improve: table size update must come first in header block

According to RFC 7531, section 4.2, HPACK dynamic table size update must
happen at the beginning of the header block.

This change will increase the number of green cases in h2spec, which other
HTTP/2 implementation such as hyper/h2(Rust) or Crystal http/2 uses
in order to check spec conformance.

References

- https://datatracker.ietf.org/doc/html/rfc7541#section-4.2
- netty/netty#12988
- https://github.com/summerwind/h2spec
- https://github.com/hyperium/h2?tab=readme-ov-file#features
- https://github.com/ysbaddaden/http2

hpack/shared/src/main/scala/org/http4s/hpack/Decoder.scala

@@ -31,12 +31,11 @@
  */
 package org.http4s.hpack;
 
-import java.io.IOException;
-import java.io.InputStream;
-
-import org.http4s.hpack.HpackUtil.IndexType;
+import org.http4s.hpack.HeaderField.HEADER_ENTRY_OVERHEAD
+import org.http4s.hpack.HpackUtil.IndexType
 
-import org.http4s.hpack.HeaderField.HEADER_ENTRY_OVERHEAD;
+import java.io.IOException
+import java.io.InputStream;
 
 private[http4s] final class Decoder(dynamicTable: DynamicTable) {
 
@@ -48,6 +47,11 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
     new IOException("invalid max dynamic table size");
   private[this] val MAX_DYNAMIC_TABLE_SIZE_CHANGE_REQUIRED =
     new IOException("max dynamic table size change required");
+  private[this] val UNEXPECTED_DYNAMIC_TABLE_SIZE_UPDATE =
+    new IOException(
+      "Dynamic table size update must happen "
+        + "at the beginning of the header block"
+    )
 
   private[this] val EMPTY = Array.emptyByteArray;
 
@@ -103,6 +107,8 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
     */
   @throws[IOException]
   def decode(in: InputStream, headerListener: HeaderListener): Unit = {
+    var seenOnlyDynamicTableSizeUpdate = true
+
     while (in.available() > 0) {
       var b: Byte = 0
       import State._
@@ -115,6 +121,7 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
           }
           if (b < 0) {
             // Indexed Header Field
+            seenOnlyDynamicTableSizeUpdate = false
             index = b & 0x7f;
             if (index == 0) {
               throw ILLEGAL_INDEX_VALUE;
@@ -125,6 +132,7 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
             }
           } else if ((b & 0x40) == 0x40) {
             // Literal Header Field with Incremental Indexing
+            seenOnlyDynamicTableSizeUpdate = false
             indexType = IndexType.INCREMENTAL;
             index = b & 0x3f;
             if (index == 0) {
@@ -138,15 +146,20 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
             }
           } else if ((b & 0x20) == 0x20) {
             // Dynamic Table Size Update
-            index = b & 0x1f;
-            if (index == 0x1f) {
-              state = State.READ_MAX_DYNAMIC_TABLE_SIZE;
+            if (seenOnlyDynamicTableSizeUpdate) {
+              index = b & 0x1f;
+              if (index == 0x1f) {
+                state = State.READ_MAX_DYNAMIC_TABLE_SIZE;
+              } else {
+                setDynamicTableSize(index);
+                state = State.READ_HEADER_REPRESENTATION;
+              }
             } else {
-              setDynamicTableSize(index);
-              state = State.READ_HEADER_REPRESENTATION;
+              throw UNEXPECTED_DYNAMIC_TABLE_SIZE_UPDATE
             }
           } else {
             // Literal Header Field without Indexing / never Indexed
+            seenOnlyDynamicTableSizeUpdate = false
             indexType = if ((b & 0x10) == 0x10) IndexType.NEVER else IndexType.NONE;
             index = b & 0x0f;
             if (index == 0) {

hpack/shared/src/test/scala/org/http4s/hpack/DecoderTest.scala

@@ -33,15 +33,15 @@ package org.http4s.hpack
 
 import cats.kernel.Eq
 import cats.syntax.all._
-
-import java.io.ByteArrayInputStream
-import java.io.IOException
-import org.junit.Before
-import org.junit.Test
 import org.http4s.hpack.HpackUtil.ISO_8859_1
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
 import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+
+import java.io.ByteArrayInputStream
+import java.io.IOException
 
 object DecoderTest {
   private val MAX_HEADER_SIZE = 8192
@@ -142,6 +142,16 @@ class DecoderTest {
     decode("2081")
   }
 
+  @Test(expected = classOf[IOException])
+  @throws[IOException]
+  def testDynamicTableSizeUpdateAfterTheBeginningOfTheBlock(): Unit =
+    decode("8120")
+
+  @Test(expected = classOf[IOException])
+  @throws[Exception]
+  def testDynamicTableSizeUpdateAfterTheBeginningOfTheBlockLong(): Unit =
+    decode("813FE11F")
+
   @Test(expected = classOf[IOException])
   @throws[Exception]
   def testTooLargeDynamicTableSizeUpdate(): Unit = {