull request May 28, 2026

Make Runtime=NET task host handshake architecture-agnostic #13890

Merged

Potential fix for pull request finding

254e310

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

github-actions Bot reviewed May 28, 2026

View reviewed changes

github-actions Bot mentioned this pull request May 28, 2026

[aw] Expert Code Review (on open) failed #13891

Open

rainersigwald approved these changes May 28, 2026

View reviewed changes

ViktorHofer merged commit 790ad1e into dotnet:main May 29, 2026
10 checks passed

This was referenced May 31, 2026

[release/10.0.4xx] Source code updates from dotnet/msbuild dotnet/dotnet#6931

Merged

[main] Source code updates from dotnet/msbuild dotnet/dotnet#6932

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Report actual launch path in MSB4216 for Runtime="NET" task host#13889

Report actual launch path in MSB4216 for Runtime="NET" task host#13889
ViktorHofer merged 3 commits into
dotnet:mainfrom
ViktorHofer:vihofer/fix-msb4216-misleading-apphost-path

ViktorHofer commented May 28, 2026 •

edited

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

github-actions Bot left a comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

ViktorHofer commented May 28, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

Uh oh!

Conversation

ViktorHofer commented May 28, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary

Change

Notes

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Copilot's findings

Uh oh!

Uh oh!

Uh oh!

Uh oh!

github-actions Bot left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

ViktorHofer commented May 28, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

ViktorHofer commented May 28, 2026 •

edited

Loading