Compose DOTNET_STARTUP_HOOKS across all Android environment sources instead of overwriting

[Copilot]

DOTNET_STARTUP_HOOKS was being rewritten by Hot Reload/trimmable typemap targets from @(RuntimeEnvironmentVariable) only, which silently dropped values provided via @(AndroidEnvironment) files. This change now composes startup hooks in two layers: contribution in targets, and final merge at environment consolidation time.

• Compose target-level startup-hook contributions before environment file generation

  • Added _ComposeDotNetStartupHooks in Xamarin.Android.Common.targets (wired before _GenerateEnvironmentFiles).
  • Collects hooks from:
    • internal contributors via @(_AndroidDotnetStartupHooks)
    • existing @(RuntimeEnvironmentVariable) entries for DOTNET_STARTUP_HOOKS
  • Removes duplicates and emits one RuntimeEnvironmentVariable value joined with :.

• Merge repeated DOTNET_STARTUP_HOOKS definitions at the final consolidation layer

  • Updated EnvironmentBuilder to merge repeated DOTNET_STARTUP_HOOKS values (from environment files and generated entries) instead of last-write-wins.
  • Uses colon-separated merge with deduplication so the final environment has one effective DOTNET_STARTUP_HOOKS value.

• Stop Hot Reload from mutating @(RuntimeEnvironmentVariable) directly

  • _AndroidConfigureHotReloadEnvironment now contributes the normalized assembly name to @(_AndroidDotnetStartupHooks) instead of remove/re-add on DOTNET_STARTUP_HOOKS.
  • Keeps RuntimeHostConfigurationOption behavior unchanged.

• Add focused coverage for cross-source merge behavior

  • Added a build test in EnvironmentContentTests that defines:
    • a runtime-provided hook (RuntimeEnvironmentVariable)
    • a file-provided hook (AndroidEnvironment)
  • Verifies final DOTNET_STARTUP_HOOKS contains both values.

<ItemGroup>
  <_AndroidDotnetStartupHooks Include="@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_STARTUP_HOOKS')->'%(Value)')" />
</ItemGroup>

<RemoveDuplicates Inputs="@(_AndroidDotnetStartupHooks)">
  <Output TaskParameter="Filtered" ItemName="_AndroidDotnetStartupHooksDistinct" />
</RemoveDuplicates>

<RuntimeEnvironmentVariable Remove="DOTNET_STARTUP_HOOKS" />
<RuntimeEnvironmentVariable Include="DOTNET_STARTUP_HOOKS"
    Value="@(_AndroidDotnetStartupHooksDistinct,':')"
    Condition="'@(_AndroidDotnetStartupHooksDistinct)' != ''" />

if (name == "DOTNET_STARTUP_HOOKS" && environmentVariables.TryGetValue (escapedName, out string? existingStartupHooks)) {
	environmentVariables [escapedName] = MergeDotNetStartupHooks (existingStartupHooks, escapedValue);
} else {
	environmentVariables [escapedName] = escapedValue;
}

[simonrozsival]

@copilot address my questions

[simonrozsival]

This target does not have Inputs and Outputs, should it have them?

[Copilot]

Good question. I kept _ComposeDotNetStartupHooks without Inputs/Outputs intentionally because it only transforms in-memory MSBuild items/properties for the current build graph; persisting incremental state there could skip needed recomposition. I also moved the final cross-file merge to EnvironmentBuilder in 120aedf, so this target stays lightweight and order-safe.

[simonrozsival]

If I understand it correctly, we will leave the original DOTNET_STARTUP_HOOKS values in the env files? Doesn't this mean that we will have multiple definitions of DOTNET_STARTUP_HOOKS in the final list of environment variables? Shouldn't we move the merging logic to the GenerateAdditionalProviderSources or EnvironmentBuilder or some other place where we eventually merge all the environment variables into a single list of definitions?

[Copilot]

You're right. I changed this in 120aedf: _ComposeDotNetStartupHooks no longer reads @(AndroidEnvironment)/@(LibraryEnvironments) lines, and EnvironmentBuilder now merges repeated DOTNET_STARTUP_HOOKS definitions while parsing the final environment inputs. This moves merging to the consolidation point and avoids duplicate final definitions.

[Copilot]

@copilot address my questions

Addressed in 120aedf. I answered both review questions and updated the implementation so final DOTNET_STARTUP_HOOKS merging happens in EnvironmentBuilder (the environment consolidation point), while _ComposeDotNetStartupHooks only prepares runtime/internal contributions.