Bazel & protobuf: how to choose a specific protoc version - protocol-buffers

Question

I am working in a project using some proto sources that are already compiled using a specific version, I also need to compile some custom protos that are cohabiting in the same project, so the protoc needs to match the one that was used to generate the other ones.
I can see in the pre-generated ones:
#if PROTOBUF_VERSION < 3009000
#if 3009002 < PROTOBUF_MIN_PROTOC_VERSION
In mines:
#if PROTOBUF_VERSION < 3017000
#if 3017000 < PROTOBUF_MIN_PROTOC_VERSION
I don't quite understand which protoc is being used, the one installed on my system is 3.19.4.
Also this is my WORKSPACE:
http_archive(
name = "rules_proto",
sha256 = "66bfdf8782796239d3875d37e7de19b1d94301e8972b3cbd2446b332429b4df1",
strip_prefix = "rules_proto-4.0.0",
urls = [
"https://mirror.bazel.build/github.com/bazelbuild/rules_proto/archive/refs/tags/4.0.0.tar.gz",
"https://github.com/bazelbuild/rules_proto/archive/refs/tags/4.0.0.tar.gz",
],
)
load("#rules_proto//proto:repositories.bzl", "rules_proto_dependencies", "rules_proto_toolchains")
rules_proto_dependencies()
rules_proto_toolchains()
http_archive(
name = "com_github_grpc_grpc",
urls = [
"https://github.com/grpc/grpc/archive/refs/tags/v1.44.0.tar.gz",
],
sha256 = "8c05641b9f91cbc92f51cc4a5b3a226788d7a63f20af4ca7aaca50d92cc94a0d",
strip_prefix = "grpc-1.44.0",
)
load("#com_github_grpc_grpc//bazel:grpc_deps.bzl", "grpc_deps")
grpc_deps()
load("#com_github_grpc_grpc//bazel:grpc_extra_deps.bzl", "grpc_extra_deps")
grpc_extra_deps()
The error I am currently getting is:
In file included from cc/tensorflow/plugin_primeclient/grappler/grappler.cc:7:
bazel-out/aarch64-fastbuild/bin/cc/tensorflow/plugin/protos/graph.pb.h:12:2: error: #error This file was generated by a newer version of protoc which is
12 | #error This file was generated by a newer version of protoc which is
| ^~~~~
bazel-out/aarch64-fastbuild/bin/cc/tensorflow/plugin/protos/graph.pb.h:13:2: error: #error incompatible with your Protocol Buffer headers. Please update
13 | #error incompatible with your Protocol Buffer headers. Please update
| ^~~~~
bazel-out/aarch64-fastbuild/bin/cc/tensorflow/plugin/protos/graph.pb.h:14:2: error: #error your headers.
14 | #error your headers.
| ^~~~~

Answer 1

I'll try and describe the general process I take when tracking dependency problems down in Bazel, as it seems to be a regular problem that you'll probably run into again.
Before Bazel does anything to do with the build it's going to look in your WORKSPACE file to see if it needs to fetch any dependencies. It might not seem like an important detail, but Bazel handles WORKSPACE dependencies from top to bottom. We can use this behaviour to override the protobuf version used. Checkout the maybe macro if your interested in how this works.
So in your WORKSPACE file, the first dependency that you have is 'rules_proto' # version 4.0.0 with the http_archive. Then you are loading two macros from 'rules_proto' here;
load("#rules_proto//proto:repositories.bzl", "rules_proto_dependencies", "rules_proto_toolchains")
rules_proto_dependencies()
rules_proto_toolchains()
So first things first let's head over to the rules_proto releases page and find your specific release. Then click on the little hash on that page (circled in red).
Then click "browse files" in the top right;
This will allow you to browse the state of that repository at that specific version. Now as you loaded "repositories.bzl" you'll want to open that up and inspect it (ctrl-F search for protobuf). You'll find that it calls another private macro i.e.
protobuf_workspace(name = "com_google_protobuf")
If we look for where that macro was loaded you'll see that you'll need to follow that through to the 'proto/private/dependencies.bzl';
load("//proto/private:dependencies.bzl", "dependencies", "maven_dependencies", "protobuf_workspace")
After opening that up and searching again for protobuf you'll find the line that specifies the protobuf version;
"com_github_protocolbuffers_protobuf": {
#...
},
So by the looks of it you are using an older version of protobuf with Bazel than what is installed on your system. So in order to override the protobuf version in Bazel you need to simply add it as a dependency in your WORKSPACE before the rules_proto repository. e.g.
# file: //:WORKSPACE
http_archive(
name = "com_github_protocolbuffers_protobuf",
# TODO: Leave this empty and Bazel will tell you what to put here when you build.
sha256 = "",
# Note: Same version as your system deps.
strip_prefix = "protobuf-3.19.4",
urls = [
# Note: Same version as your system deps.
"https://github.com/protocolbuffers/protobuf/releases/download/v3.19.4/protobuf-all-3.19.4.tar.gz"
],
)
http_archive(
name = "rules_proto",
# The rest of the WORKSPACE...