[OTLP] Use ConditionalWeakTable for cache

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufOtlpResourceSerializer.cs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 using System.Buffers;
-using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
 using OpenTelemetry.Resources;
 
 namespace OpenTelemetry.Exporter.OpenTelemetryProtocol.Implementation.Serializer;
@@ -12,7 +12,7 @@ internal static class ProtobufOtlpResourceSerializer
     private const int ReserveSizeForLength = 4;
     private const int InitialBufferSize = 2048;
 
-    private static readonly ConcurrentDictionary<Resource, byte[]> CachedResourceBytes = new();
+    private static readonly ConditionalWeakTable<Resource, byte[]> CachedResourceBytes = new();
 
     private static ReadOnlySpan<byte> EmptyResourceBytes => [0x0A, 0x80, 0x80, 0x80, 0x00];
 
@@ -24,16 +24,21 @@ internal static int WriteResource(byte[] buffer, int writePosition, Resource? re
             return writePosition + EmptyResourceBytes.Length;
         }
 
-        var cached = CachedResourceBytes.GetOrAdd(resource, static r => SerializeResourceToBytes(r));
+#if NET10_0_OR_GREATER
+        var cached = CachedResourceBytes.GetOrAdd(resource, SerializeResourceToBytes);
+#else
+        var cached = CachedResourceBytes.GetValue(resource, SerializeResourceToBytes);
+#endif
+
         Buffer.BlockCopy(cached, 0, buffer, writePosition, cached.Length);
         return writePosition + cached.Length;
     }
 
     private static byte[] SerializeResourceToBytes(Resource resource)
     {
         var pool = ArrayPool<byte>.Shared;
-
         var buffer = pool.Rent(InitialBufferSize);
+
         try
         {
             while (true)

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpResourceTests.cs

@@ -1,6 +1,7 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
+using System.Runtime.CompilerServices;
 using OpenTelemetry.Exporter.OpenTelemetryProtocol.Implementation.Serializer;
 using OpenTelemetry.Proto.Trace.V1;
 using OpenTelemetry.Resources;
@@ -65,4 +66,30 @@ public void ToOtlpResourceTest(bool includeServiceNameInResource)
             Assert.DoesNotContain(otlpResource.Attributes, kvp => kvp.Key == ResourceSemanticConventions.AttributeServiceName);
         }
     }
+
+    [Fact]
+    public void WriteResourceDoesNotKeepResourceAlive()
+    {
+        var reference = CreateSerializedResourceWeakReference();
+
+        GC.Collect();
+        GC.WaitForPendingFinalizers();
+        GC.Collect();
+
+        Assert.False(reference.TryGetTarget(out _), "Resource should not be kept alive after serialization.");
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static WeakReference<Resource> CreateSerializedResourceWeakReference()
+    {
+        var resource = ResourceBuilder.CreateEmpty()
+            .AddAttributes([new("key", "value")])
+            .Build();
+
+        var buffer = new byte[1024];
+
+        _ = ProtobufOtlpResourceSerializer.WriteResource(buffer, 0, resource);
+
+        return new WeakReference<Resource>(resource);
+    }
 }