bcr_compatibility: Support selecting top modules from BCR (bazelbuild…

…#2094)

buildkite/bazel-central-registry/README.md

@@ -26,6 +26,8 @@ A new build can be triggered via the [BCR Bazel Compatibility Test](https://buil
 
     Examples: `[email protected],rules_java@latest`, `rules_*@latest`, `protobuf@<29.0-rc1`
 
+* `SELECT_TOP_BCR_MODULES`: (Optional) Set this env var to select the top N most important modules from the BCR for testing (based on their PageRank values). This will override the `MODULE_SELECTIONS` env var.
+
 * `SMOKE_TEST_PERCENTAGE`: (Optional) Specifies a percentage of selected modules to be randomly sampled for smoke testing.
 
     For example, if `MODULE_SELECTIONS=rules_*@latest` and `SMOKE_TEST_PERCENTAGE=10`, then 10% of modules with name starting with `rules_` will be randomly selected.

buildkite/bazel-central-registry/bcr_compatibility.py

@@ -65,6 +65,14 @@ def select_modules_from_env_vars():
     MODULE_SELECTIONS = os.environ.get('MODULE_SELECTIONS', '')
     SMOKE_TEST_PERCENTAGE = os.environ.get('SMOKE_TEST_PERCENTAGE', None)
 
+    top_n = os.environ.get('SELECT_TOP_BCR_MODULES')
+    if top_n:
+        output = subprocess.check_output(
+            ["bazel", "run", "//tools:module_analyzer", "--", "--name-only", f"--top_n={top_n}"],
+        )
+        top_modules = output.decode("utf-8").split()
+        MODULE_SELECTIONS = ','.join([f"{module}@latest" for module in top_modules])
+
     if not MODULE_SELECTIONS:
         return []