Enable TaskHost Callbacks by default#13579
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JanProvaznik merged 2 commits intomainfrom Apr 21, 2026
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Callbacks are now always enabled via packet version negotiation. Remove the temporary escape hatch env var and cross-version fallback tests. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
This PR finalizes default enablement of IBuildEngine callback forwarding for out-of-proc TaskHost execution by making the feature depend purely on the negotiated packet protocol version (and bumping that version), removing the temporary environment-variable escape hatch.
Changes:
- Bump TaskHost/node packet protocol version to 4 to represent callback-capable communication.
- Remove
MSBUILDENABLETASKHOSTCALLBACKS(Traits escape hatch) and rely on packet version for callback support gating. - Update TaskHost callback integration tests to stop force-enabling callbacks via env var and remove “callbacks disabled” regression tests.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| src/Shared/INodePacket.cs | Bumps PacketVersion to 4 and updates the version history comment. |
| src/MSBuild/OutOfProcTaskHostNode.cs | Removes env-var override; callback support now depends on packet version only. |
| src/Framework/Traits.cs | Removes the EnableTaskHostCallbacks escape-hatch environment variable. |
| src/Build.UnitTests/BackEnd/TaskHostCallback_Tests.cs | Removes env-var setup and deletes tests for the “callbacks not supported” path. |
Comments suppressed due to low confidence (1)
src/Build.UnitTests/BackEnd/TaskHostCallback_Tests.cs:112
- This file previously covered the “callbacks not supported”/cross-version path (MSB5022 for IsRunningMultipleNodes/BuildProjectFile and NotImplementedException for RequestCores). With callbacks now enabled by default, those regression tests were removed, leaving the fallback behavior untested even though the production code still has explicit
!CallbacksSupportedbranches. Consider reintroducing at least one unit-style test that forcesCallbacksSupportedfalse (e.g., by constructing a minimal TaskHostConfiguration and setting a lower parent packet version) to keep the cross-version compatibility behavior from regressing.
/// <summary>
/// Verifies RequestCores callback works when task is explicitly run in TaskHost via TaskHostFactory.
/// The first RequestCores call should always return at least 1 (the implicit core).
/// </summary>
[Fact]
JanProvaznik
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Apr 21, 2026
This was referenced Apr 21, 2026
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Fixes #12863
Context
final enablement without escape hatch
Notes
validated by vmr and vs expinsert