include gen_dir irrespective of sources_dir in protoc_gen

[perezd]

This is an attempt at fixing #9731.

This code was introduced Jan 28th of this year in this PR:
#9438

However, there's generated code (via a genrule) may be included in a protoc invocation without a proper source_dir.

[perezd]

Hoping @comius can weigh in on this change as I'm less familiar with it, and he was the originator of the code.

[comius]

LGTM

[comius]

Envoy is still building with this patched in, integration tests pass.

bazel test --config=remote --override_repository=com_google_protobuf=/.../protobuf  //test/integration:all

[mikex-oss]

Hi, it looks like this merge breaks our project. I was trying to upgrade from release v3.20.0 (https://github.com/protocolbuffers/protobuf/releases/download/v3.20.0/protobuf-all-3.20.0.tar.gz) to the latest (https://github.com/protocolbuffers/protobuf/releases/download/v21.12/protobuf-all-21.12.tar.gz). Someone from the protobuf team mentioned that this merge was possibly related. I checked by running on this commit (fails) and its parent (passes).

Issue: Bazel build of py_proto_library with generated source fails.

BUILD file:

load("@com_google_protobuf//:protobuf.bzl", "py_proto_library")

#  py_proto_library depend on the source proto being in the same package as the consuming rule
genrule(
    name = "generated_proto",
    srcs = ["@external_repo//path/to:foo.proto"],
    outs = ["foo.proto"],
    cmd = "cp $< $@",
)

py_proto_library(
    name = "foo_py_pb2",
    srcs = ["foo.proto"],
)

Before:

$ bazel build //my_project/proto:foo_py_pb2
INFO: Analyzed target //my_project/proto:foo_py_pb2 (42 packages loaded, 710 targets configured).
INFO: Found 1 target...
...
INFO: From ProtoCompile my_project/proto/foo_pb2.py:
<path>/sandbox/linux-sandbox/1047/execroot/my_project_repo
Target //my_project/proto:foo_py_pb2 up-to-date:
  bazel-bin/my_project/proto/foo_pb2.py
INFO: Elapsed time: 75.079s, Critical Path: 16.77s
INFO: 202 processes: 2 internal, 200 linux-sandbox.
INFO: Build completed successfully, 202 total actions

After this merge:

ERROR: <path>/my_project/proto/BUILD:206:17: ProtoCompile my_project/proto/foo_pb2.py failed: (Exit 1): bash failed: error executing command /bin/bash -c ... (remaining 1 argument skipped)

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
foo.proto: File does not reside within any path specified using --proto_path (or -I).  You must specify a --proto_path which encompasses this file.  Note that the proto_path must be an exact prefix of the .proto file names -- protoc is too dumb to figure out when two paths (e.g. absolute and relative) are equivalent (it's harder than you think).
mv: cannot stat 'bazel-out/k8-fastbuild/bin/foo_pb2.py': No such file or directory
<path>/sandbox/linux-sandbox/510/execroot/my_project_repo
Target //my_project/proto:foo_py_pb2 failed to build

Can someone take a look?