Fix the issue where putobject requests sometimes is hanging because 100 continue response is not being read automatically

Author: zoewangg

.changes/next-release/bugfix-NettyNIOAsyncHTTPClient-5b20e7b.json

@@ -0,0 +1,5 @@
+{
+    "category": "Netty NIO Async HTTP Client", 
+    "type": "bugfix", 
+    "description": "Fix the issue where streaming requests with `Expect: 100-continue` header sometimes are hanging because 100Continue response message is not being read automatically. See [#459](https://github.com/aws/aws-sdk-java-v2/issues/459)"
+}

http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/NettyRequestExecutor.java

@@ -182,14 +182,29 @@ private void writeRequest(HttpRequest request) {
                    }
                });
 
-        // FullDuplex calls need to start reading at the same time we make the request.
-        if (context.executeRequest().fullDuplex()) {
+        if (shouldExplicitlyTriggerRead()) {
             channel.pipeline().addFirst(new ReadTimeoutHandler(context.configuration().readTimeoutMillis(),
                                                                TimeUnit.MILLISECONDS));
             channel.read();
         }
     }
 
+    /**
+     * It should explicitly trigger Read for the following situations:
+     *
+     * - FullDuplex calls need to start reading at the same time we make the request.
+     * - Request with "Expect: 100-continue" header should read the 100 continue response.
+     *
+     * @return true if it should explicitly read from channel
+     */
+    private boolean shouldExplicitlyTriggerRead() {
+        boolean is100ContinueExpected = context.executeRequest().request()
+                                               .firstMatchingHeader("Expect")
+                                               .filter(b -> b.equalsIgnoreCase("100-continue")).isPresent();
+
+        return context.executeRequest().fullDuplex() || is100ContinueExpected;
+    }
+
     private URI endpoint() {
         return context.executeRequest().request().getUri();
     }