Fix HPACK table size update sequencing

httpcore5-h2/src/main/java/org/apache/hc/core5/http2/hpack/HPackEncoder.java

@@ -55,12 +55,14 @@ public final class HPackEncoder {
     private final CharsetEncoder charsetEncoder;
     private ByteBuffer tmpBuf;
     private int maxTableSize;
+    private int minTableSize;
 
     HPackEncoder(final OutboundDynamicTable dynamicTable, final CharsetEncoder charsetEncoder) {
         this.dynamicTable = Objects.requireNonNull(dynamicTable);
         this.huffmanBuf = new ByteArrayBuffer(128);
         this.charsetEncoder = charsetEncoder;
         this.maxTableSize = this.dynamicTable.getMaxSize();
+        this.minTableSize = -1;
     }
 
     HPackEncoder(final OutboundDynamicTable dynamicTable, final Charset charset) {
@@ -144,6 +146,19 @@ private void ensureCapacity(final int extra) {
         }
     }
 
+    private void encodeMaxTableSizeUpdates(final ByteArrayBuffer dst) {
+        if (this.minTableSize == -1) {
+            return;
+        }
+        encodeInt(dst, 5, this.minTableSize, 0x20);
+        this.dynamicTable.setMaxSize(this.minTableSize);
+        if (this.maxTableSize != this.minTableSize) {
+            encodeInt(dst, 5, this.maxTableSize, 0x20);
+            this.dynamicTable.setMaxSize(this.maxTableSize);
+        }
+        this.minTableSize = -1;
+    }
+
     int encodeString(
             final ByteArrayBuffer dst,
             final CharSequence charSequence, final int off, final int len,
@@ -262,11 +277,7 @@ void encodeHeader(
     void encodeHeader(
             final ByteArrayBuffer dst, final String name, final String value, final boolean sensitive,
             final boolean noIndexing, final boolean useHuffman) throws CharacterCodingException {
-        //send receiver the updated dynamic table size
-        if (maxTableSize != this.dynamicTable.getMaxSize()) {
-            encodeInt(dst, 5, maxTableSize, 0x20);
-            this.dynamicTable.setMaxSize(maxTableSize);
-        }
+        encodeMaxTableSizeUpdates(dst);
 
         final HPackRepresentation representation;
         if (sensitive) {
@@ -342,6 +353,13 @@ public int getMaxTableSize() {
     public void setMaxTableSize(final int maxTableSize) {
         Args.notNegative(maxTableSize, "Max table size");
         this.maxTableSize = maxTableSize;
+        if (this.minTableSize == -1) {
+            if (maxTableSize != this.dynamicTable.getMaxSize()) {
+                this.minTableSize = maxTableSize;
+            }
+        } else if (maxTableSize < this.minTableSize) {
+            this.minTableSize = maxTableSize;
+        }
     }
 
-}
+}

httpcore5-h2/src/test/java/org/apache/hc/core5/http2/hpack/TestHPackEncoder.java

@@ -0,0 +1,88 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.core5.http2.hpack;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+
+import org.apache.hc.core5.util.ByteArrayBuffer;
+import org.junit.jupiter.api.Test;
+
+class TestHPackEncoder {
+
+    @Test
+    void testMultipleTableSizeUpdatesSignalMinimumFirstThenFinal() throws Exception {
+        final OutboundDynamicTable dynamicTable = new OutboundDynamicTable(4096);
+        final HPackEncoder encoder = new HPackEncoder(dynamicTable, StandardCharsets.US_ASCII);
+        final ByteArrayBuffer dst = new ByteArrayBuffer(128);
+
+        encoder.setMaxTableSize(1024);
+        encoder.setMaxTableSize(2048);
+
+        // No local resize until the next header block is encoded.
+        assertEquals(4096, dynamicTable.getMaxSize());
+        assertEquals(2048, encoder.getMaxTableSize());
+
+        encoder.encodeHeader(dst, "x-test", "abc", false, false, false);
+
+        // First 6 bytes must be:
+        //   1024 => 0x3f 0xe1 0x07
+        //   2048 => 0x3f 0xe1 0x0f
+        final byte[] actual = Arrays.copyOf(dst.array(), dst.length());
+        assertArrayEquals(new byte[]{
+                0x3f, (byte) 0xe1, 0x07,
+                0x3f, (byte) 0xe1, 0x0f
+        }, Arrays.copyOf(actual, 6));
+
+        // Local encoder state must also end at the final advertised size.
+        assertEquals(2048, dynamicTable.getMaxSize());
+    }
+
+    @Test
+    void testShrinkThenRestoreOriginalStillEmitsTwoUpdates() throws Exception {
+        final OutboundDynamicTable dynamicTable = new OutboundDynamicTable(4096);
+        final HPackEncoder encoder = new HPackEncoder(dynamicTable, StandardCharsets.US_ASCII);
+        final ByteArrayBuffer dst = new ByteArrayBuffer(128);
+
+        encoder.setMaxTableSize(1024);
+        encoder.setMaxTableSize(4096);
+
+        encoder.encodeHeader(dst, "x-test", "abc", false, false, false);
+
+        final byte[] actual = Arrays.copyOf(dst.array(), dst.length());
+        assertArrayEquals(new byte[]{
+                0x3f, (byte) 0xe1, 0x07,
+                0x3f, (byte) 0xe1, 0x1f
+        }, Arrays.copyOf(actual, 6));
+
+        assertEquals(4096, dynamicTable.getMaxSize());
+    }
+
+}