Merge pull request #3280 from dmargala/bugfix/flex-nodes-slurm-partition-default

[bugfix] Let Slurm decide partition in flexible node allocation if no default partition is defined

Author: vkarak

reframe/core/schedulers/slurm.py

@@ -308,6 +308,21 @@ def _get_default_partition(self):
 
         return None
 
+    def _get_actual_partition(self, options):
+        try:
+            completed = _run_strict(
+                ' '.join(['srun'] + options + ['--test-only', 'true'])
+            )
+            partition_match = re.search(r'partition (?P<partition>\S+)\s+',
+                                        completed.stderr)
+            if partition_match:
+                return partition_match.group('partition')
+
+        except SpawnedProcessError as e:
+            self.log('could not retrieve actual partition')
+
+        return None
+
     def _merge_files(self, job):
         with osext.change_dir(job.workdir):
             out_glob = glob.glob(job.stdout + '_*')
@@ -345,11 +360,15 @@ def filternodes(self, job, nodes):
             nodes &= self._get_reservation_nodes(reservation)
             self.log(f'[F] Filtering nodes by reservation {reservation}: '
                      f'available nodes now: {len(nodes)}')
-
         if partitions:
             partitions = set(partitions.strip().split(','))
         else:
-            default_partition = self._get_default_partition()
+            # Use a default partition if one is configured. Otherwise,
+            # fallback to the partition Slurm chooses for this set of options.
+            default_partition = (
+                self._get_default_partition() or
+                self._get_actual_partition(options)
+            )
             partitions = {default_partition} if default_partition else set()
             self.log(
                 f'[F] No partition specified; using {default_partition!r}'