[SPARK-25921][PYSPARK] Fix barrier task run without BarrierTaskContext while python worker reuse

## What changes were proposed in this pull request?

Running a barrier job after a normal spark job causes the barrier job to run without a BarrierTaskContext. This is because while python worker reuse, BarrierTaskContext._getOrCreate() will still return a TaskContext after firstly submit a normal spark job, we'll get a `AttributeError: 'TaskContext' object has no attribute 'barrier'`. Fix this by adding check logic in BarrierTaskContext._getOrCreate() and make sure it will return BarrierTaskContext in this scenario.

## How was this patch tested?

Add new UT in pyspark-core.

Closes apache#22962 from xuanyuanking/SPARK-25921.

Authored-by: Yuanjian Li <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>
(cherry picked from commit c00e72f)
Signed-off-by: Wenchen Fan <[email protected]>

Author: xuanyuanking

python/pyspark/taskcontext.py

@@ -147,8 +147,8 @@ def __init__(self):
     @classmethod
     def _getOrCreate(cls):
         """Internal function to get or create global BarrierTaskContext."""
-        if cls._taskContext is None:
-            cls._taskContext = BarrierTaskContext()
+        if not isinstance(cls._taskContext, BarrierTaskContext):
+            cls._taskContext = object.__new__(cls)
         return cls._taskContext
 
     @classmethod

python/pyspark/tests.py

@@ -614,6 +614,21 @@ def context_barrier(x):
         times = rdd.barrier().mapPartitions(f).map(context_barrier).collect()
         self.assertTrue(max(times) - min(times) < 1)
 
+    def test_barrier_with_python_worker_reuse(self):
+        """
+        Verify that BarrierTaskContext.barrier() with reused python worker.
+        """
+        self.sc._conf.set("spark.python.work.reuse", "true")
+        rdd = self.sc.parallelize(range(4), 4)
+        # start a normal job first to start all worker
+        result = rdd.map(lambda x: x ** 2).collect()
+        self.assertEqual([0, 1, 4, 9], result)
+        # make sure `spark.python.work.reuse=true`
+        self.assertEqual(self.sc._conf.get("spark.python.work.reuse"), "true")
+
+        # worker will be reused in this barrier job
+        self.test_barrier()
+
     def test_barrier_infos(self):
         """
         Verify that BarrierTaskContext.getTaskInfos() returns a list of all task infos in the