[AIRFLOW-XXX] KubernetesPodOperator: fix _extract_xcom for larger json files

[ddelange]

When calling update() and peek_stdout() immediately after write_stdin(),
the cat command was sometimes not done yet with writing to stdin. Hence,
an incomplete (and broken) json file was returned.

[2019-12-11 16:16:50,586] {logging_mixin.py:112} INFO - [2019-12-11 16:16:50,586] {pod_launcher.py:217} INFO - Running command... cat /airflow/xcom/return.json
[2019-12-11 16:16:50,630] {logging_mixin.py:112} INFO - [2019-12-11 16:16:50,630] {pod_launcher.py:217} INFO - Running command... kill -s SIGINT 1
[2019-12-11 16:16:57,143] {logging_mixin.py:112} INFO - [2019-12-11 16:16:57,143] {local_task_job.py:124} INFO - Time since last heartbeat(0.01 s) < heartrate(10.0 s), sleeping for 9.99135 s
[2019-12-11 16:17:02,048] {logging_mixin.py:112} INFO - [2019-12-11 16:17:02,047] {pod_launcher.py:132} INFO - <broken json string here>
[2019-12-11 16:17:02,076] {taskinstance.py:1058} ERROR - Unterminated string starting at: line 1 column 31378 (char 31377)
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 930, in _run_raw_task
    result = task_copy.execute(context=context)
  File "/usr/local/lib/python3.7/site-packages/airflow/contrib/operators/kubernetes_pod_operator.py", line 161, in execute
    get_logs=self.get_logs)
  File "/usr/local/lib/python3.7/site-packages/airflow/contrib/kubernetes/pod_launcher.py", line 117, in run_pod
    return self._monitor_pod(pod, get_logs)
  File "/usr/local/lib/python3.7/site-packages/airflow/contrib/kubernetes/pod_launcher.py", line 133, in _monitor_pod
    result = json.loads(result)
  File "/usr/local/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/usr/local/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/lib/python3.7/json/decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 31378 (char 31377)
[2019-12-11 16:17:02,080] {taskinstance.py:1089} INFO - Marking task as FAILED.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (v1-10-test@8ad2f60). Click here to learn what that means.
The diff coverage is 100%.

@@              Coverage Diff              @@
##             v1-10-test    #6797   +/-   ##
=============================================
  Coverage              ?   81.15%           
=============================================
  Files                 ?      519           
  Lines                 ?    35348           
  Branches              ?        0           
=============================================
  Hits                  ?    28687           
  Misses                ?     6661           
  Partials              ?        0

Impacted Files	Coverage Δ
airflow/contrib/kubernetes/pod_launcher.py	92.53% <100%> (ø)
airflow/utils/dag_processing.py	92.19% <100%> (ø)
airflow/jobs/backfill_job.py	92.6% <100%> (ø)

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