fix: cc_toolchain should prefer protoc when prebuilt flag is flipped.

[thesayyn]

cc_toolchain now respects the prefer_prebuilt_protoc flag. When the flag is set to true, the toolchain uses the full protoc binary instead of protoc_minimal, enabling prebuilt protoc support.

Follow up to this is to have minimal protoc released as an artifact and use that.

[mkruskal-google]

Can we actually do a positive check that this uses the prebuilt? Negative regex checks are kindof brittle (a typo trivially passes) and this might start failing if we switch to a prebuilt protoc_minimal

[meteorcloudy]

Can we also verify java_proto_library and py_proto_library?

[zhangskz]

nit: analysis

[Wyverald]

@meteorcloudy reported the following:

I'm using protobuf 33.x with changes from #25168 to build proto libraries in Bazel. Building with the following flags:

common --per_file_copt=external/.*protobuf.*/src/google/protobuf/compiler/main.cc@--THIS_CC_TOOLCHAIN_IS_BROKEN
common --host_per_file_copt=external/.*protobuf.*/src/google/protobuf/compiler/main.cc@--THIS_CC_TOOLCHAIN_IS_BROKEN
common --incompatible_enable_proto_toolchain_resolution
common --@com_google_protobuf//bazel/toolchains:prefer_prebuilt_protoc

For //src/main/protobuf:command_server_cc_proto, it failed with

In file included from bazel-out/k8-fastbuild/bin/src/main/protobuf/command_server.pb.h:34:
bazel-out/k8-fastbuild/bin/src/main/protobuf/failure_details.pb.h:16:2: error: #error "Protobuf C++ gencode is built with an incompatible version of"
   16 | #error "Protobuf C++ gencode is built with an incompatible version of"
      |  ^~~~~

For //src/main/protobuf:command_server_java_proto, it still tried to compile the protoc from source, which triggered:
gcc: error: unrecognized command-line option '--THIS_CC_TOOLCHAIN_IS_BROKEN'

[alexeagle]

@Wyverald @meteorcloudy that's expected, the cc implementation has a hard requirement of exact version match, and there isn't a published prebuild for the HEAD pre-release version of the repo. It's the same reason @thesayyn mentions that the newly added test target is tagged manual and is expected behavior.

[Wyverald]

@Wyverald @meteorcloudy that's expected, the cc implementation has a hard requirement of exact version match, and there isn't a published prebuild for the HEAD pre-release version of the repo. It's the same reason @thesayyn mentions that the newly added test target is tagged manual and is expected behavior.

Ah, thanks for the context! Just for my information, where is protobuf pulling prebuilt protoc's from? I'm assuming that publishing a prebuilt protoc is part of the protobuf release process (so 33.3 would come with a prebuilt protoc)?

[alexeagle]

https://github.com/protocolbuffers/protobuf/pull/24115/changes#diff-b4f2e9f64ea668b0d49eb76f523b1e449fe7add0a23ada273e93552769d5bf02 is a good place to look. Yes, the release process swaps out this file with the tagged version and integrity hashes of the binaries, prior to that we can only fake the version