Add macOSVersion helper

src/platform.ts

@@ -1,3 +1,7 @@
+import * as semver from "semver";
+
+import type { ActionState } from "./action-common";
+
 /** Platform identifiers used in CodeQL bundle asset names. */
 export enum BundlePlatform {
   Linux64 = "linux64",
@@ -24,3 +28,28 @@ export function getBundlePlatform(
       return undefined;
   }
 }
+
+/**
+ * Tries to determine the version of macOS.
+ *
+ * @returns
+ *  The version as either a semantic version object, the raw version string
+ *  if it is not a semantic version, or `undefined` if we are not on macOS.
+ */
+export function macOSVersion(
+  action: ActionState<["Base"]>,
+): semver.SemVer | string | undefined {
+  // Skip if we are not running on macOS.
+  if (action.platform !== "darwin") {
+    return undefined;
+  }
+
+  // Try to parse the OS version string.
+  const version = semver.parse(action.osRelease);
+
+  if (version === null) {
+    return action.osRelease;
+  }
+
+  return version;
+}