Fix OTLP traces export routed through APM Unix domain socket instead of OTLP endpoint

tracer/src/Datadog.Trace/Agent/ManagedApiOtlp.cs

@@ -8,13 +8,12 @@
 #nullable enable
 
 using System;
-using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
-using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using Datadog.Trace.Agent.Transports;
 using Datadog.Trace.Configuration;
-using Datadog.Trace.DogStatsd;
+using Datadog.Trace.Logging;
 
 namespace Datadog.Trace.Agent;
 
@@ -23,6 +22,8 @@ namespace Datadog.Trace.Agent;
 /// </summary>
 internal sealed class ManagedApiOtlp : IApi
 {
+    private static readonly IDatadogLogger Log = DatadogLogging.GetLoggerFor<ManagedApiOtlp>();
+
     private IApi _api;
 
     public ManagedApiOtlp(TracerSettings settings)
@@ -34,7 +35,7 @@ public ManagedApiOtlp(TracerSettings settings)
         [MemberNotNull(nameof(_api))]
         void UpdateApi(TracerSettings settings, ExporterSettings exporterSettings)
         {
-            var apiRequestFactory = TracesTransportStrategy.Get(exporterSettings);
+            var apiRequestFactory = CreateOtlpRequestFactory(exporterSettings);
             var api = new ApiOtlp(apiRequestFactory, settings, exporterSettings);
             Interlocked.Exchange(ref _api!, api);
         }
@@ -49,6 +50,21 @@ public Task<bool> SendTracesAsync(ArraySegment<byte> traces, int numberOfTraces,
 
     public Task<bool> SendStatsAsync(StatsBuffer stats, long bucketDuration)
         => Volatile.Read(ref _api).SendStatsAsync(stats, bucketDuration);
+
+    /// <summary>
+    /// Creates an <see cref="IApiRequestFactory"/> for OTLP export using the OTLP endpoint
+    /// directly, bypassing the APM trace transport strategy. This is necessary because
+    /// <see cref="TracesTransportStrategy"/> selects the transport based on the APM agent
+    /// configuration (e.g. the APM Unix domain socket), which is incorrect for OTLP export
+    /// that should use the OTLP-specific endpoint.
+    /// </summary>
+    internal static IApiRequestFactory CreateOtlpRequestFactory(ExporterSettings exporterSettings)
+    {
+        var otlpEndpoint = exporterSettings.OtlpTracesEndpoint;
+        var baseEndpoint = new Uri($"{otlpEndpoint.Scheme}://{otlpEndpoint.Authority}");
+        Log.Debug("Using " + nameof(HttpClientRequestFactory) + " for OTLP traces transport to {Endpoint}", baseEndpoint);
+        return new HttpClientRequestFactory(baseEndpoint, AgentHttpHeaderNames.DefaultHeaders);
+    }
 }
 
 #endif

tracer/test/Datadog.Trace.Tests/Agent/ManagedApiOtlpTests.cs

@@ -0,0 +1,90 @@
+// <copyright file="ManagedApiOtlpTests.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>
+
+#if NET6_0_OR_GREATER
+
+using System;
+using System.Collections.Specialized;
+using Datadog.Trace.Agent;
+using Datadog.Trace.Agent.Transports;
+using Datadog.Trace.Configuration;
+using Datadog.Trace.Configuration.Telemetry;
+using FluentAssertions;
+using Xunit;
+
+namespace Datadog.Trace.Tests.Agent;
+
+public class ManagedApiOtlpTests
+{
+    [Fact]
+    public void CreateOtlpRequestFactory_WhenApmUsesUnixDomainSocket_UsesHttpForOtlp()
+    {
+        // Simulate the Linux scenario: APM UDS exists at the default path,
+        // and the OTLP endpoint defaults to http://localhost:4318
+        var source = BuildSource("OTEL_EXPORTER_OTLP_ENDPOINT:http://localhost:4318");
+        Func<string, bool> fileExists = path => path == ExporterSettings.DefaultTracesUnixDomainSocket;
+        var exporterSettings = new ExporterSettings(source, fileExists, NullConfigurationTelemetry.Instance);
+
+        // Verify the APM transport is UDS (this is the precondition that caused the bug)
+        exporterSettings.TracesTransport.Should().Be(TracesTransportType.UnixDomainSocket);
+
+        var factory = ManagedApiOtlp.CreateOtlpRequestFactory(exporterSettings);
+
+        // The factory must be a plain HttpClientRequestFactory, not a
+        // SocketHandlerRequestFactory (which would route through the APM UDS)
+        factory.Should().BeOfType<HttpClientRequestFactory>();
+    }
+
+    [Theory]
+    [InlineData("http://localhost:4318", "http://localhost:4318/v1/traces")]
+    [InlineData("http://otel-collector:4318", "http://otel-collector:4318/v1/traces")]
+    [InlineData("http://localhost:9999", "http://localhost:9999/v1/traces")]
+    public void CreateOtlpRequestFactory_UsesCorrectOtlpEndpoint(string otlpEndpoint, string expectedTracesEndpoint)
+    {
+        var source = BuildSource($"OTEL_EXPORTER_OTLP_ENDPOINT:{otlpEndpoint}");
+        Func<string, bool> fileExists = path => path == ExporterSettings.DefaultTracesUnixDomainSocket;
+        var exporterSettings = new ExporterSettings(source, fileExists, NullConfigurationTelemetry.Instance);
+
+        // Verify the OTLP endpoint is correct
+        exporterSettings.OtlpTracesEndpoint.Should().Be(new Uri(expectedTracesEndpoint));
+
+        var factory = ManagedApiOtlp.CreateOtlpRequestFactory(exporterSettings);
+
+        // The factory should target the OTLP endpoint's authority
+        var expectedBase = new Uri($"{new Uri(expectedTracesEndpoint).Scheme}://{new Uri(expectedTracesEndpoint).Authority}");
+        factory.GetEndpoint("/v1/traces").Should().Be(new Uri(expectedBase, "/v1/traces"));
+    }
+
+    [Fact]
+    public void CreateOtlpRequestFactory_WithExplicitTracesEndpoint_UsesSignalEndpoint()
+    {
+        var source = BuildSource(
+            "OTEL_EXPORTER_OTLP_ENDPOINT:http://general-endpoint:4318",
+            "OTEL_EXPORTER_OTLP_TRACES_ENDPOINT:http://traces-endpoint:4318/v1/traces");
+        var exporterSettings = new ExporterSettings(source, _ => false, NullConfigurationTelemetry.Instance);
+
+        exporterSettings.OtlpTracesEndpoint.Should().Be(new Uri("http://traces-endpoint:4318/v1/traces"));
+
+        var factory = ManagedApiOtlp.CreateOtlpRequestFactory(exporterSettings);
+
+        factory.Should().BeOfType<HttpClientRequestFactory>();
+        factory.GetEndpoint("/v1/traces").Should().Be(new Uri("http://traces-endpoint:4318/v1/traces"));
+    }
+
+    private static NameValueConfigurationSource BuildSource(params string[] config)
+    {
+        var configNameValues = new NameValueCollection();
+
+        foreach (var item in config)
+        {
+            var separatorIndex = item.IndexOf(':');
+            configNameValues.Add(item.Substring(0, separatorIndex), item.Substring(separatorIndex + 1));
+        }
+
+        return new NameValueConfigurationSource(configNameValues);
+    }
+}
+
+#endif