Move DefaultServiceName to MutableSettings (#7530)

## Summary of changes

- Expose `DefaultServiceName` on `MutableSettings` instead of on
`TracerManager`
- Expose `MutableSettings` on `PerTraceSettings`

## Reason for change

The `DefaultServiceName` depends on `ServiceName`, which can change at
runtime, so `MutableSettings` seems like the best place for it
(eventually we will only have a single `TracerManager` and
`TracerSettings` per lifetime

> Note that this also solves an existing edge-case bug when customers do
config in code and already-finished traces are serialized with the
"incorrect" default service name.

## Implementation details

Mostly commit-by-commit but:

- Move the "fallback application name" calculation to a helper class
- Expose the fallback application name on `TracerSettings`
- Created as a `Lazy<>` because this calculation can be kind of
expensive, and isn't necessary if the customer specifies a service name.
  - Exposed on `TracerSettings` because it doesn't change
- Expose `DefaultServiceName` on `MutableSettings` instead of
`TracerManager`.
- Update usages to point to the new location

Additionally, there are some places where I believe we were
"incorrectly" using the `DD_SERVICE` value, and ignoring the real
fallback name.
- [x] Service Discovery's `StoreTracerMetadata` - @anna-git can you
confirm if I'm correct that this _should_ be using the "calculated"
service name as a fallback?
- [x] `TraceExporterConfiguration` - @ganeshnj, same question, can you
confirm that we should be passing the "calculated" service name, not
just the "explictly set" service name?

One additional aspect I think we should consider: 
- Currently we're calling `NormalizeService(serviceName)` in a few
places. Is there any reason we shouldn't be _always_ normalizing the
service name?
- e.g. why don't we normalize `DefaultServiceName` automatically?

## Test coverage

Covered by existing tests generally

## Other details

https://datadoghq.atlassian.net/browse/LANGPLAT-819

Part of a config stack

- #7522
- #7525
- #7530 👈
- #7532
- #7543
- #7544

Author: andrewlock

tracer/src/Datadog.Trace/Ci/TestOptimizationTracerManager.cs

@@ -38,7 +38,6 @@ public TestOptimizationTracerManager(
             ITelemetryController telemetry,
             IDiscoveryService discoveryService,
             DataStreamsManager dataStreamsManager,
-            string defaultServiceName,
             IGitMetadataTagsProvider gitMetadataTagsProvider,
             ITraceSampler traceSampler,
             ISpanSampler spanSampler,
@@ -56,7 +55,6 @@ public TestOptimizationTracerManager(
                 telemetry,
                 discoveryService,
                 dataStreamsManager,
-                defaultServiceName,
                 gitMetadataTagsProvider,
                 traceSampler,
                 spanSampler,
@@ -156,7 +154,6 @@ public LockedManager(
                 ITelemetryController telemetry,
                 IDiscoveryService discoveryService,
                 DataStreamsManager dataStreamsManager,
-                string defaultServiceName,
                 IGitMetadataTagsProvider gitMetadataTagsProvider,
                 ITraceSampler traceSampler,
                 ISpanSampler spanSampler,
@@ -174,7 +171,6 @@ public LockedManager(
                 telemetry,
                 discoveryService,
                 dataStreamsManager,
-                defaultServiceName,
                 gitMetadataTagsProvider,
                 traceSampler,
                 spanSampler,

tracer/src/Datadog.Trace/Ci/TestOptimizationTracerManagerFactory.cs

@@ -47,7 +47,6 @@ protected override TracerManager CreateTracerManagerFrom(
             ITelemetryController telemetry,
             IDiscoveryService discoveryService,
             DataStreamsManager dataStreamsManager,
-            string defaultServiceName,
             IGitMetadataTagsProvider gitMetadataTagsProvider,
             ITraceSampler traceSampler,
             ISpanSampler spanSampler,
@@ -59,10 +58,10 @@ protected override TracerManager CreateTracerManagerFrom(
             telemetry.RecordTestOptimizationSettings(_settings);
             if (_testOptimizationTracerManagement.UseLockedTracerManager)
             {
-                return new TestOptimizationTracerManager.LockedManager(settings, agentWriter, scopeManager, statsd, runtimeMetrics, logSubmissionManager, telemetry, discoveryService, dataStreamsManager, defaultServiceName, gitMetadataTagsProvider, traceSampler, spanSampler, remoteConfigurationManager, dynamicConfigurationManager, tracerFlareManager, spanEventsManager);
+                return new TestOptimizationTracerManager.LockedManager(settings, agentWriter, scopeManager, statsd, runtimeMetrics, logSubmissionManager, telemetry, discoveryService, dataStreamsManager, gitMetadataTagsProvider, traceSampler, spanSampler, remoteConfigurationManager, dynamicConfigurationManager, tracerFlareManager, spanEventsManager);
             }
 
-            return new TestOptimizationTracerManager(settings, agentWriter, scopeManager, statsd, runtimeMetrics, logSubmissionManager, telemetry, discoveryService, dataStreamsManager, defaultServiceName, gitMetadataTagsProvider, traceSampler, spanSampler, remoteConfigurationManager, dynamicConfigurationManager, tracerFlareManager, spanEventsManager);
+            return new TestOptimizationTracerManager(settings, agentWriter, scopeManager, statsd, runtimeMetrics, logSubmissionManager, telemetry, discoveryService, dataStreamsManager, gitMetadataTagsProvider, traceSampler, spanSampler, remoteConfigurationManager, dynamicConfigurationManager, tracerFlareManager, spanEventsManager);
         }
 
         protected override ITelemetryController CreateTelemetryController(TracerSettings settings, IDiscoveryService discoveryService)

tracer/src/Datadog.Trace/Configuration/DynamicConfigurationManager.cs

@@ -247,14 +247,12 @@ private ApplyDetails[] ConfigurationUpdated(
         private void ApplyMergedConfiguration(List<RemoteConfiguration> remoteConfigurations)
         {
             // Get current service/environment for filtering
-            var currentSettings = Tracer.Instance.Settings;
-            var serviceName = currentSettings.ServiceName;
-            var environment = currentSettings.Environment ?? Tracer.Instance.DefaultServiceName;
+            var currentSettings = Tracer.Instance.CurrentTraceSettings.Settings;
 
             var mergedConfigJToken = ApmTracingConfigMerger.MergeConfigurations(
                 remoteConfigurations,
-                serviceName,
-                environment);
+                serviceName: currentSettings.ServiceName,
+                environment: currentSettings.Environment);
 
             var configurationSource = new DynamicConfigConfigurationSource(mergedConfigJToken, ConfigurationOrigins.RemoteConfig);
 

tracer/src/Datadog.Trace/Configuration/MutableSettings.cs

@@ -47,6 +47,7 @@ private MutableSettings(
         bool startupDiagnosticLogEnabled,
         string? environment,
         string? serviceName,
+        string defaultServiceName,
         string? serviceVersion,
         HashSet<string> disabledIntegrationNames,
         ReadOnlyDictionary<string, string> grpcTags,
@@ -74,6 +75,7 @@ private MutableSettings(
         StartupDiagnosticLogEnabled = startupDiagnosticLogEnabled;
         Environment = environment;
         ServiceName = serviceName;
+        DefaultServiceName = defaultServiceName;
         ServiceVersion = serviceVersion;
         DisabledIntegrationNames = disabledIntegrationNames;
         GrpcTags = grpcTags;
@@ -158,11 +160,19 @@ private MutableSettings(
     public string? Environment { get; }
 
     /// <summary>
-    /// Gets the service name applied to top-level spans and used to build derived service names.
+    /// Gets the user-specified service name for the application. You should typically
+    /// favor <see cref="DefaultServiceName"/> which includes the calculated application
+    /// name where an explicit service name is not provided.
     /// </summary>
     /// <seealso cref="ConfigurationKeys.ServiceName"/>
     public string? ServiceName { get; }
 
+    /// <summary>
+    /// Gets the service name applied to top-level spans and used to build derived service names.
+    /// Composed based on <see cref="ServiceName"/> if provided, or a fallback value
+    /// </summary>
+    public string DefaultServiceName { get; }
+
     /// <summary>
     /// Gets the version tag applied to all spans.
     /// </summary>
@@ -713,6 +723,7 @@ public static MutableSettings CreateUpdatedMutableSettings(
             startupDiagnosticLogEnabled: startupDiagnosticLogEnabled,
             environment: environment,
             serviceName: serviceName,
+            defaultServiceName: serviceName ?? tracerSettings.FallbackApplicationName,
             serviceVersion: serviceVersion,
             disabledIntegrationNames: disabledIntegrationNames,
             grpcTags: grpcTags,
@@ -1046,6 +1057,7 @@ public static MutableSettings CreateInitialMutableSettings(
             startupDiagnosticLogEnabled: startupDiagnosticLogEnabled,
             environment: environment,
             serviceName: serviceName,
+            defaultServiceName: serviceName ?? tracerSettings.FallbackApplicationName,
             serviceVersion: serviceVersion,
             disabledIntegrationNames: disabledIntegrationNames,
             grpcTags: grpcTags,

tracer/src/Datadog.Trace/Configuration/PerTraceSettings.cs

@@ -16,13 +16,14 @@ internal class PerTraceSettings
     {
         private readonly ConcurrentDictionary<string, string> _serviceNameCache = new();
 
-        public PerTraceSettings(ITraceSampler? traceSampler, ISpanSampler? spanSampler, IReadOnlyDictionary<string, string> serviceNames, NamingSchema schema)
+        public PerTraceSettings(ITraceSampler? traceSampler, ISpanSampler? spanSampler, IReadOnlyDictionary<string, string> serviceNames, NamingSchema schema, MutableSettings mutableSettings)
         {
             TraceSampler = traceSampler;
             SpanSampler = spanSampler;
             ServiceNames = serviceNames;
             Schema = schema;
             HasResourceBasedSamplingRule = (traceSampler?.HasResourceBasedSamplingRule ?? false) || (spanSampler?.HasResourceBasedSamplingRule ?? false);
+            Settings = mutableSettings;
         }
 
         public ITraceSampler? TraceSampler { get; }
@@ -35,6 +36,8 @@ public PerTraceSettings(ITraceSampler? traceSampler, ISpanSampler? spanSampler,
 
         public bool HasResourceBasedSamplingRule { get; }
 
+        public MutableSettings Settings { get; }
+
         internal string GetServiceName(Tracer tracer, string serviceName)
         {
             if (ServiceNames.TryGetValue(serviceName, out var name))

tracer/src/Datadog.Trace/Configuration/TracerSettings.cs

@@ -17,6 +17,7 @@
 using Datadog.Trace.LibDatadog;
 using Datadog.Trace.Logging;
 using Datadog.Trace.Logging.DirectSubmission;
+using Datadog.Trace.PlatformHelpers;
 using Datadog.Trace.Propagators;
 using Datadog.Trace.Sampling;
 using Datadog.Trace.SourceGenerators;
@@ -32,12 +33,10 @@ namespace Datadog.Trace.Configuration
     public record TracerSettings
     {
         private static readonly IDatadogLogger Log = DatadogLogging.GetLoggerFor<TracerSettings>();
-        private static readonly HashSet<string> DefaultExperimentalFeatures = new HashSet<string>()
-        {
-            "DD_TAGS"
-        };
+        private static readonly HashSet<string> DefaultExperimentalFeatures = ["DD_TAGS"];
 
         private readonly IConfigurationTelemetry _telemetry;
+        private readonly Lazy<string> _fallbackApplicationName;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="TracerSettings"/> class with default values.
@@ -646,6 +645,9 @@ not null when string.Equals(x, "lowmemory", StringComparison.OrdinalIgnoreCase)
                 config.WithKeys(ConfigurationKeys.GraphQLErrorExtensions).AsString(),
                 commaSeparator);
 
+            // We create a lazy here because this is kind of expensive, and we want to avoid calling it if we can
+            _fallbackApplicationName = new(() => ApplicationNameHelpers.GetFallbackApplicationName(this));
+
             InitialMutableSettings = MutableSettings.CreateInitialMutableSettings(source, telemetry, errorLog, this);
             MutableSettings = InitialMutableSettings;
         }
@@ -662,6 +664,8 @@ not null when string.Equals(x, "lowmemory", StringComparison.OrdinalIgnoreCase)
 
         internal MutableSettings MutableSettings { get; init; }
 
+        internal string FallbackApplicationName => _fallbackApplicationName.Value;
+
         /// <inheritdoc cref="MutableSettings.Environment"/>
         public string? Environment => MutableSettings.Environment;
 

tracer/src/Datadog.Trace/Debugger/DebuggerManager.cs

@@ -88,12 +88,12 @@ private string GetServiceName(TracerSettings tracerSettings)
         {
             try
             {
-                return TraceUtil.NormalizeTag(tracerSettings.ServiceName ?? TracerManager.Instance.DefaultServiceName);
+                return TraceUtil.NormalizeTag(tracerSettings.MutableSettings.DefaultServiceName);
             }
             catch (Exception e)
             {
                 Log.Error(e, "Could not set `DynamicInstrumentationHelper.ServiceName`.");
-                return TracerManager.Instance.DefaultServiceName;
+                return tracerSettings.MutableSettings.DefaultServiceName;
             }
         }
 

tracer/src/Datadog.Trace/LibDatadog/ServiceDiscovery/ServiceDiscoveryHelper.cs

@@ -36,7 +36,7 @@ internal static StoreMetadataResult StoreTracerMetadata(TracerSettings tracerSet
                     TracerConstants.Language,
                     TracerConstants.ThreePartVersion,
                     Environment.MachineName,
-                    tracerSettings.ServiceName,
+                    tracerSettings.MutableSettings.DefaultServiceName,
                     tracerSettings.Environment,
                     tracerSettings.ServiceVersion);
 

tracer/src/Datadog.Trace/PlatformHelpers/ApplicationNameHelpers.cs

@@ -0,0 +1,90 @@
+// <copyright file="ApplicationNameHelpers.cs" company="Datadog">
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2 License.
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
+// </copyright>
+
+#nullable enable
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+using Datadog.Trace.Configuration;
+using Datadog.Trace.Logging;
+using Datadog.Trace.Util;
+
+namespace Datadog.Trace.PlatformHelpers;
+
+internal static class ApplicationNameHelpers
+{
+    private const string UnknownServiceName = "UnknownService";
+    private static readonly IDatadogLogger Log = DatadogLogging.GetLoggerFor(typeof(ApplicationNameHelpers));
+
+    public static string GetFallbackApplicationName(TracerSettings settings)
+        => GetApplicationName(settings) ?? UnknownServiceName;
+
+    /// <summary>
+    /// Gets a fallback "application name" for the executing application by looking at
+    /// the hosted app name (.NET Framework on IIS only), assembly name, and process name.
+    /// </summary>
+    /// <returns>The default service name.</returns>
+    private static string? GetApplicationName(TracerSettings settings)
+    {
+        try
+        {
+            if ((settings.IsRunningInAzureAppService || settings.IsRunningInAzureFunctions) &&
+                settings.AzureAppServiceMetadata?.SiteName is { } siteName)
+            {
+                return siteName;
+            }
+
+            if (settings.LambdaMetadata is { IsRunningInLambda: true, ServiceName: var serviceName })
+            {
+                return serviceName;
+            }
+
+            try
+            {
+                if (TryLoadAspNetSiteName(out siteName))
+                {
+                    return siteName;
+                }
+            }
+            catch (Exception ex)
+            {
+                // Unable to call into System.Web.dll
+                Log.Error(ex, "Unable to get application name through ASP.NET settings");
+            }
+
+            return Assembly.GetEntryAssembly()?.GetName().Name ??
+                   ProcessHelpers.GetCurrentProcessName();
+        }
+        catch (Exception ex)
+        {
+            Log.Error(ex, "Error creating default service name");
+            return null;
+        }
+    }
+
+    private static bool TryLoadAspNetSiteName([NotNullWhen(true)] out string? siteName)
+    {
+#if NETFRAMEWORK
+        try
+        {
+            // System.Web.dll is only available on .NET Framework
+            if (System.Web.Hosting.HostingEnvironment.IsHosted)
+            {
+                // if this app is an ASP.NET application, return "SiteName/ApplicationVirtualPath".
+                // note that ApplicationVirtualPath includes a leading slash.
+                siteName = (System.Web.Hosting.HostingEnvironment.SiteName + System.Web.Hosting.HostingEnvironment.ApplicationVirtualPath).TrimEnd('/');
+                return true;
+            }
+        }
+        catch (TypeLoadException ex)
+        {
+            Log.Warning(ex, "Unable to determine ASP.NET site name: HostingEnvironment type could not be loaded. This is expected when running ASP.NET Core on the .NET Framework CLR, which is not supported");
+        }
+#endif
+        siteName = null;
+        return false;
+    }
+}

tracer/src/Datadog.Trace/Tracer.cs

@@ -151,7 +151,7 @@ public IScope ActiveScope
         /// <summary>
         /// Gets the default service name for traces where a service name is not specified.
         /// </summary>
-        public string DefaultServiceName => TracerManager.DefaultServiceName;
+        public string DefaultServiceName => CurrentTraceSettings.Settings.DefaultServiceName;
 
         /// <summary>
         /// Gets the git metadata provider.