Properly clear metadata when encoding/decoding payloads

src/Temporalio/Worker/WorkflowCodecHelper.cs

@@ -1,5 +1,6 @@
 using System.Linq;
 using System.Threading.Tasks;
+using Google.Protobuf;
 using Google.Protobuf.Collections;
 using Temporalio.Api.Common.V1;
 using Temporalio.Bridge.Api.ActivityResult;
@@ -223,6 +224,7 @@ private static async Task EncodeAsync(IPayloadCodec codec, Payload payload)
             var encodedList = await codec.EncodeAsync(new Payload[] { payload }).ConfigureAwait(false);
             var encoded = encodedList.Single();
             payload.Metadata.Clear();
+            payload.Data = ByteString.Empty;
             payload.MergeFrom(encoded);
         }
 
@@ -324,6 +326,7 @@ private static async Task DecodeAsync(IPayloadCodec codec, Payload payload)
             // We are gonna require a single result here
             var decoded = await codec.DecodeAsync(new Payload[] { payload }).ConfigureAwait(false);
             payload.Metadata.Clear();
+            payload.Data = ByteString.Empty;
             payload.MergeFrom(decoded.Single());
         }
     }

tests/Temporalio.Tests/Worker/WorkflowWorkerTests.cs

@@ -5,17 +5,20 @@
 
 using System.Collections.Concurrent;
 using System.Linq.Expressions;
+using System.Text;
 using System.Threading.Tasks.Dataflow;
 using Microsoft.Extensions.Logging;
 using Temporalio.Activities;
 using Temporalio.Api.Common.V1;
 using Temporalio.Api.Enums.V1;
+using Temporalio.Api.History.V1;
 using Temporalio.Client;
 using Temporalio.Client.Schedules;
 using Temporalio.Common;
 using Temporalio.Converters;
 using Temporalio.Exceptions;
 using Temporalio.Runtime;
+using Temporalio.Tests.Converters;
 using Temporalio.Worker;
 using Temporalio.Workflows;
 using Xunit;
@@ -5736,6 +5739,60 @@
         });
     }
 
+    [Workflow]
+    public class NullWithCodecWorkflow
+    {
+        [Activity]
+        public static string? ReturnsNull() => null;
+
+        [WorkflowRun]
+        public async Task<string?> RunAsync(string? input)
+        {
+            Assert.Null(input);
+            return await Workflow.ExecuteActivityAsync(
+                () => ReturnsNull(),
+                new() { StartToCloseTimeout = TimeSpan.FromSeconds(10) });
+        }
+    }
+
+    [Fact]
+    public async Task ExecuteWorkflowAsync_NullWithCodec_EncodedProperly()
+    {
+        // Need client with codec
+        var newOptions = (TemporalClientOptions)Client.Options.Clone();
+        newOptions.DataConverter = DataConverter.Default with
+        {
+            PayloadCodec = new Base64PayloadCodec(),
+        };
+        var client = new TemporalClient(Client.Connection, newOptions);
+
+        // Run worker
+        await ExecuteWorkerAsync<NullWithCodecWorkflow>(
+            async worker =>
+            {
+                var handle = await client.StartWorkflowAsync(
+                    (NullWithCodecWorkflow wf) => wf.RunAsync(null),
+                    new(id: $"workflow-{Guid.NewGuid()}", taskQueue: worker.Options.TaskQueue!));
+                var result = await handle.GetResultAsync();
+                Assert.Null(result);
+                // Confirm the codec is in use in history with the activity result
+                ActivityTaskCompletedEventAttributes? attrs = null;
+                await foreach (var evt in handle.FetchHistoryEventsAsync())
+                {
+                    if (evt.ActivityTaskCompletedEventAttributes is { } taskAttrs)
+                    {
+                        attrs = taskAttrs;
+                        break;
+                    }
+                }
+                Assert.Equal(
+                    Base64PayloadCodec.EncodingName,
+                    attrs?.Result?.Payloads_?.Single()?.Metadata?.GetValueOrDefault("encoding")?.ToStringUtf8());
+            },
+            new TemporalWorkerOptions().AddAllActivities<NullWithCodecWorkflow>(null),
+            client);
+    }
+
     internal static Task AssertTaskFailureContainsEventuallyAsync(
         WorkflowHandle handle, string messageContains)
     {