[internal] Reject CI builds if any .rej files exist

.rej are the byproduct of failed patches via git-apply or artifacts of
the backporting process through jasper. Any build that has them should
be rejected early.

tasks/jenkins.js

@@ -16,6 +16,7 @@ module.exports = function (grunt) {
     grunt.option('os-packages', true);
 
     grunt.task.run(compact([
+      'rejectRejFiles',
       'test',
       process.env.JOB_NAME === 'kibana_core' ? 'build' : null
     ]));

tasks/reject_rej_files.js

@@ -0,0 +1,30 @@
+import { startsWith } from 'lodash';
+
+// Fails if any .rej files are found
+// .rej files are an artifact from a failed git-apply or a jasper backport
+
+// This check is intentionally performed on the files in the repo rather than
+// on the files that are to be committed.
+
+export default grunt => {
+  grunt.registerTask('rejectRejFiles', 'Reject any git-apply .rej files', () => {
+    const ignoredTopLevelDirs = [
+      'esvm',
+      'installedPlugins',
+      'node_modules',
+      'optimize'
+    ];
+
+    const matchBase = true;
+    const filter = file => (
+      ignoredTopLevelDirs.every(dir => !startsWith(file, dir))
+    );
+
+    const files = grunt.file.expand({ filter, matchBase }, '*.rej');
+    if (files.length > 0) {
+      const err = `.rej files are not allowed:\n${files.join('\n')}`;
+      grunt.log.error(err);
+      return false;
+    }
+  });
+};