BCR: Block fewer builds (#2717)

Only block builds with shell/batch commands in their yml configs, not
all yml changes.

Co-authored-by: Xùdōng Yáng <wyverald@gmail.com>

buildkite/bazel-central-registry/bcr_presubmit.py

diff --git a/buildkite/bazel-central-registry/bcr_presubmit.py b/buildkite/bazel-central-registry/bcr_presubmit.py
index 0740937..972d8e6 100755
--- a/buildkite/bazel-central-registry/bcr_presubmit.py
+++ b/buildkite/bazel-central-registry/bcr_presubmit.py
@@ -555,7 +555,7 @@
             with open(get_presubmit_yml(module_name, module_version), "rt") as f:
                 presubmit_content = f.read()
 
-            if "shell_commands" in presubmit_content:
+            if bazelci.contains_user_commands(presubmit_content):
                 risky_presubmit_yml_files.add(f"modules/{module_name}/{module_version}/presubmit.yml")
 
             configs = get_anonymous_module_task_config(module_name, module_version)

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index fbef75d..3ac234b 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -3305,6 +3305,13 @@
 
     modified_files = get_modified_files(os.getenv("BUILDKITE_COMMIT"))
     modified_config_files = [f for f in modified_files if is_config_file(f)]
+
+    if os.getenv("BUILDKITE_PIPELINE_SLUG", "") == "bazel-central-registry":
+        # Blocking all yml changes in BCR PRs would be too annoying,
+        # so we only block those with shell/batch commands.
+        # This is the same behavior as in bcr_presubmit.py.
+        modified_config_files = [p for p in modified_config_files if is_risky_bcr_config(p)]
+
     if modified_config_files:
         steps.append(
             {
@@ -3320,6 +3327,15 @@
     return ".bazelci" in path or path.endswith(".yml")
 
 
+def is_risky_bcr_config(path):
+    with open(path, "rt") as f:
+        return contains_user_commands(f.read())
+
+
+def contains_user_commands(yml_content):
+    return  "shell_commands" in yml_content or "batch_commands" in yml_content
+
+
 def create_buildifier_step(buildifier_config):
     buildifier_env_vars = {}
     if isinstance(buildifier_config, str):