Write grpc-web trailer names to response as lowercase

examples/Browser/Server/wwwroot/Scripts/index.js

@@ -30,10 +30,15 @@ streamInput.onclick = function () {
         request.setName(nameInput.value);
 
         streamingCall = client.sayHellos(request, {});
-        streamingCall.on('data', function (response) {
+        streamingCall.on('data', (response) => {
             resultText.innerHTML += htmlEscape(response.getMessage()) + '<br />';
         });
-        streamingCall.on('end', function () {
+        streamingCall.on('status', (status) => {
+            if (status.code == 0) {
+                resultText.innerHTML += 'Done';
+            } else {
+                resultText.innerHTML += 'Error: ' + htmlEscape(status.details);
+            }
         });
     } else {
         streamingCall.cancel();

src/Grpc.AspNetCore.Web/Internal/GrpcWebProtocolHelpers.cs

@@ -117,7 +117,17 @@ private static void WriteTrailersContent(Span<byte> buffer, IHeaderDictionary tr
                 {
                     if (value != null)
                     {
-                        var position = Encoding.ASCII.GetBytes(kv.Key, currentBuffer);
+                        // Get lower-case ASCII bytes for the key.
+                        // gRPC-Web protocol says that names should be lower-case and grpc-web JS client
+                        // will check for 'grpc-status' and 'grpc-message' in trailers with lower-case key.
+                        // https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-WEB.md#protocol-differences-vs-grpc-over-http2
+                        for (int i = 0; i < kv.Key.Length; i++)
+                        {
+                            char c = kv.Key[i];
+                            currentBuffer[i] = (byte)((uint)(c - 'A') <= ('Z' - 'A') ? c | 0x20 : c);
+                        }
+
+                        var position = kv.Key.Length;
 
                         currentBuffer[position++] = Colon;
                         currentBuffer[position++] = Space;

test/Grpc.AspNetCore.Server.Tests/Web/GrpcWebProtocolHelpersTests.cs

@@ -74,6 +74,31 @@ public void WriteTrailers_OneTrailer_WrittenToOutput()
             Assert.AreEqual("one: two\r\n", text);
         }
 
+        [Test]
+        public void WriteTrailers_OneTrailerMixedCase_WrittenToOutputLowerCase()
+        {
+            // Arrange
+            var trailers = new HeaderDictionary();
+            trailers.Add("One", "Two");
+            var output = new ArrayBufferWriter<byte>();
+
+            // Act
+            GrpcWebProtocolHelpers.WriteTrailers(trailers, output);
+
+            // Assert
+            Assert.AreEqual(15, output.WrittenSpan.Length);
+
+            Assert.AreEqual(128, output.WrittenSpan[0]);
+            Assert.AreEqual(0, output.WrittenSpan[1]);
+            Assert.AreEqual(0, output.WrittenSpan[2]);
+            Assert.AreEqual(0, output.WrittenSpan[3]);
+            Assert.AreEqual(10, output.WrittenSpan[4]);
+
+            var text = Encoding.ASCII.GetString(output.WrittenSpan.Slice(5));
+
+            Assert.AreEqual("one: Two\r\n", text);
+        }
+
         [Test]
         public void WriteTrailers_MultiValueTrailer_WrittenToOutput()
         {