Set property to enable streaming in WASM and gRPC-Web

src/Grpc.Net.Client.Web/GrpcWebHandler.cs

@@ -37,6 +37,8 @@ namespace Grpc.Net.Client.Web
     /// </remarks>
     public sealed class GrpcWebHandler : DelegatingHandler
     {
+        internal const string WebAssemblyEnableStreamingResponseKey = "WebAssemblyEnableStreamingResponse";
+
         /// <summary>
         /// Gets or sets the HTTP version to use when making gRPC-Web calls.
         /// <para>
@@ -114,6 +116,16 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
         private async Task<HttpResponseMessage> SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
         {
             request.Content = new GrpcWebRequestContent(request.Content, GrpcWebMode);
+
+            // Set WebAssemblyEnableStreamingResponse to true on gRPC-Web request.
+            // https://github.com/mono/mono/blob/a0d69a4e876834412ba676f544d447ec331e7c01/sdks/wasm/framework/src/System.Net.Http.WebAssemblyHttpHandler/WebAssemblyHttpHandler.cs#L149
+            //
+            // This must be set so WASM will stream the response. Without this setting the WASM HTTP handler will only
+            // return content once the entire response has been downloaded. This breaks server streaming.
+            //
+            // https://github.com/mono/mono/issues/18718
+            request.Properties[WebAssemblyEnableStreamingResponseKey] = true;
+
             if (HttpVersion != null)
             {
                 // This doesn't guarantee that the specified version is used. Some handlers will ignore it.

test/Grpc.Net.Client.Tests/Web/GrpcWebHandlerTests.cs

@@ -87,13 +87,64 @@ public async Task HttpVersion_Set_HttpRequestMessageVersionChanged()
             Assert.AreEqual(HttpVersion.Version20, response.Version);
         }
 
+        [Test]
+        public async Task SendAsync_GrpcCall_ResponseStreamingPropertySet()
+        {
+            // Arrange
+            var request = new HttpRequestMessage
+            {
+                Version = HttpVersion.Version20,
+                Content = new ByteArrayContent(Array.Empty<byte>())
+                {
+                    Headers = { ContentType = new MediaTypeHeaderValue("application/grpc") }
+                }
+            };
+            var testHttpHandler = new TestHttpHandler();
+            var grpcWebHandler = new GrpcWebHandler(GrpcWebMode.GrpcWeb, testHttpHandler);
+            var messageInvoker = new HttpMessageInvoker(grpcWebHandler);
+
+            // Act
+            await messageInvoker.SendAsync(request, CancellationToken.None);
+
+            // Assert
+            Assert.AreEqual(true, testHttpHandler.WebAssemblyEnableStreamingResponse);
+        }
+
+        [Test]
+        public async Task SendAsync_NonGrpcCall_ResponseStreamingPropertyNotSet()
+        {
+            // Arrange
+            var request = new HttpRequestMessage
+            {
+                Version = HttpVersion.Version20,
+                Content = new ByteArrayContent(Array.Empty<byte>())
+                {
+                    Headers = { ContentType = new MediaTypeHeaderValue("application/text") }
+                }
+            };
+            var testHttpHandler = new TestHttpHandler();
+            var grpcWebHandler = new GrpcWebHandler(GrpcWebMode.GrpcWeb, testHttpHandler);
+            var messageInvoker = new HttpMessageInvoker(grpcWebHandler);
+
+            // Act
+            await messageInvoker.SendAsync(request, CancellationToken.None);
+
+            // Assert
+            Assert.AreEqual(null, testHttpHandler.WebAssemblyEnableStreamingResponse);
+        }
+
         private class TestHttpHandler : HttpMessageHandler
         {
             public Version? RequestVersion { get; private set; }
+            public bool? WebAssemblyEnableStreamingResponse { get; private set; }
 
             protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
             {
                 RequestVersion = request.Version;
+                if (request.Properties.TryGetValue(GrpcWebHandler.WebAssemblyEnableStreamingResponseKey, out var enableStreaming))
+                {
+                    WebAssemblyEnableStreamingResponse = (bool)enableStreaming;
+                }
 
                 return Task.FromResult(new HttpResponseMessage()
                 {