Preserve virtual task environments in TaskHosts

[rainersigwald]

Related issue(s):

Fixes: #14986

copilot writes:

Context

Under -mt, a task can add an environment variable before any TaskHost launches without changing the parent's process-global environment. The variable reaches the first task configuration, but TaskHost reconciliation compares that configuration with its inherited startup environment and treats every difference as a bitness adjustment. It consequently replaces the supplied value with the startup value, including deleting a newly added variable. The first task's environment was incorrectly serving as a proxy for the launcher's environment at process creation.

For static-graph restore, this strips MSBUILDCOORDINATORGRANTID from the RestoreTaskEx TaskHost. The nested NuGet process then requests capacity held by its waiting parent, causing a deadlock.

Changes Made

Make the supplied task environment authoritative for ordinary variables. Restrict startup reconciliation to the six documented Windows WOW64 architecture/Program Files variables, retaining their existing forward and reverse mappings. Unix hosts apply none of these Windows-specific adjustments. This corrects the reconciliation policy rather than special-casing the coordinator token or adding a launch-time environment exchange.

Add regression coverage for environment additions/replacements/removals, task mutations, host reuse, concurrent build managers, cross-bitness execution, nested coordinator grants, and static-graph restore. Update the related documentation.

Compatibility

The correction uses existing Change Wave 18.12. MSBUILDDISABLEFEATURESFROMVERSION=18.12 restores legacy reconciliation and can reintroduce the deadlock. No public APIs, diagnostics, or packet formats change. The modern .NET and .NET Framework hosts receive the fix; the separate legacy CLR2 host is unchanged.

Testing

Validation of the original fix, before subsequent merges from main:

• Before the fix, three MT regression cases failed as expected: addition lost, replacement reverted, and removal resurrected.
• build.cmd -v quiet succeeded with no warnings or errors. Bootstrap help and the repository sample build also succeeded.
• 91 focused cases passed across net472 and net11.0. Coordinator cases require both parent and child to receive grants under a one-node budget; static-graph restore uses cleared package sources.
• Two existing apphost checks passed with SourceLink disabled to bypass an SDK task-construction failure also reproduced under legacy reconciliation.

Not rerun after the latest merges from main. Actual Unix execution remains for CI; the policy tests cover the Unix branch.

Dependencies and Follow-up

None. This does not depend on enlightening NuGet's static-graph restore task.