Unmarshaling data that was marshaled using Google's proto.Marshal() omits protobuf extensions

[dylan-bourque]

It appears that there is an incompatibility between Google's proto.Marhsal() and Gogo's proto.Unmarshal().

Given a protobuf structure with an extension (assigned with proto.SetExtension()), if I marshal it to binary using Google's proto.Marshal() function then unmarshal it using Gogo's proto.Unmarshal() the resulting structure has its XXX_InternalExtensions field set to nil.

Possibly relevant, I am building a "legacy" project in GOPATH mode (explicitly not using Go modules).

Relevant Go environment:

go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/dbourque/Library/Caches/go-build"
GOENV="/Users/dbourque/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOOS="darwin"
GOPATH="/Users/dbourque/dev/golang"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.14.3/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.14.3/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CGO_ENABLED="1"
GOMOD=""

Package	Version	Commit
github.com/gogo/protobuf	v1.3.1	5628607
github.com/golang/protobuf	v1.4.2	d04d7b157bb510b1e0c10132224b616ac0e26b17
google.golang.org/protobuf	v1.24.0 + 6 commits	69839c78c3baff8f1fb37f37b24127ecae185e03