RFC: Add ServiceProviderAccessor to allow access to IServiceProvider from Interceptor

src/Temporalio.Extensions.Hosting/IServiceProviderAccessor.cs

@@ -0,0 +1,16 @@
+using System;
+
+namespace Temporalio.Extensions.Hosting
+{
+    /// <summary>
+    /// Provides access to the current, scoped <see cref="IServiceProvider"/> if
+    /// one is available.
+    /// </summary>
+    public interface IServiceProviderAccessor
+    {
+        /// <summary>
+        /// Gets or sets the current service provider.
+        /// </summary>
+        IServiceProvider? ServiceProvider { get; set; }
+    }
+}

src/Temporalio.Extensions.Hosting/ServiceProviderAccessor.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Threading;
+
+namespace Temporalio.Extensions.Hosting
+{
+    /// <summary>
+    /// Provides an implementation of <see cref="IServiceProvider" /> based on
+    /// the current execution context.
+    /// </summary>
+    public class ServiceProviderAccessor : IServiceProviderAccessor
+    {
+        private static readonly AsyncLocal<ServiceProviderHolder> ServiceProviderCurrent = new();
+
+        /// <inheritdoc/>
+        public IServiceProvider? ServiceProvider
+        {
+            get => ServiceProviderCurrent.Value?.ServiceProvider;
+
+            set
+            {
+                var holder = ServiceProviderCurrent.Value;
+                if (holder != null)
+                {
+                    // Clear current IServiceProvider trapped in the AsyncLocals, as its done.
+                    holder.ServiceProvider = null;
+                }
+
+                if (value != null)
+                {
+                    // Use an object indirection to hold the IServiceProvider in the AsyncLocal,
+                    // so it can be cleared in all ExecutionContexts when its cleared.
+                    ServiceProviderCurrent.Value = new ServiceProviderHolder { ServiceProvider = value };
+                }
+            }
+        }
+
+        private sealed class ServiceProviderHolder
+        {
+            public IServiceProvider? ServiceProvider { get; set; }
+        }
+    }
+}

src/Temporalio.Extensions.Hosting/ServiceProviderExtensions.cs

@@ -68,6 +68,14 @@ public static ActivityDefinition CreateTemporalActivityDefinition(
 #else
                 var scope = provider.CreateScope();
 #endif
+                IServiceProviderAccessor? serviceProviderAccessor =
+                    scope.ServiceProvider.GetService<IServiceProviderAccessor>();
+
+                if (serviceProviderAccessor is not null)
+                {
+                    serviceProviderAccessor.ServiceProvider = scope.ServiceProvider;
+                }
+
                 try
                 {
                     object? result;
@@ -111,6 +119,10 @@ public static ActivityDefinition CreateTemporalActivityDefinition(
                 }
                 finally
                 {
+                    if (serviceProviderAccessor is not null)
+                    {
+                        serviceProviderAccessor.ServiceProvider = null;
+                    }
 #if NET6_0_OR_GREATER
                     await scope.DisposeAsync().ConfigureAwait(false);
 #else