This PR contains the following updates:

Release Notes

v3.12.0

Compare Source

Protocol Compiler

• [experimental] Singular, non-message typed fields in proto3 now support
  presence tracking. This is enabled by adding the "optional" field label and
  passing the --experimental_allow_proto3_optional flag to protoc.
  • For usage info, see docs/field_presence.md.
  • During this experimental phase, code generators should update to support
    proto3 presence, see docs/implementing_proto3_presence.md for instructions.
• Allow duplicate symbol names when multiple descriptor sets are passed on
  the command-line, to match the behavior when multiple .proto files are passed.
• Deterministic protoc --descriptor_set_out (#​7175)

C++

• [experimental] Added proto3 presence support.
• New descriptor APIs to support proto3 presence.
• Fix for #​7463 in -rc1 (core dump mixing optional and singular fields in proto3)
• Enable Arenas by default on all .proto files.
• Documented that users are not allowed to subclass Message or MessageLite.
• Mark generated classes as final; inheriting from protos is strongly discouraged.
• Add stack overflow protection for text format with unknown fields.
• Add accessors for map key and value FieldDescriptors.
• Add FieldMaskUtil::FromFieldNumbers().
• MessageDifferencer: use ParsePartial() on Any fields so the diff does not
  fail when there are missing required fields.
• ReflectionOps::Merge(): lookup messages in the right factory, if it can.
• Added Descriptor::WellKnownTypes enum and Descriptor::well_known_type()
  accessor as an easier way of determining if a message is a Well-Known Type.
• Optimized RepeatedField::Add() when it is used in a loop.
• Made proto move/swap more efficient.
• De-virtualize the GetArena() method in MessageLite.
• Improves performance of json_stream_parser.cc by factor 1000 (#​7230)
• bug: #​7076 undefine Windows OUT and OPTIONAL macros (#​7087)
• Fixed a bug in FieldDescriptor::DebugString() that would erroneously print
  an "optional" label for a field in a oneof.
• Fix bug in parsing bool extensions that assumed they are always 1 byte.
• Fix off-by-one error in FieldOptions::ByteSize() when extensions are present.
• Clarified the comments to show an example of the difference between
  Descriptor::extension and DescriptorPool::FindAllExtensions.
• Add a compiler option 'code_size' to force optimize_for=code_size on all
  protos where this is possible.

Java

• [experimental] Added proto3 presence support.
• Fix for #​7480 in -rc1 ("TextFormat and JsonFormat ignore experimental proto3 optional enums")
• Fix for #​7505 in -rc1 (" toString() returns incorrect ascii when there are duplicate keys in a map")
• Mark java enum _VALUE constants as @​Deprecated if the enum field is deprecated
• reduce size for enums with allow_alias set to true.
• Sort map fields alphabetically by the field's key when printing textproto.
• TextFormat.merge() handles Any as top level type.
• Throw a descriptive IllegalArgumentException when calling
  getValueDescriptor() on enum special value UNRECOGNIZED instead of
  ArrayIndexOutOfBoundsException.
• Fixed an issue with JsonFormat.printer() where setting printingEnumsAsInts()
  would override the configuration passed into includingDefaultValueFields().
• Implement overrides of indexOf() and contains() on primitive lists returned
  for repeated fields to avoid autoboxing the list contents.
• Add overload to FieldMaskUtil.fromStringList that accepts a descriptor.
• [bazel] Move Java runtime/toolchains into //java (#​7190)

Python

• [experimental] Added proto3 presence support.
• [experimental] fast import protobuf module, only works with cpp generated code linked in.
• Truncate 'float' fields to 4 bytes of precision in setters for pure-Python
  implementation (C++ extension was already doing this).
• Fixed a memory leak in C++ bindings.
• Added a deprecation warning when code tries to create Descriptor objects
  directly.
• Fix unintended comparison between bytes and string in descriptor.py.
• Avoid printing excess digits for float fields in TextFormat.
• Remove Python 2.5 syntax compatibility from the proto compiler generated _pb2.py module code.
• Drop 3.3, 3.4 and use single version docker images for all python tests (#​7396)

JavaScript

• Fix js message pivot selection (#​6813)

PHP

• Persistent Descriptor Pool (#​6899)
• Implement lazy loading of php class for proto messages (#​6911)
• Correct @​return in Any.unpack docblock (#​7089)
• Ignore unknown enum value when ignore_unknown specified (#​7455)

Ruby

• [experimental] Implemented proto3 presence for Ruby. (#​7406)
• Stop building binary gems for ruby <2.5 (#​7453)
• Fix for wrappers with a zero value (#​7195)
• Fix for JSON serialization of 0/empty-valued wrapper types (#​7198)
• Call "Class#new" over rb_class_new_instance in decoding (#​7352)
• Build extensions for Ruby 2.7 (#​7027)
• assigning 'nil' to submessage should clear the field. (#​7397)

C

• [experimental] Add support for proto3 presence fields in C# (#​7382)
• Cleanup various bits of Google.Protobuf (#​6674)
• Fix conformance test failures for Google.Protobuf (#​6910)
• Fix latest ArgumentException for C# extensions (#​6938)
• Remove unnecessary branch from ReadTag (#​7289)
• Enforce recursion depth checking for unknown fields (#​7132)
• Mark GetOption API as obsolete and expose the "GetOptions()" method
  on descriptors instead (#​7491)
• Remove Has/Clear members for C# message fields in proto2 (#​7429)

Objective-C

• [experimental] ObjC Proto3 optional support (#​7421)
• Block subclassing of generated classes (#​7124)
• Use references to Obj C classes instead of names in descriptors. (#​7026)
• Revisit how the WKTs are bundled with ObjC. (#​7173)

Other

• Add a proto_lang_toolchain for javalite (#​6882)
• [bazel] Update gtest and deprecate //external:{gtest,gtest_main} (#​7237)
• Add application note for explicit presence tracking. (#​7390)
• Howto doc for implementing proto3 presence in a code generator. (#​7407)

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