[Hosting] Throw startup exceptions (#4006)

* Allow the hosting startup to throw for valid issues.

* CHANGELOG stub.

* CHANGELOG patch.

* Warning cleanup.

* Code review.

src/OpenTelemetry.Extensions.Hosting/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* If the OpenTelemetry SDK cannot start it will now throw exceptions and prevent
+  the host from starting.
+  ([#4006](https://github.com/open-telemetry/opentelemetry-dotnet/pull/4006))
+
 ## 1.4.0-rc.1
 
 Released 2022-Dec-12

src/OpenTelemetry.Extensions.Hosting/Implementation/HostingExtensionsEventSource.cs

@@ -15,7 +15,6 @@
 // </copyright>
 
 using System.Diagnostics.Tracing;
-using OpenTelemetry.Internal;
 
 namespace OpenTelemetry.Extensions.Hosting.Implementation
 {
@@ -27,34 +26,16 @@ namespace OpenTelemetry.Extensions.Hosting.Implementation
     {
         public static HostingExtensionsEventSource Log = new();
 
-        [NonEvent]
-        public void FailedInitialize(Exception ex)
+        [Event(1, Message = "OpenTelemetry TracerProvider was not found in application services. Tracing will remain disabled.", Level = EventLevel.Warning)]
+        public void TracerProviderNotRegistered()
         {
-            if (this.IsEnabled(EventLevel.Error, EventKeywords.All))
-            {
-                this.FailedInitialize(ex.ToInvariantString());
-            }
+            this.WriteEvent(1);
         }
 
-        [NonEvent]
-        public void FailedOpenTelemetrySDK(Exception ex)
+        [Event(2, Message = "OpenTelemetry MeterProvider was not found in application services. Metrics will remain disabled.", Level = EventLevel.Warning)]
+        public void MeterProviderNotRegistered()
         {
-            if (this.IsEnabled(EventLevel.Error, EventKeywords.All))
-            {
-                this.FailedOpenTelemetrySDK(ex.ToInvariantString());
-            }
-        }
-
-        [Event(1, Message = "An exception occurred while initializing OpenTelemetry Tracing. OpenTelemetry tracing will remain disabled. Exception: '{0}'.", Level = EventLevel.Error)]
-        public void FailedInitialize(string exception)
-        {
-            this.WriteEvent(1, exception);
-        }
-
-        [Event(2, Message = "An exception occurred while retrieving OpenTelemetry Tracer. OpenTelemetry tracing will remain disabled. Exception: '{0}'.", Level = EventLevel.Error)]
-        public void FailedOpenTelemetrySDK(string exception)
-        {
-            this.WriteEvent(2, exception);
+            this.WriteEvent(2);
         }
     }
 }

src/OpenTelemetry.Extensions.Hosting/Implementation/TelemetryHostedService.cs

@@ -33,16 +33,9 @@ internal sealed class TelemetryHostedService : IHostedService
 
     public Task StartAsync(CancellationToken cancellationToken)
     {
-        try
-        {
-            // The sole purpose of this HostedService is to ensure all
-            // instrumentations, exporters, etc., are created and started.
-            Initialize(this.serviceProvider);
-        }
-        catch (Exception ex)
-        {
-            HostingExtensionsEventSource.Log.FailedOpenTelemetrySDK(ex);
-        }
+        // The sole purpose of this HostedService is to ensure all
+        // instrumentations, exporters, etc., are created and started.
+        Initialize(this.serviceProvider);
 
         return Task.CompletedTask;
     }
@@ -57,11 +50,15 @@ internal sealed class TelemetryHostedService : IHostedService
         Debug.Assert(serviceProvider != null, "serviceProvider was null");
 
         var meterProvider = serviceProvider.GetService<MeterProvider>();
-        var tracerProvider = serviceProvider.GetService<TracerProvider>();
-
-        if (meterProvider == null && tracerProvider == null)
+        if (meterProvider == null)
         {
-            throw new InvalidOperationException("Could not resolve either MeterProvider or TracerProvider through application ServiceProvider, OpenTelemetry SDK has not been initialized.");
+            HostingExtensionsEventSource.Log.MeterProviderNotRegistered();
+        }
+
+        var tracerProvider = serviceProvider.GetService<TracerProvider>();
+        if (tracerProvider == null)
+        {
+            HostingExtensionsEventSource.Log.TracerProviderNotRegistered();
         }
     }
 }

test/OpenTelemetry.Extensions.Hosting.Tests/TelemetryHostedServiceTests.cs

@@ -0,0 +1,84 @@
+// <copyright file="TelemetryHostedServiceTests.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using Microsoft.Extensions.Hosting;
+using OpenTelemetry.Trace;
+using Xunit;
+
+namespace OpenTelemetry.Extensions.Hosting.Tests;
+
+public class TelemetryHostedServiceTests
+{
+    [Fact]
+    public async Task StartWithoutProvidersDoesNotThrow()
+    {
+        var builder = new HostBuilder().ConfigureServices(services =>
+        {
+            services.AddOpenTelemetry()
+                .StartWithHost();
+        });
+
+        var host = builder.Build();
+
+        await host.StartAsync().ConfigureAwait(false);
+
+        await host.StopAsync().ConfigureAwait(false);
+    }
+
+    [Fact]
+    public async Task StartWithExceptionsThrows()
+    {
+        bool expectedInnerExceptionThrown = false;
+
+        var builder = new HostBuilder().ConfigureServices(services =>
+        {
+            services.AddOpenTelemetry()
+                .WithTracing(builder =>
+                {
+                    builder.ConfigureBuilder((sp, sdkBuilder) =>
+                    {
+                        try
+                        {
+                            // Note: This throws because services cannot be
+                            // registered after IServiceProvider has been
+                            // created.
+                            sdkBuilder.SetSampler<MySampler>();
+                        }
+                        catch (NotSupportedException)
+                        {
+                            expectedInnerExceptionThrown = true;
+                            throw;
+                        }
+                    });
+                })
+                .StartWithHost();
+        });
+
+        var host = builder.Build();
+
+        await Assert.ThrowsAsync<NotSupportedException>(() => host.StartAsync()).ConfigureAwait(false);
+
+        await host.StopAsync().ConfigureAwait(false);
+
+        Assert.True(expectedInnerExceptionThrown);
+    }
+
+    private sealed class MySampler : Sampler
+    {
+        public override SamplingResult ShouldSample(in SamplingParameters samplingParameters)
+            => new SamplingResult(SamplingDecision.RecordAndSample);
+    }
+}