scheduler.py: Implement operation cancellation

buildgrid/server/scheduler.py

@@ -128,3 +128,13 @@ class Scheduler:
     def get_job_operation(self, job_name):
         """Returns the operation associated to job."""
         return self.jobs[job_name].operation
+
+    def cancel_job_operation(self, job_name):
+        """"Cancels the underlying operation of a given job.
+
+        This will also cancel any job's lease that may have been issued.
+
+        Args:
+            job_name (str): name of the job holding the operation to cancel.
+        """
+        self.jobs[job_name].cancel_operation()