fix: cc_toolchain should prefer protoc when prebuilt flag is flipped. (#25168)

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.

Closes #25168

COPYBARA_INTEGRATE_REVIEW=#25168 from thesayyn:cc_toolchain_prebuilt 30451be
FUTURE_COPYBARA_INTEGRATE_REVIEW=#25168 from thesayyn:cc_toolchain_prebuilt 30451be
PiperOrigin-RevId: 853356620

Author: thesayyn

BUILD.bazel

@@ -566,7 +566,13 @@ proto_lang_toolchain(
     command_line = "--cpp_out=$(OUT)",
     plugin = "//src/google/protobuf/compiler/cpp:protoc-gen-cpp",
     plugin_format_flag = "--plugin=protoc-gen-cpp=%s",
-    protoc_minimal_do_not_use = "//src/google/protobuf/compiler:protoc_minimal",
+    protoc_minimal_do_not_use = select({
+        # Set minimal protoc if prefer_prebuilt_protoc.flag_set is true since a prebuilt for protoc_minimal is not available
+        # TODO: Add a flag to switch between minimal and full protoc with proto_lang_toolchain once we have a minimal protoc binary.
+        # Setting this attribute to None will make the toolchain to pick the full protoc binary.
+        "//bazel/toolchains:prefer_prebuilt_protoc.flag_set": None,
+        "//conditions:default": "//src/google/protobuf/compiler:protoc_minimal",
+    }),
     runtime = "//src/google/protobuf",
     visibility = ["//visibility:public"],
 )

bazel/tests/BUILD

@@ -1,6 +1,8 @@
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("//bazel:proto_descriptor_set.bzl", "proto_descriptor_set")
 load("//bazel:proto_library.bzl", "proto_library")
 load(":bazel_proto_library_tests.bzl", "bazel_proto_library_test_suite")
+load(":cc_toolchain_tests.bzl", "cc_toolchain_test_suite")
 load(":java_proto_library_tests.bzl", "java_proto_library_test_suite")
 load(":proto_common_check_collocated_tests.bzl", "proto_common_check_collocated_test_suite")
 load(":proto_common_compile_tests.bzl", "proto_common_compile_test_suite")
@@ -19,8 +21,17 @@ proto_common_check_collocated_test_suite(name = "proto_common_check_collocated_t
 
 bazel_proto_library_test_suite(name = "bazel_proto_library_test_suite")
 
+cc_toolchain_test_suite(name = "cc_toolchain_test_suite")
+
 java_proto_library_test_suite(name = "java_proto_library_test_suite")
 
+bzl_library(
+    name = "cc_toolchain_tests_bzl",
+    srcs = ["cc_toolchain_tests.bzl"],
+    parse_tests = False,
+    visibility = ["//visibility:private"],
+)
+
 proto_library(
     name = "empty_proto_library",
 )

bazel/tests/cc_toolchain_tests.bzl

@@ -0,0 +1,81 @@
+# Protocol Buffers - Google's data interchange format
+# Copyright 2025 Google Inc.  All rights reserved.
+#
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+"""Tests for cc_toolchain prebuilt protoc configuration."""
+
+load("@rules_testing//lib:analysis_test.bzl", "analysis_test", "test_suite")
+load("@rules_testing//lib:truth.bzl", "matching")
+load("@rules_testing//lib:util.bzl", "util")
+load("//bazel:proto_library.bzl", "proto_library")
+load("//bazel/tests/testdata:compile_rule.bzl", "compile_rule")
+
+_PREFER_PREBUILT_PROTOC = str(Label("//bazel/toolchains:prefer_prebuilt_protoc"))
+
+def cc_toolchain_test_suite(name):
+    test_suite(
+        name = name,
+        tests = [
+            _test_cc_toolchain_uses_full_protoc_when_prefer_prebuilt_flag_set,
+            _test_cc_toolchain_uses_protoc_minimal_by_default,
+        ],
+    )
+
+def _test_cc_toolchain_uses_full_protoc_when_prefer_prebuilt_flag_set(name):
+    util.helper_target(
+        proto_library,
+        name = name + "_proto",
+        srcs = ["A.proto"],
+    )
+    util.helper_target(
+        compile_rule,
+        name = name + "_compile",
+        proto_dep = ":" + name + "_proto",
+        toolchain = "//:cc_toolchain",
+    )
+
+    analysis_test(
+        name = name,
+        target = name + "_compile",
+        impl = _test_cc_toolchain_uses_full_protoc_when_prefer_prebuilt_flag_set_impl,
+        config_settings = {_PREFER_PREBUILT_PROTOC: True},
+    )
+
+def _test_cc_toolchain_uses_full_protoc_when_prefer_prebuilt_flag_set_impl(env, target):
+    # Find the compile action
+    action = env.expect.that_target(target).action_named("GenProto")
+
+    # When prefer_prebuilt_protoc is True, protoc_minimal_do_not_use is None,
+    # so the cc_toolchain should use the full protoc (not protoc_minimal).
+    # The protoc path should end with "/protoc" not contain "protoc_minimal"
+    action.argv().contains_predicate(matching.str_matches("*/protoc"))
+    action.argv().not_contains_predicate(matching.str_matches("*protoc_minimal*"))
+
+def _test_cc_toolchain_uses_protoc_minimal_by_default(name):
+    util.helper_target(
+        proto_library,
+        name = name + "_proto",
+        srcs = ["A.proto"],
+    )
+    util.helper_target(
+        compile_rule,
+        name = name + "_compile",
+        proto_dep = ":" + name + "_proto",
+        toolchain = "//:cc_toolchain",
+    )
+
+    analysis_test(
+        name = name,
+        target = name + "_compile",
+        impl = _test_cc_toolchain_uses_protoc_minimal_by_default_impl,
+    )
+
+def _test_cc_toolchain_uses_protoc_minimal_by_default_impl(env, target):
+    # Find the compile action
+    action = env.expect.that_target(target).action_named("GenProto")
+
+    # By default (prefer_prebuilt_protoc is False), protoc_minimal_do_not_use is set,
+    # so the cc_toolchain should use protoc_minimal.
+    action.argv().contains_predicate(matching.str_matches("*protoc_minimal*"))

bazel/toolchains/BUILD

@@ -53,7 +53,11 @@ bool_flag(
 config_setting(
     name = "prefer_prebuilt_protoc.flag_set",
     flag_values = {":prefer_prebuilt_protoc": "true"},
-    visibility = ["//bazel/private/toolchains/prebuilt:__pkg__"],
+    visibility = [
+        "//bazel/private/toolchains/prebuilt:__pkg__",
+        # Needed by cc_toolchain to switch between minimal and full protoc
+        "//:__pkg__",
+    ],
 )
 
 # The public API users set to disable the validation action failing.

examples/example_without_cc_toolchain/.bazelrc

@@ -1,6 +1,6 @@
 # Simulate a non-functional CC toolchain
-common --per_file_copt=external/.*protobuf.*@--THIS_CC_TOOLCHAIN_IS_BROKEN
-common --host_per_file_copt=external/.*protobuf.*@--THIS_CC_TOOLCHAIN_IS_BROKEN
+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
 # But, users should be able to use pre-built protoc toolchains instead.
 common --incompatible_enable_proto_toolchain_resolution
-common --@com_google_protobuf//bazel/toolchains:prefer_prebuilt_protoc
+common --@com_google_protobuf//bazel/toolchains:prefer_prebuilt_protoc

examples/example_without_cc_toolchain/BUILD.bazel

@@ -1,6 +1,17 @@
+load("@com_google_protobuf//bazel:cc_proto_library.bzl", "cc_proto_library")
 load("@com_google_protobuf//bazel:proto_library.bzl", "proto_library")
 
 proto_library(
     name = "empty_proto",
     srcs = ["empty.proto"],
 )
+
+cc_proto_library(
+    name = "cc_empty_proto",
+    # We already have a anaylsis test that asserts that the cc_proto_library
+    # uses the correct prebuilt toolchain, this cc_proto_library will never
+    # compile since the prebuilt toolchain is always behind the HEAD therefore
+    # will end up in version skew compilation errors.
+    tags = ["manual"],
+    deps = [":empty_proto"],
+)