Python-slack-sdk: Enable automatic retry by a handy way

[seratch]

What's this?

WebClient does not retry HTTP requests in any case. Developers may want to have auto-retries for intermittent and/or recoverable errors. We can consider adding a new option (or its underlying function) for auto-retry on the library side.

Basic Design Idea

The primary option should be a retry configuration like urllib3 offers: https://urllib3.readthedocs.io/en/latest/reference/urllib3.util.html

retry = Retry(connect=5, read=2, redirect=5, ratelimited=10)
client = WebClient(token="xoxb-", retry=retry)

WebClient can translate the configuration to an error handling function under the hood. The possible function interface can be as below (note: this is a completely pseudo code):

def retry_handler(client, req, error, retry_num) -> Union[SlackResponse, Exception]:
     if is_recoverable(error):
         return client.retry(req)
     else:
         return error

client = WebClient(token="xoxb-", retry_handler=retry_handler)

Most developers just need retry config but some may want to take full control of the logic (e.g., implementing exponential backoff retries etc).

Factors to Consider

• [ ] Provide the same interface to both AsyncWebClient and WebClient; this means the compatibility with AIOHTTP must be evaluated before deciding the interface
• [ ] Provide the option to WebClient and WebhookClient
• [ ] Enable overwriting per endpoint; developers may want to turn retries on only for a particular endpoint
• [ ] Smooth integration with Bolt for Python; the design should not block giving the config from App class constructor
• [ ] Handle ratelimited errors appropriately

... what else?

Category (place an x in each of the [ ])

• [x] slack_sdk.web.WebClient (Web API client)
• [x] slack_sdk.webhook.WebhookClient (Incoming Webhook, response_url sender)
• [ ] slack_sdk.models (UI component builders)
• [ ] slack_sdk.oauth (OAuth Flow Utilities)
• [ ] slack_sdk.rtm.RTMClient (RTM client)
• [ ] slack_sdk.signature (Request Signature Verifier)

Requirements

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[ashvinnihalani]

Also a note, the retry should not only retry the command but retry the recreating the connection. Often times Slack responds with a connection reset by peer when you are trying mass calls and to recover you need to reinitialize the client.

[seratch]

Due to my capacity for the next minor release, I've moved this feature from 3.5 to 3.6.

[averypmc]

Would love to see this feature!