Prepare UnknownFieldSet for replace std::string with absl::string_view:

 - Add overloads that take `absl::Cord` and `std::string&&` as inputs, putting the burden in the implementation instead of users.
 - Add overload that returns `absl::Span<char>` for callers that need higher performance requirements where we can avoid copies altogether.
 - Hide the APIs that return `std::string*` when the breaking change is enabled (via `-D PROTOBUF_TEMPORARY_ENABLE_STRING_VIEW_RETURN_TYPE`).

PiperOrigin-RevId: 655600399

Author: protobuf-github-bot

src/google/protobuf/arena_unittest.cc

@@ -652,7 +652,7 @@ TEST(ArenaTest, UnknownFields) {
   arena_message_3->mutable_unknown_fields()->AddVarint(1000, 42);
   arena_message_3->mutable_unknown_fields()->AddFixed32(1001, 42);
   arena_message_3->mutable_unknown_fields()->AddFixed64(1002, 42);
-  arena_message_3->mutable_unknown_fields()->AddLengthDelimited(1003);
+  arena_message_3->mutable_unknown_fields()->AddLengthDelimited(1003, "");
   arena_message_3->mutable_unknown_fields()->DeleteSubrange(0, 2);
   arena_message_3->mutable_unknown_fields()->DeleteByNumber(1002);
   arena_message_3->mutable_unknown_fields()->DeleteByNumber(1003);

src/google/protobuf/descriptor.cc

@@ -8921,11 +8921,12 @@ bool DescriptorBuilder::OptionInterpreter::InterpretSingleOption(
         new UnknownFieldSet());
     switch ((*iter)->type()) {
       case FieldDescriptor::TYPE_MESSAGE: {
-        std::string* outstr =
-            parent_unknown_fields->AddLengthDelimited((*iter)->number());
-        ABSL_CHECK(unknown_fields->SerializeToString(outstr))
+        std::string outstr;
+        ABSL_CHECK(unknown_fields->SerializeToString(&outstr))
             << "Unexpected failure while serializing option submessage "
             << debug_msg_name << "\".";
+        parent_unknown_fields->AddLengthDelimited((*iter)->number(),
+                                                  std::move(outstr));
         break;
       }
 

src/google/protobuf/parse_context.cc

@@ -12,6 +12,7 @@
 
 #include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 #include "google/protobuf/message_lite.h"
 #include "google/protobuf/repeated_field.h"
 #include "google/protobuf/wire_format_lite.h"
@@ -265,6 +266,23 @@ const char* EpsCopyInputStream::ReadCordFallback(const char* ptr, int size,
   return ptr;
 }
 
+const char* EpsCopyInputStream::ReadCharsFallback(const char* ptr,
+                                                  absl::Span<char> out) {
+  char* out_ptr = out.data();
+  ptr = AppendSize(ptr, out.size(), [&](const char* p, int s) {
+    memcpy(out_ptr, p, s);
+    out_ptr += s;
+  });
+
+  // If we had an error, set the leftover memory to make sure we don't leak
+  // uninit data in the object.
+  if (ABSL_PREDICT_FALSE(ptr == nullptr)) {
+    memset(out_ptr, 0xCD, out.data() + out.size() - out_ptr);
+  }
+
+  return ptr;
+}
+
 
 const char* EpsCopyInputStream::InitFrom(io::ZeroCopyInputStream* zcis) {
   zcis_ = zcis;

src/google/protobuf/parse_context.h

@@ -212,6 +212,15 @@ class PROTOBUF_EXPORT EpsCopyInputStream {
     return ReadCordFallback(ptr, size, cord);
   }
 
+  PROTOBUF_NODISCARD const char* ReadChars(const char* ptr,
+                                           absl::Span<char> out) {
+    if (out.size() <= static_cast<size_t>(buffer_end_ + kSlopBytes - ptr)) {
+      memcpy(out.data(), ptr, out.size());
+      return ptr + out.size();
+    }
+    return ReadCharsFallback(ptr, out);
+  }
+
 
   template <typename Tag, typename T>
   PROTOBUF_NODISCARD const char* ReadRepeatedFixed(const char* ptr,
@@ -369,6 +378,7 @@ class PROTOBUF_EXPORT EpsCopyInputStream {
   const char* AppendStringFallback(const char* ptr, int size, std::string* str);
   const char* ReadStringFallback(const char* ptr, int size, std::string* str);
   const char* ReadCordFallback(const char* ptr, int size, absl::Cord* cord);
+  const char* ReadCharsFallback(const char* ptr, absl::Span<char> out);
   static bool ParseEndsInSlopRegion(const char* begin, int overrun, int depth);
   bool StreamNext(const void** data) {
     bool res = zcis_->Next(data, &size_);

src/google/protobuf/unknown_field_set.cc

@@ -11,10 +11,15 @@
 
 #include "google/protobuf/unknown_field_set.h"
 
+#include <cstring>
+#include <string>
+#include <utility>
+
 #include "absl/log/absl_check.h"
 #include "absl/strings/cord.h"
 #include "absl/strings/internal/resize_uninitialized.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 #include "google/protobuf/extension_set.h"
 #include "google/protobuf/generated_message_tctable_impl.h"
 #include "google/protobuf/io/coded_stream.h"
@@ -117,13 +122,44 @@ void UnknownFieldSet::AddFixed64(int number, uint64_t value) {
   field.data_.fixed64_ = value;
 }
 
+void UnknownFieldSet::AddLengthDelimited(int number, const absl::Cord& value) {
+  auto out = AddLengthDelimitedUninitialized(number, value.size());
+  for (absl::string_view part : value.Chunks()) {
+    memcpy(out.data(), part.data(), part.size());
+    out.remove_prefix(part.size());
+  }
+}
+
+absl::Span<char> UnknownFieldSet::AddLengthDelimitedUninitialized(int number,
+                                                                  size_t size) {
+  auto& field = *fields_.Add();
+  field.number_ = number;
+  field.SetType(UnknownField::TYPE_LENGTH_DELIMITED);
+  std::string* str = field.data_.string_value =
+      Arena::Create<std::string>(arena());
+  absl::strings_internal::STLStringResizeUninitialized(str, size);
+  return absl::Span<char>(*str);
+}
+
+template <int&...>
+void UnknownFieldSet::AddLengthDelimited(int number, std::string&& value) {
+  auto& field = *fields_.Add();
+  field.number_ = number;
+  field.SetType(UnknownField::TYPE_LENGTH_DELIMITED);
+  field.data_.string_value =
+      Arena::Create<std::string>(arena(), std::move(value));
+}
+template void UnknownFieldSet::AddLengthDelimited(int, std::string&&);
+
+#if !defined(PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE)
 std::string* UnknownFieldSet::AddLengthDelimited(int number) {
   auto& field = *fields_.Add();
   field.number_ = number;
   field.SetType(UnknownField::TYPE_LENGTH_DELIMITED);
   field.data_.string_value = Arena::Create<std::string>(arena());
   return field.data_.string_value;
 }
+#endif  // PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE
 
 UnknownFieldSet* UnknownFieldSet::AddGroup(int number) {
   auto& field = *fields_.Add();
@@ -284,10 +320,10 @@ class UnknownFieldParserHelper {
   }
   const char* ParseLengthDelimited(uint32_t num, const char* ptr,
                                    ParseContext* ctx) {
-    std::string* s = unknown_->AddLengthDelimited(num);
     int size = ReadSize(&ptr);
     GOOGLE_PROTOBUF_PARSER_ASSERT(ptr);
-    return ctx->ReadString(ptr, size, s);
+    return ctx->ReadChars(ptr,
+                          unknown_->AddLengthDelimitedUninitialized(num, size));
   }
   const char* ParseGroup(uint32_t num, const char* ptr, ParseContext* ctx) {
     return ctx->ParseGroupInlined(ptr, num * 8 + 3, [&](const char* ptr) {

src/google/protobuf/unknown_field_set.h

@@ -24,6 +24,7 @@
 #include "absl/log/absl_check.h"
 #include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/span.h"
 #include "google/protobuf/arena.h"
 #include "google/protobuf/io/coded_stream.h"
 #include "google/protobuf/io/zero_copy_stream_impl_lite.h"
@@ -47,6 +48,8 @@ class InternalMetadata;           // metadata_lite.h
 class WireFormat;                 // wire_format.h
 class MessageSetFieldSkipperUsingCord;
 // extension_set_heavy.cc
+class UnknownFieldParserHelper;
+struct UnknownFieldSetTestPeer;
 
 #if defined(PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE)
 using UFSStringView = absl::string_view;
@@ -87,7 +90,13 @@ class PROTOBUF_EXPORT UnknownField {
   inline void set_fixed32(uint32_t value);
   inline void set_fixed64(uint64_t value);
   inline void set_length_delimited(absl::string_view value);
+  // template to avoid ambiguous overload resolution.
+  template <int&...>
+  inline void set_length_delimited(std::string&& value);
+  inline void set_length_delimited(const absl::Cord& value);
+#if !defined(PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE)
   inline std::string* mutable_length_delimited();
+#endif  // PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE
   inline UnknownFieldSet* mutable_group();
 
   inline size_t GetLengthDelimitedSize() const;
@@ -191,7 +200,14 @@ class PROTOBUF_EXPORT UnknownFieldSet {
   void AddFixed32(int number, uint32_t value);
   void AddFixed64(int number, uint64_t value);
   void AddLengthDelimited(int number, absl::string_view value);
+  // template to avoid ambiguous overload resolution.
+  template <int&...>
+  void AddLengthDelimited(int number, std::string&& value);
+  void AddLengthDelimited(int number, const absl::Cord& value);
+
+#if !defined(PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE)
   std::string* AddLengthDelimited(int number);
+#endif  // PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE
   UnknownFieldSet* AddGroup(int number);
 
   // Adds an unknown field from another set.
@@ -233,6 +249,10 @@ class PROTOBUF_EXPORT UnknownFieldSet {
       : UnknownFieldSet(arena) {}
 
  private:
+  friend internal::WireFormat;
+  friend internal::UnknownFieldParserHelper;
+  friend internal::UnknownFieldSetTestPeer;
+
   using InternalArenaConstructable_ = void;
   using DestructorSkippable_ = void;
 
@@ -241,6 +261,14 @@ class PROTOBUF_EXPORT UnknownFieldSet {
 
   Arena* arena() { return fields_.GetArena(); }
 
+  // Returns a buffer of `size` chars for the user to fill in.
+  // The buffer is potentially uninitialized memory. Failing to write to it
+  // might lead to undefined behavior when reading it later.
+  // Prefer the overloads above when possible. Calling this API without
+  // validating the `size` parameter can lead to unintentional memory usage and
+  // potential OOM.
+  absl::Span<char> AddLengthDelimitedUninitialized(int number, size_t size);
+
   void ClearFallback();
   void SwapSlow(UnknownFieldSet* other);
 
@@ -275,7 +303,7 @@ inline void WriteVarint(uint32_t num, uint64_t val, UnknownFieldSet* unknown) {
 }
 inline void WriteLengthDelimited(uint32_t num, absl::string_view val,
                                  UnknownFieldSet* unknown) {
-  unknown->AddLengthDelimited(num)->assign(val.data(), val.size());
+  unknown->AddLengthDelimited(num, val);
 }
 
 PROTOBUF_EXPORT
@@ -331,7 +359,10 @@ inline UnknownField* UnknownFieldSet::mutable_field(int index) {
 
 inline void UnknownFieldSet::AddLengthDelimited(int number,
                                                 const absl::string_view value) {
-  AddLengthDelimited(number)->assign(value.data(), value.size());
+  auto field = AddLengthDelimitedUninitialized(number, value.size());
+  if (!value.empty()) {
+    memcpy(field.data(), value.data(), value.size());
+  }
 }
 
 inline int UnknownField::number() const { return static_cast<int>(number_); }
@@ -376,10 +407,21 @@ inline void UnknownField::set_length_delimited(const absl::string_view value) {
   assert(type() == TYPE_LENGTH_DELIMITED);
   data_.string_value->assign(value.data(), value.size());
 }
+template <int&...>
+inline void UnknownField::set_length_delimited(std::string&& value) {
+  assert(type() == TYPE_LENGTH_DELIMITED);
+  *data_.string_value = std::move(value);
+}
+inline void UnknownField::set_length_delimited(const absl::Cord& value) {
+  assert(type() == TYPE_LENGTH_DELIMITED);
+  absl::CopyCordToString(value, data_.string_value);
+}
+#if !defined(PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE)
 inline std::string* UnknownField::mutable_length_delimited() {
   assert(type() == TYPE_LENGTH_DELIMITED);
   return data_.string_value;
 }
+#endif  // PROTOBUF_FUTURE_STRING_VIEW_RETURN_TYPE
 inline UnknownFieldSet* UnknownField::mutable_group() {
   assert(type() == TYPE_GROUP);
   return data_.group_;
@@ -397,6 +439,8 @@ inline size_t UnknownField::GetLengthDelimitedSize() const {
 
 inline void UnknownField::SetType(Type type) { type_ = type; }
 
+extern template void UnknownFieldSet::AddLengthDelimited(int, std::string&&);
+
 namespace internal {
 
 // Add specialization of InternalMetadata::Container to provide arena support.