Add remote feature flag for cache plugins via environment variable

[Copilot]

Summary

Adds "CachePlugins" feature flag to control cache plugin execution (Quickbuild, etc.) at runtime via the MSBUILD_CACHEPLUGINS_DISABLED environment variable. Enabled by default for backward compatibility, can be remotely disabled by Visual Studio if plugin issues arise.

Customer Impact

Provides kill switch for cache plugins when MSBuild cannot control their versioning. VS can use remote feature flags to set the environment variable and disable cache plugins. No impact when enabled (default state). When disabled, builds proceed without caching.

Regression?

No. Feature enabled by default maintains existing behavior.

Testing

• 100 existing project cache tests pass
• 2 new tests verify flag correctly enables/disables cache plugin initialization and usage via environment variable
• Tested graph builds, VS scenarios, and ShouldUseCache code paths

Risk

Low. Changes are defensive checks that gate existing functionality. Three guarded entry points:

• ProjectCacheService.InitializePluginsForGraph() - graph build initialization
• ProjectCacheService.InitializePluginsForVsScenario() - VS scenario initialization
• ProjectCacheService.ShouldUseCache() - runtime cache queries

Feature checks MSBUILD_CACHEPLUGINS_DISABLED environment variable. When set to "1" or "true", returns FeatureStatus.NotAvailable and cache plugins are disabled. When not set or empty, returns FeatureStatus.Available (default).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Wrap Quickbuild with a remote feature flag to be able to disable it.</issue_title>
<issue_description>MSBuild doesn't control versioning for this Quickbuild plugging.
Wrapping this with the remote flag would give us move control if something goes wrong with this dependency.</issue_description>

Comments on the Issue (you are @copilot in this section)

@rainersigwald I'd say not "quickbuild" but "any cache plugin"

• Fixes Wrap Quickbuild with a remote feature flag to be able to disable it. #11723

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[YuliiaKovalova]

@copilot edit it by considering how remote flags work in VS (e.g:
if (featureFlags.IsFeatureEnabled("MSBuild.VSTelemetryDisabled", false))
{
Environment.SetEnvironmentVariable("MSBUILD_TELEMETRY_OPTOUT", "1", EnvironmentVariableTarget.Process);
}
})

it should go through env variable setup

[Copilot]

@copilot edit it by considering how remote flags work in VS (e.g:
if (featureFlags.IsFeatureEnabled("MSBuild.VSTelemetryDisabled", false))
{
...

Updated to use environment variable pattern. The feature is now controlled via MSBUILD_CACHEPLUGINS_DISABLED environment variable, which VS can set through remote feature flags. When set to "1" or "true", cache plugins are disabled. Commit db114bd.

[YuliiaKovalova]

why it's not a part of CreateDefaultFeatureStatusMap?

[YuliiaKovalova]

@copilot why it's not a part of CreateDefaultFeatureStatusMap?

[Copilot]

The CachePlugins feature needs to check the MSBUILD_CACHEPLUGINS_DISABLED environment variable at runtime, which is different from other features that have static values in the map. The environment variable approach allows VS to dynamically control the feature through remote flags by setting the env var before MSBuild starts, following the same pattern as MSBUILD_TELEMETRY_OPTOUT. If we put it in CreateDefaultFeatureStatusMap(), it would only check the env var once at initialization, rather than on each call to CheckFeatureAvailability().