Fix - Http3FrameCodec decode fail during unknown settings

**Motivation:**
The change in
[https://github.com/netty/netty/pull/15909](https://github.com/netty/netty/pull/15909)
is merged but not yet released. While working on HTTP/3 connections I
noticed a logical issue: when an unknown settings key is received, the
current implementation returns a value, which causes Http3FrameCodec to
treat it as an error (because settings.put(key, value) returns the
previous value for known keys and null for new ones). For unknown keys
we should ignore them, so we now always return null to achieve the
expected behavior.

**Modification:**
* [x] settings.put(key, value) now returns null for unknown keys (non
null triggers an error in Http3FrameCodec decoding).

* [x] Added unit tests to verify this.

> Note: Http3CodecTest didn’t catch the issue previously because it used
frames produced by the same put logic, so both encoding and decoding
behaved consistently and the issue was hidden.

Port of netty/netty#15998

Author: normanmaurer

src/main/java/io/netty/incubator/codec/http3/Http3SettingIdentifier.java

@@ -90,11 +90,7 @@ public enum Http3SettingIdentifier {
     private final long id;
 
     private static final Map<Long, Http3SettingIdentifier> LOOKUP = Collections.unmodifiableMap(
-            Arrays.stream(values())
-                    .collect(Collectors.toMap(
-                            Http3SettingIdentifier::id,
-                            Function.identity()
-                    ))
+            Arrays.stream(values()).collect(Collectors.toMap(Http3SettingIdentifier::id, Function.identity()))
     );
 
     Http3SettingIdentifier(long id) {

src/main/java/io/netty/incubator/codec/http3/Http3Settings.java

@@ -119,7 +119,7 @@ public final class Http3Settings implements Iterable<Map.Entry<Long, Long>> {
      * Creates a new instance
      */
     public Http3Settings() {
-        this.settings = new LongObjectHashMap<>(4);
+        this.settings = new LongObjectHashMap<>(Http3SettingIdentifier.values().length);
     }
 
     /**
@@ -164,7 +164,7 @@ public Long put(long key, Long value) {
 
         // When Non-Standard/Unknown settings identifier identifier present - Ignore
         if (identifier == null) {
-            return value;
+            return null;
         }
 
         //Validation

src/test/java/io/netty/incubator/codec/http3/Http3SettingsTest.java

@@ -32,7 +32,6 @@
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
-import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 
 /**
  * unit tests for {@link Http3Settings}.
@@ -264,4 +263,46 @@ void testCopyFromNullThrows() {
         Http3Settings settings = new Http3Settings();
         assertThrows(NullPointerException.class, () -> settings.putAll(null));
     }
+
+    @Test
+    void duplicateSettingsValuesInsideFrameTest() {
+        Http3SettingsFrame settingsFrame = new DefaultHttp3SettingsFrame();
+        assertNull(settingsFrame.put(Http3SettingIdentifier.HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY.id(), 100L));
+        assertNull(settingsFrame.put(Http3SettingIdentifier.HTTP3_SETTINGS_QPACK_BLOCKED_STREAMS.id(), 1L));
+        assertNull(settingsFrame.put(Http3SettingIdentifier.HTTP3_SETTINGS_MAX_FIELD_SECTION_SIZE.id(), 128L));
+        assertNull(settingsFrame.put(Http3SettingIdentifier.HTTP3_SETTINGS_ENABLE_CONNECT_PROTOCOL.id(), 0L));
+        assertNull(settingsFrame.put(Http3SettingIdentifier.HTTP3_SETTINGS_H3_DATAGRAM.id(), 1L));
+        //known headers should not contain duplicate so give non-null
+        // which is used in http3framecodec to throw error
+        assertNotNull(settingsFrame.put(Http3SettingIdentifier.HTTP3_SETTINGS_H3_DATAGRAM.id(), 1L));
+        // Ensure we can encode and decode all sizes correctly.
+        // unknown settings id/key will be ignored
+        assertNull(settingsFrame.put(63, 63L));
+        assertNull(settingsFrame.put(16383, 16383L));
+        assertNull(settingsFrame.put(1073741823, 1073741823L));
+        assertNull(settingsFrame.put(4611686018427387903L, 4611686018427387903L));
+        //even duplicates of unknown ignored as we ignore unknown
+        assertNull(settingsFrame.put(4611686018427387903L, 4611686018427387903L));
+    }
+
+    @Test
+    void duplicateSettingsValuesInsideHttp3SettingsTest() {
+        Http3Settings http3Settings = new Http3Settings();
+        assertNull(http3Settings.put(Http3SettingIdentifier.HTTP3_SETTINGS_QPACK_MAX_TABLE_CAPACITY.id(), 100L));
+        assertNull(http3Settings.put(Http3SettingIdentifier.HTTP3_SETTINGS_QPACK_BLOCKED_STREAMS.id(), 1L));
+        assertNull(http3Settings.put(Http3SettingIdentifier.HTTP3_SETTINGS_MAX_FIELD_SECTION_SIZE.id(), 128L));
+        assertNull(http3Settings.put(Http3SettingIdentifier.HTTP3_SETTINGS_ENABLE_CONNECT_PROTOCOL.id(), 0L));
+        assertNull(http3Settings.put(Http3SettingIdentifier.HTTP3_SETTINGS_H3_DATAGRAM.id(), 1L));
+        //known headers should not contain duplicate so give non-null
+        // which is used in http3framecodec to throw error
+        assertNotNull(http3Settings.put(Http3SettingIdentifier.HTTP3_SETTINGS_H3_DATAGRAM.id(), 1L));
+        // Ensure we can encode and decode all sizes correctly.
+        // unknown settings id/key will be ignored
+        assertNull(http3Settings.put(63, 63L));
+        assertNull(http3Settings.put(16383, 16383L));
+        assertNull(http3Settings.put(1073741823, 1073741823L));
+        assertNull(http3Settings.put(4611686018427387903L, 4611686018427387903L));
+        //even duplicates of unknown ignored as we ignore unknown
+        assertNull(http3Settings.put(4611686018427387903L, 4611686018427387903L));
+    }
 }