golang: allow downloads during instalation of gRPC protobuf plugins (#…

…21614)

As described in #21529, the Go
rules are setting `GOPROXY=off` during the invocations of `go install`
to install the gRPC protobuf plugins because that is the default for
`GoSdkProcess`. The plugin builds are failing because `GOPROXY=off`
prevents `go` from contacting the Go module proxy to learn about module
deprecations. (Unlike other uses of `GoSdkProcess`, the gRPC protobuf
plugins are relying on `go install` to do a full build of the plugin
sources, which is unlike the use of `GoSdkProcess` in the rest of the Go
backend rules.)

Solution: Set `allow_downloads=True` (which is already done for a
preceding `go mod download`) to allow `go` to contact the Go module
proxy as needed by not setting `GOPROXY=off`.

Fixes #21529.

docs/notes/2.25.x.md

@@ -85,6 +85,11 @@ The default version of the [Pex](https://docs.pex-tool.org/) tool has been updat
 
 The `pants.backend.experimental.python.lint.ruff.check` backend [now supports](https://github.com/pantsbuild/pants/pull/21783) including [Ruff's output file as a report](https://www.pantsbuild.org/2.25/docs/python/overview/linters-and-formatters#bandit-flake8-pylint-and-ruff-report-files).
 
+#### Go
+
+Fix a bug with the setup of the gRPC protobuf plugins where a `go install` invocation was prevented from accessing the Go module proxy during its build of those plugins, which caused those builds to fail.
+
+
 ##### NEW: Python for OpenAPI
 
 A new experimental `pants.backend.experimental.openapi.codegen.python` backend

src/python/pants/backend/codegen/protobuf/go/rules.py

@@ -599,6 +599,8 @@ async def setup_go_protoc_plugin() -> _SetupGoProtocPlugin:
                 input_digest=download_sources_result.output_digest,
                 output_files=["gopath/bin/protoc-gen-go"],
                 description="Build Go protobuf plugin for `protoc`.",
+                # Allow `go` to contact the Go module proxy since it will run its own build.
+                allow_downloads=True,
             ),
         ),
         Get(
@@ -611,6 +613,8 @@ async def setup_go_protoc_plugin() -> _SetupGoProtocPlugin:
                 input_digest=download_sources_result.output_digest,
                 output_files=["gopath/bin/protoc-gen-go-grpc"],
                 description="Build Go gRPC protobuf plugin for `protoc`.",
+                # Allow `go` to contact the Go module proxy since it will run its own build.
+                allow_downloads=True,
             ),
         ),
     )