Fix error thrown in presence of placeholder tasks

When there are placeholder tasks in an executor for a node then there is no
build number yet assigned and attempting to access that key in the dictionary
will fail.

This fix filters out any tasks which are instances of that class before
proceeding.

Change-Id: Ie5b237c3e6023c1822f91e5982ec84f1a363130c
Closes-Bug: #1659787

jenkins/__init__.py

@@ -1146,7 +1146,7 @@ class Jenkins(object):
                     raise
             for executor in info['executors']:
                 executable = executor['currentExecutable']
-                if executable:
+                if executable and 'PlaceholderTask' not in executable.get('_class', ''):
                     executor_number = executor['number']
                     build_number = executable['number']
                     url = executable['url']

tests/test_build.py

@@ -417,3 +417,40 @@ class JenkinsListRunningBuildsTest(JenkinsTestBase):
         builds = self.j.get_running_builds()
         # Should treat the slave as not running any builds
         self.assertEqual([], builds)
+
+    @patch.object(jenkins.Jenkins, 'get_node_info')
+    @patch.object(jenkins.Jenkins, 'get_nodes')
+    def test_placeholder_task_in_queue(self, nodes_mock, node_info_mock):
+        nodes_to_return = [{
+            'name': "foo-slave", 'offline': False
+        }]
+        nodes_mock.return_value = nodes_to_return
+        node_info_to_return = {
+            "executors": [
+                {
+                    "currentExecutable": None,
+                    "currentWorkUnit": None,
+                    "idle": True,
+                    "likelyStuck": False,
+                    "number": 1,
+                    "progress": -1
+                },
+                {
+                    'currentExecutable': {
+                        '_class': (
+                            'org.jenkinsci.plugins.workflow.support.steps.'
+                            'ExecutorStepExecution$PlaceholderTask$'
+                            'PlaceholderExecutable'
+                        )
+                    },
+                    'currentWorkUnit': {},
+                    'idle': False,
+                    'likelyStuck': False,
+                    'number': 1,
+                    'progress': 0
+                }
+            ],
+        }
+        node_info_mock.return_value = node_info_to_return
+        builds = self.j.get_running_builds()
+        self.assertEqual([], builds)