Fix download filename for RC versions

protoc/private/BUILD.bazel

@@ -1,7 +1,10 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+load(":prebuilt_protoc_toolchain_test.bzl", "release_version_to_artifact_name_test_suite")
 
 bzl_library(
     name = "versions",
     srcs = ["versions.bzl"],
     visibility = ["//protoc:__subpackages__"],
 )
+
+release_version_to_artifact_name_test_suite()

protoc/private/prebuilt_protoc_toolchain.bzl

@@ -22,14 +22,27 @@ GOOGLE_PROTOBUF_DEP_EDGES = {
     "wrappers": [],
 }
 
+def release_version_to_artifact_name(release_version, platform):
+    # versions have a "v" prefix like "v28.0"
+    stripped_version = release_version.removeprefix("v")
+
+    # release candidate versions like "v29.0-rc3" have artifact names
+    # like "protoc-29.0-rc-3-osx-x86_64.zip"
+    artifact_version = stripped_version.replace("rc", "rc-")
+
+    return "{}-{}-{}.zip".format(
+        "protoc",
+        artifact_version,
+        platform,
+    )
+
 def _prebuilt_protoc_repo_impl(rctx):
     release_version = rctx.attr.version
     if release_version == "LATEST":
         release_version = PROTOC_VERSIONS.keys()[0]
 
-    filename = "{}-{}-{}.zip".format(
-        "protoc",
-        release_version.removeprefix("v"),
+    filename = release_version_to_artifact_name(
+        release_version,
         rctx.attr.platform,
     )
     url = "https://github.com/protocolbuffers/protobuf/releases/download/{}/{}".format(

protoc/private/prebuilt_protoc_toolchain_test.bzl

@@ -0,0 +1,31 @@
+"""Unit test suite for version-to-artifact-name resolution helper.
+
+Based on the example at https://bazel.build/rules/testing#testing-starlark-utilities
+"""
+
+load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
+load(":prebuilt_protoc_toolchain.bzl", "release_version_to_artifact_name")
+load(":versions.bzl", "PROTOC_VERSIONS")
+
+def _release_version_to_artifact_name_test_impl(ctx):
+    env = unittest.begin(ctx)
+    count = 0
+
+    for release_version in PROTOC_VERSIONS:
+        artifact_name = release_version_to_artifact_name(release_version, "linux-x86_64")
+        artifact_dict = PROTOC_VERSIONS[release_version]
+
+        # there should be a linux-x86_64 artifact in every release
+        asserts.true(env, artifact_name in artifact_dict, "'{}' not found for release version '{}'".format(artifact_name, release_version))
+        count += 1
+
+    asserts.true(env, count > 0, "no versions tested")
+    return unittest.end(env)
+
+release_version_to_artifact_name_test = unittest.make(_release_version_to_artifact_name_test_impl)
+
+def release_version_to_artifact_name_test_suite():
+    unittest.suite(
+        "release_version_to_artifact_name_tests",
+        release_version_to_artifact_name_test,
+    )