Firebase db listener cannot handle with internet disconnetions

首先检查 issue 中所示的 SSEClient 和 Event 类，然后在 db.reference().listen() 执行期间或之后断开网络以复现故障。跟踪 listener 如何处理 stream 中断和重新连接；当 listener 恢复接收 Firebase 更新，并且 listen() 之后的代码不会被无限期阻塞时，即表示完成。

[READ] Step 1: Are you in the right place?

• For issues or feature requests related to the code in this repository
  file a Github issue.
  • If this is a feature request make sure the issue title starts with "FR:".
• For general technical questions, post a question on StackOverflow
  with the firebase tag.
• For general Firebase discussion, use the firebase-talk
  google group.
• For help troubleshooting your application that does not fall under one
  of the above categories, reach out to the personalized
  Firebase support channel.

[REQUIRED] Step 2: Describe your environment

• Operating System version: Ubuntu 18 and Raspbian
• Firebase SDK version: lasted realesed with pip install
• Library version: lasted realesed with pip install
• Firebase Product: firebase db stream (listener)

[REQUIRED] Step 3: Describe the problem

db.reference().listen() hang all code, do not run the rest of code, if when during connection of listener get internet disconnected, or if streamming allready connected and get disconnected for more than 2~3min and reconnect, do not get any updates from firebase.
i was searching for the root of this problem, and realised SSEclient have timeout default set by 3000, and the Event class retry time is set by None. Im not sure about this but maybe this is the problem?

Steps to reproduce:

To simulate the error we can just pull the cable or simply disconnect the internet from the system, wait 2 minutes to 3 minutes to reconnect it, after the reconnection, update any end-point that the streamming is listening. We can notice that we will not receive any updates.

It is also possible to simulate it during the streaming connection opening disconnect the internet and wait 2 to 3 minutes again and reconnect. We will not receive any updates from the firebase, and it will also be noticed that when this occurs, the codes below the opening stream do not run even wait for long time.

Relevant Code:

class SSEClient(object):
'''SSE client implementation.'''

def __init__(self, url, session, retry=3000, **kwargs):
    """Initializes the SSEClient.
    Args:
      url: The remote url to connect to.
      session: The requests session.
      retry: The retry interval in milliseconds (optional).
      **kwargs: Extra kwargs that will be sent to ``requests.get()`` (optional).
    """

class Event(object):
'''Event represents the events fired by SSE.'''

sse_line_pattern = re.compile('(?P<name>[^:]*):?( ?(?P<value>.*))?')

def __init__(self, data='', event_type='message', event_id=None, retry=None):
    self.data = data
    self.event_type = event_type
    self.event_id = event_id
    self.retry = retry

// TODO(you): code here to reproduce the problem

import firebase_admin
from firebase_admin import db
from firebase_admin import credentials

# Init Firebase.
dir_path = os.path.dirname(os.path.realpath(__file__))
cred = credentials.Certificate(dir_path + '/serviceAccountKey.json')
firebase_admin.initialize_app(cred, {
    'databaseURL': DB_URL
})

try:
    streaming = db.reference('path').listen(callback)
except ApiCallError:
    timer = 0

# did this code above to handle with disconnections.
timer = 100
while True:
    if internet_on():
        if timer == 0:
            try:
                streaming.close()
           except:
                #we do not have streaming connections open.
                pass
            try:
                streaming = db.reference('path').listen(callback)
                timer = 100
            except ApiCallError:
                timer = 0
    else:
        time.sleep(1)
        timer -= 1 if timer > 0 else 0