Let GC collect localSettings as soon as possible (#281)

Motivation:

There is no need to hold a reference to the localSettings after we sent
it as we only do once.

Modifications:

- Null out localSettings as soon as possible to save memory

Result:

Less memory usage in long living connections

src/main/java/io/netty/incubator/codec/http3/Http3ClientConnectionHandler.java

@@ -21,8 +21,6 @@
 
 import java.util.function.LongFunction;
 
-import static io.netty.incubator.codec.http3.Http3SettingsFrame.HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY;
-
 public final class Http3ClientConnectionHandler extends Http3ConnectionHandler {
 
     private final LongFunction<ChannelHandler> pushStreamHandlerFactory;
@@ -69,8 +67,7 @@ void initBidirectionalStream(ChannelHandlerContext ctx, QuicStreamChannel channe
 
     @Override
     void initUnidirectionalStream(ChannelHandlerContext ctx, QuicStreamChannel streamChannel) {
-        final Long maxTableCapacity = remoteControlStreamHandler.localSettings()
-                .get(HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY);
+        final long maxTableCapacity = maxTableCapacity();
         streamChannel.pipeline().addLast(
                 new Http3UnidirectionalStreamInboundClientHandler(codecFactory,
                         localControlStreamHandler, remoteControlStreamHandler,

src/main/java/io/netty/incubator/codec/http3/Http3ConnectionHandler.java

@@ -43,6 +43,8 @@ public abstract class Http3ConnectionHandler extends ChannelInboundHandlerAdapte
     final QpackEncoder qpackEncoder;
     private boolean controlStreamCreationInProgress;
 
+    final long maxTableCapacity;
+
     /**
      * Create a new instance.
      * @param server                                {@code true} if server-side, {@code false} otherwise.
@@ -71,7 +73,7 @@ public abstract class Http3ConnectionHandler extends ChannelInboundHandlerAdapte
             // Just use the maximum value we can represent via a Long.
             maxFieldSectionSize = Long.MAX_VALUE;
         }
-        long maxTableCapacity = localSettings.getOrDefault(HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY, 0);
+        this.maxTableCapacity = localSettings.getOrDefault(HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY, 0);
         int maxBlockedStreams = toIntExact(localSettings.getOrDefault(HTTP3_SETTINGS_QPACK_BLOCKED_STREAMS, 0));
         qpackDecoder = new QpackDecoder(maxTableCapacity, maxBlockedStreams);
         qpackEncoder = new QpackEncoder();
@@ -193,6 +195,10 @@ public void channelRead(ChannelHandlerContext ctx, Object msg) {
      */
     abstract void initUnidirectionalStream(ChannelHandlerContext ctx, QuicStreamChannel streamChannel);
 
+    long maxTableCapacity() {
+        return maxTableCapacity;
+    }
+
     /**
      * Always returns {@code false} as it keeps state.
      */

src/main/java/io/netty/incubator/codec/http3/Http3ControlStreamOutboundHandler.java

@@ -28,10 +28,10 @@
 final class Http3ControlStreamOutboundHandler
         extends Http3FrameTypeDuplexValidationHandler<Http3ControlStreamFrame> {
     private final boolean server;
-    private final Http3SettingsFrame localSettings;
     private final ChannelHandler codec;
     private Long sentMaxPushId;
     private Long sendGoAwayId;
+    private Http3SettingsFrame localSettings;
 
     Http3ControlStreamOutboundHandler(boolean server, Http3SettingsFrame localSettings, ChannelHandler codec) {
         super(Http3ControlStreamFrame.class);
@@ -40,15 +40,6 @@ final class Http3ControlStreamOutboundHandler
         this.codec = ObjectUtil.checkNotNull(codec, "codec");
     }
 
-    /**
-     * Returns the local settings that were sent on the control stream.
-     *
-     * @return the local {@link Http3SettingsFrame}.
-     */
-    Http3SettingsFrame localSettings() {
-        return localSettings;
-    }
-
     /**
      * Returns the last id that was sent in a MAX_PUSH_ID frame or {@code null} if none was sent yet.
      *
@@ -69,8 +60,13 @@ public void channelActive(ChannelHandlerContext ctx) {
         // Add the encoder and decoder in the pipeline so we can handle Http3Frames. This needs to happen after
         // we did write the type via a ByteBuf.
         ctx.pipeline().addFirst(codec);
+
+        assert localSettings != null;
         // If writing of the local settings fails let's just teardown the connection.
-        closeOnFailure(ctx.writeAndFlush(DefaultHttp3SettingsFrame.copyOf(localSettings)));
+        closeOnFailure(ctx.writeAndFlush(localSettings));
+
+        // Let the GC collect localSettings.
+        localSettings = null;
 
         ctx.fireChannelActive();
     }

src/main/java/io/netty/incubator/codec/http3/Http3ServerConnectionHandler.java

@@ -81,8 +81,7 @@ void initBidirectionalStream(ChannelHandlerContext ctx, QuicStreamChannel stream
 
     @Override
     void initUnidirectionalStream(ChannelHandlerContext ctx, QuicStreamChannel streamChannel) {
-        final Long maxTableCapacity = remoteControlStreamHandler.localSettings()
-                .get(HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY);
+        final long maxTableCapacity = maxTableCapacity();
         streamChannel.pipeline().addLast(
                 new Http3UnidirectionalStreamInboundServerHandler(codecFactory,
                         localControlStreamHandler, remoteControlStreamHandler,