Use source-generated local proxy in ServiceJsonRpcDescriptor.Construc…

[olegtk]

Startup.SingleFile perf test indicates 20 methods are being jitted in localRpcProxies_8869a195-770f-62ef-3da5-47c5c92f39b3, which are RPC contract interfaces:

They do have source generator setup ([JsonRpcContract]), but still cause jitting because ServiceJsonRpcDescriptor.ConstructLocalProxy ignores it.

ServiceJsonRpcDescriptor.ConstructLocalProxy unconditionally went through LocalProxyGeneration, which emits a dynamic 'localRpcProxies_' assembly via Reflection.Emit and JIT's a fresh proxy method per RPC method. The vs-servicehub source generator has been emitting [LocalProxyMappingAttribute]-annotated ProxyBase subclasses for every [JsonRpcContract] interface for a while, but only ServiceJsonRpcPolyTypeDescriptor consulted them; the legacy descriptor never did, so contracts that opted into [JsonRpcContract] (e.g. RpcContracts. in DevDiv/VS) still paid the JIT cost on hot startup paths.

Make the legacy descriptor try ProxyBase.TryCreateProxy first, falling back to LocalProxyGeneration when no compatible source-generated proxy is registered or when the target lacks one of the additional service interfaces. The fallback path preserves the existing ServiceCompositionException-wrapped failure mode.

The Disposed (and the ConstructLocalProxy() null-return) changes only exist because, once you route the legacy descriptor through ProxyBase, the existing ServiceJsonRpcDescriptor_ProxyTests test class fails on three tests that exercise behavior ProxyBase didn't implement correctly:

INotifyDisposable_HandlerAddedAfterDisposal — handler added after Dispose() must fire immediately
INotifyDisposable_DisposalReleasesRef — handlers must be released for GC after Dispose()
TargetImplementsIJsonRpcLocalProxy — ConstructLocalProxy() returns null when target lacks T

Also fix two pre-existing correctness gaps in ProxyBase that this change exposed via the legacy descriptor's test suite: (1) IJsonRpcLocalProxy.ConstructLocalProxy() now returns null when the wrapped target doesn't implement T (matching the IL-emit proxy contract) instead of throwing InvalidCastException; (2) the INotifyDisposable.Disposed event uses an Interlocked sentinel pattern so that handlers added after Dispose() fire immediately and are released for GC.

[Copilot]

Pull request overview

This pull request updates the legacy ServiceJsonRpcDescriptor local-proxy construction path to prefer existing source-generated ProxyBase proxies (registered via LocalProxyMappingAttribute on [JsonRpcContract] interfaces), falling back to the existing Reflection.Emit-based proxy generation when source-gen isn’t applicable.

Changes:

• Updated ServiceJsonRpcDescriptor.ConstructLocalProxy<T> to try ProxyBase.TryCreateProxy first (with compatibility checks for any additional service interfaces), then fall back to LocalProxyGeneration.
• Fixed ProxyBase.IJsonRpcLocalProxy.ConstructLocalProxy<T>() to return null (instead of throwing) when the wrapped target doesn’t implement T.
• Improved ProxyBase disposal notification semantics by using an interlocked sentinel-backed handler store so handlers added after disposal fire immediately and aren’t retained.

Show a summary per file

File	Description
test/Microsoft.ServiceHub.Framework.Tests/ServiceJsonRpcDescriptor_ProxyTests.cs	Adds tests validating source-generated proxy selection and dynamic-proxy fallback behavior.
src/Microsoft.ServiceHub.Framework/ServiceJsonRpcDescriptor.cs	Prefers source-generated local proxies when available, preserving fallback behavior to dynamic proxy generation.
src/Microsoft.ServiceHub.Framework/Reflection/ProxyBase.cs	Aligns ConstructLocalProxy<T>() with documented null-return semantics and strengthens Disposed event handler lifetime/ordering behavior.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0

[AArnott]

This looks good, but it exposes a problem in that source generatored proxies may implement too many interfaces, leading to casts passing that ought to fail.
We have a similar issue in StreamJsonRpc which we've designed to solve, but that solution isn't present in this repo. I'm using Copilot to add to your PR to address this.