[WASI] add trailing header support to WasiHTTPHandler

src/libraries/System.Net.Http/src/System/Net/Http/WasiHttpHandler/WasiHttpHandler.cs

@@ -60,7 +60,7 @@ public async Task<HttpResponseMessage> SendRequestAsync(HttpRequestMessage reque
                 // we will leave scope of this method
                 // we need to pass the ownership of the request and this wrapper to the response (via response content stream)
                 // unless we know that we are not streaming anymore
-                incomingStream = new WasiInputStream(this, incomingResponse.Consume());// passing self ownership, passing body ownership
+                incomingStream = new WasiInputStream(this, incomingResponse.Consume(), response);// passing self ownership, passing body ownership
                 response.Content = new StreamContent(incomingStream); // passing incomingStream ownership to SendAsync() caller
                 WasiHttpInterop.ConvertResponseHeaders(incomingResponse, response);
 

src/libraries/System.Net.Http/src/System/Net/Http/WasiHttpHandler/WasiHttpInterop.cs

@@ -178,6 +178,16 @@ public static void ConvertResponseHeaders(IncomingResponse incomingResponse, Htt
             }
         }
 
+        public static HttpResponseHeaders ConvertTrailingResponseHeaders(Fields headers)
+        {
+            var result = new HttpResponseHeaders();
+            foreach ((var key, var value) in headers.Entries())
+            {
+                result.Add(key, Encoding.UTF8.GetString(value));
+            }
+            return result;
+        }
+
         private static bool IsContentHeader(string headerName)
         {
             return HeaderDescriptor.TryGet(headerName, out HeaderDescriptor descriptor) && (descriptor.HeaderType & HttpHeaderType.Content) != 0;

src/libraries/System.Net.Http/src/System/Net/Http/WasiHttpHandler/WasiInputStream.cs

@@ -18,6 +18,7 @@ internal sealed class WasiInputStream : Stream
         private WasiRequestWrapper wrapper; // owned by this instance
         private IncomingBody body; // owned by this instance
         private InputStream stream; // owned by this instance
+        private HttpResponseMessage response;
 
         private int offset;
         private byte[]? buffer;
@@ -28,11 +29,16 @@ internal sealed class WasiInputStream : Stream
         public override bool CanWrite => false;
         public override bool CanSeek => false;
 
-        public WasiInputStream(WasiRequestWrapper wrapper, IncomingBody body)
+        public WasiInputStream(
+            WasiRequestWrapper wrapper,
+            IncomingBody body,
+            HttpResponseMessage response
+        )
         {
             this.wrapper = wrapper;
             this.body = body;
             this.stream = body.Stream();
+            this.response = response;
         }
 
         ~WasiInputStream()
@@ -111,6 +117,7 @@ CancellationToken cancellationToken
                         if (((StreamError)e.Value).Tag == StreamError.CLOSED)
                         {
                             otherSideClosed = true;
+                            await ReadTrailingHeaders(cancellationToken).ConfigureAwait(false);
                             return 0;
                         }
                         else
@@ -158,6 +165,46 @@ public override async ValueTask<int> ReadAsync(
             return result;
         }
 
+        private async Task ReadTrailingHeaders(CancellationToken cancellationToken)
+        {
+            isClosed = true;
+            stream.Dispose();
+            using var futureTrailers = IncomingBody.Finish(body);
+            while (true)
+            {
+                var trailers = futureTrailers.Get();
+                if (trailers is null)
+                {
+                    cancellationToken.ThrowIfCancellationRequested();
+                    await WasiHttpInterop
+                        .RegisterWasiPollable(futureTrailers.Subscribe(), cancellationToken)
+                        .ConfigureAwait(false);
+                }
+                else
+                {
+                    var inner = ((Result<Result<Fields?, ErrorCode>, None>)trailers!).AsOk;
+                    if (inner.IsOk)
+                    {
+                        using var headers = inner.AsOk;
+                        if (headers is not null)
+                        {
+                            response.StoreReceivedTrailingHeaders(
+                                WasiHttpInterop.ConvertTrailingResponseHeaders(headers)
+                            );
+                        }
+
+                        break;
+                    }
+                    else
+                    {
+                        throw new HttpRequestException(
+                            WasiHttpInterop.ErrorCodeToString(inner.AsErr)
+                        );
+                    }
+                }
+            }
+        }
+
         #region PlatformNotSupported
 
         public override void Flush()