Update grpc protoc plugin to be compliant of proto3 field presence

[stanley-cheung]

Fixes #23030

Proto3 introduced the optional syntax to support field presence tracking in release 3.12.0. We need to add support in our grpc protoc plugin.

See this doc for more details. Basically we need to add a function GetSupportedFeatures() to our grpc protoc plugins to mark our compliance.

This PR also has to update the third_party/protobuf link to the v3.12.2 commit.

Internal bug: b/156243646

A couple of recent issues to keep an eye on in terms of interactions with this PR:

• [Merged] Update boringssl #23012: update boringssl
• [No longer needed] Pin protobuf as temporary workaround to artifact breakage #23015: ruby pinning protobuf dependency version (so this is a potential blocker too)

Current blockers:

• [Resolved] Objective-C bazel target failed to build at 3.12.x protocolbuffers/protobuf#7532: Objective-C bazel build issue
  • Fix Fix :protobuf_objc bazel target protocolbuffers/protobuf#7538 merged
• [Resolved] error: expected primary-expression before '__attribute__' when expand macro in port_def.inc protocolbuffers/protobuf#6659: Python 2.7 + Windows build issue. More investigation at Windows Artifact build failed for Python 2.7 after upgrade to Protobuf 3.12.0 #23011
  • Fix Cherry-pick of df719723: Simplify template exporting macros protocolbuffers/protobuf#7539 merged
• [Resolved] Pin protobuf as temporary workaround to artifact breakage #23015: Ruby protobuf dropping (or not) 2.5 support
  • Fix Restore binary builds of Ruby 2.3 and 2.4 (cherry-pick of #7529) protocolbuffers/protobuf#7531 merged
• [Resolved] Build error in portability test with Alpine + CONFIG=dbg
  • Resolved by updating our docker base image from Alpine 3.5 to 3.9
• Waiting for Protobuf team to cut a new 3.12.2 release.

[stanley-cheung]

There are some Objective-C failures, that may be due to a new change in Protobuf in 3.12 protocolbuffers/protobuf#7173

The error is

ERROR: /Volumes/BuildData/tmpfs/src/github/grpc/workspace_objc_macos_opt_native/src/objective-c/BUILD:240:1: C++ compilation of rule '//src/objective-c:proto_objc_rpc_internal_testing' failed (Exit 1) wrapped_clang failed: error executing command external/local_config_cc/wrapped_clang -arch x86_64 '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -O0 -DDEBUG ... (remaining 71 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
In file included from src/objective-c/ProtoRPC/ProtoRPCLegacy.m:26:
In file included from external/com_google_protobuf/objectivec/GPBProtocolBuffers.h:44:
external/com_google_protobuf/objectivec/GPBWellKnownTypes.h:44:10: fatal error: 'GPBAny.pbobjc.h' file not found
 #import "GPBAny.pbobjc.h"
         ^~~~~~~~~~~~~~~~~
1 error generated.
Target //src/objective-c/examples:Sample failed to build

https://source.cloud.google.com/results/invocations/59fa0d35-8ca1-48b1-8ea0-d6230b7cc42a/targets/grpc%2Fcore%2Fpull_request%2Fmacos%2Fgrpc_basictests_objc_examples/log

Logged an issue to protobuf: protocolbuffers/protobuf#7532 to see if it's an issue with their :protobuf_objc target in the 3.12.x branch.

[Edit:] Fixed by protocolbuffers/protobuf#7538

[stanley-cheung]

There seems to be an issue with Python27 + Windows build after the upgrade to Protobuf 3.12 too

third_party\protobuf\src/google/protobuf/port_def.inc:457:54: error: expected primary-expression before ')' token

   PROTOBUF_EXPORT_TEMPLATE_##which##_##style(export, )

...

FAILED: build_artifact.python_windows_x64_Python27

Seems to be caused by this PR: protocolbuffers/protobuf#7344, and originally protocolbuffers/protobuf#6535

Log: https://source.cloud.google.com/results/invocations/b95cf1f2-191b-4fdb-8554-413524992d91/targets/grpc%2Fcore%2Fpull_request%2Fwindows%2Fgrpc_build_artifacts/log

Logged a separate issue #23011 to track this issue. Current idea is that mingw is being used to compile for Python 2.7, instead of msvc for Python 3. We are trying to see if there's a way to update mingw to fix this particular issue.

[Edit:] Fixed by protocolbuffers/protobuf#7539

[jtattermusch]

@srini100 note that Stanley is already updating protobuf submodule as part of this PR.

[stanley-cheung]

This is looking better. Both the Objective-C and the Python issues have found a fix. We just need protobuf team to merge in my fixes and tag a new release. Will keep monitoring that.

[stanley-cheung]

All the issues are resolved as of this point, after protobuf team merged in my two fixes. All the tests are passing except for this known flake #23027.

I will wait for the protobuf team to cut a new release 3.12.2, which will possibly happen after the long weekend.

[srini100]

LGTM

[jtattermusch]

LGTM.

[karthikravis]

Unfortunately, this PR breaks some build targets internally - https://g3c.corp.google.com/results/invocations/c787c170-933c-4d84-88db-908739d593c1/targets/%2F%2Fgooglex%2Fdaedalus%2Fembedded%2Fbase%2Fnet%2Fngrpc2%2Fclient:channel_test/log

I will create a revert PR.