Allow onComplete before request in PipeliningServerHandler

http-server-netty/src/main/java/io/micronaut/http/server/netty/handler/PipeliningServerHandler.java

@@ -797,6 +797,7 @@ private final class StreamingOutboundHandler extends OutboundHandler implements
         private final OutboundAccess outboundAccess;
         private HttpResponse initialMessage;
         private Subscription subscription;
+        private boolean earlyComplete = false;
         private boolean writtenLast = false;
 
         StreamingOutboundHandler(OutboundAccess outboundAccess, HttpResponse initialMessage) {
@@ -814,7 +815,13 @@ void writeSome() {
                 write(initialMessage, false, false);
                 initialMessage = null;
             }
-            subscription.request(1);
+            if (earlyComplete) {
+                // onComplete has been called before the first writeSome. Trigger onComplete
+                // handling again.
+                onComplete();
+            } else {
+                subscription.request(1);
+            }
         }
 
         @Override
@@ -881,7 +888,9 @@ public void onComplete() {
             }
 
             if (outboundHandler != this) {
-                throw new IllegalStateException("onComplete before request?");
+                // onComplete can be called immediately after onSubscribe, before request.
+                earlyComplete = true;
+                return;
             }
 
             outboundHandler = null;

http-server-netty/src/test/groovy/io/micronaut/http/server/netty/handler/PipeliningServerHandlerSpec.groovy

@@ -21,6 +21,7 @@ import io.netty.handler.codec.http.HttpRequest
 import io.netty.handler.codec.http.HttpResponse
 import io.netty.handler.codec.http.HttpResponseStatus
 import io.netty.handler.codec.http.HttpVersion
+import io.netty.handler.codec.http.LastHttpContent
 import reactor.core.publisher.Flux
 import reactor.core.publisher.Sinks
 import spock.lang.Issue
@@ -290,6 +291,45 @@ class PipeliningServerHandlerSpec extends Specification {
         completeOnCancel << [true, false]
     }
 
+    def 'empty streaming response while in queue'() {
+        given:
+        def resp = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK)
+        resp.headers().add(HttpHeaderNames.TRANSFER_ENCODING, HttpHeaderValues.CHUNKED)
+        def sink = Sinks.many().unicast().<HttpContent>onBackpressureBuffer()
+        def ch = new EmbeddedChannel(new PipeliningServerHandler(new RequestHandler() {
+            int i = 0
+
+            @Override
+            void accept(ChannelHandlerContext ctx, HttpRequest request, PipeliningServerHandler.OutboundAccess outboundAccess) {
+                if (i++ == 0) {
+                    outboundAccess.writeStreamed(resp, sink.asFlux())
+                } else {
+                    outboundAccess.writeStreamed(resp, Flux.empty())
+                }
+            }
+
+            @Override
+            void handleUnboundError(Throwable cause) {
+                cause.printStackTrace()
+            }
+        }))
+
+        when:
+        ch.writeInbound(new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, "/"))
+        ch.writeInbound(new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, "/"))
+        then:
+        ch.readOutbound() == null
+
+        when:
+        sink.tryEmitComplete()
+        then:
+        ch.readOutbound() == resp
+        ch.readOutbound() == LastHttpContent.EMPTY_LAST_CONTENT
+        ch.readOutbound() == resp
+        ch.readOutbound() == LastHttpContent.EMPTY_LAST_CONTENT
+        ch.readOutbound() == null
+    }
+
     static class MonitorHandler extends ChannelOutboundHandlerAdapter {
         int flush = 0
         int read = 0