Fix handling of very long user-agent strings (#28376)

* fix encoding of very long user-agent strings

* fuzzer found example

src/core/ext/transport/chttp2/transport/hpack_encoder.cc

@@ -26,6 +26,7 @@
 #include <cstdint>
 
 #include "hpack_constants.h"
+#include "hpack_encoder_table.h"
 
 /* This is here for grpc_is_binary_header
  * TODO(murgatroid99): Remove this
@@ -759,6 +760,12 @@ void HPackCompressor::Framer::Encode(GrpcTimeoutMetadata,
 }
 
 void HPackCompressor::Framer::Encode(UserAgentMetadata, const Slice& slice) {
+  if (slice.length() > HPackEncoderTable::MaxEntrySize()) {
+    EmitLitHdrWithNonBinaryStringKeyNotIdx(
+        StaticSlice::FromStaticString(UserAgentMetadata::key()).c_slice(),
+        slice.c_slice());
+    return;
+  }
   if (!slice.is_equivalent(compressor_->user_agent_)) {
     compressor_->user_agent_ = slice.Ref();
     compressor_->user_agent_index_ = 0;

src/core/ext/transport/chttp2/transport/hpack_encoder_table.cc

@@ -22,7 +22,7 @@ namespace grpc_core {
 
 uint32_t HPackEncoderTable::AllocateIndex(size_t element_size) {
   uint32_t new_index = tail_remote_index_ + table_elems_ + 1;
-  GPR_DEBUG_ASSERT(element_size < 65536);
+  GPR_DEBUG_ASSERT(element_size <= MaxEntrySize());
 
   if (element_size > max_table_size_) {
     while (table_size_ > 0) {

src/core/ext/transport/chttp2/transport/hpack_encoder_table.h

@@ -29,6 +29,8 @@ class HPackEncoderTable {
  public:
   HPackEncoderTable() : elem_size_(hpack_constants::kInitialTableEntries) {}
 
+  static constexpr size_t MaxEntrySize() { return 65535; }
+
   // Reserve space in table for the new element, evict entries if needed.
   // Return the new index of the element. Return 0 to indicate not adding to
   // table.