Listeners for main server process

[ahopkins]

Stems from discussion in #2015.

This is a POC for adding listeners that only are triggered in the main server process. There is no testing yet, and I have not done any edge cases (ASGI should throw error).

Eventually, this should be replaced by #1630.

---

@app.listener("main_process_start")
async def process_start(app, loop):
    print("process_start")

@app.listener("main_process_stop")
async def process_stop(app, loop):
    print("process_stop")

Single server

[2021-02-02 00:51:12 +0200] [566407] [INFO] Goin' Fast @ http://127.0.0.1:9999
process_start
[2021-02-02 00:51:12 +0200] [566407] [INFO] Starting worker [566407]
^C[2021-02-02 00:51:12 +0200] [566407] [INFO] Stopping worker [566407]
process_stop
[2021-02-02 00:51:12 +0200] [566407] [INFO] Server Stopped

Multiple servers

[2021-02-02 00:51:18 +0200] [566483] [INFO] Goin' Fast @ http://127.0.0.1:9999
process_start
[2021-02-02 00:51:18 +0200] [566484] [INFO] Starting worker [566484]
[2021-02-02 00:51:18 +0200] [566485] [INFO] Starting worker [566485]
^C^C[2021-02-02 00:51:22 +0200] [566483] [INFO] Received signal SIGTERM. Shutting down.
[2021-02-02 00:51:22 +0200] [566483] [INFO] Received signal SIGINT. Shutting down.
[2021-02-02 00:51:22 +0200] [566485] [INFO] Stopping worker [566485]
[2021-02-02 00:51:22 +0200] [566484] [INFO] Stopping worker [566484]
process_stop
[2021-02-02 00:51:22 +0200] [566483] [INFO] Server Stopped

[codecov]

Codecov Report

Merging #2018 (a712586) into master (170177f) will increase coverage by 0.036%.
The diff coverage is 96.552%.

@@              Coverage Diff              @@
##            master     #2018       +/-   ##
=============================================
+ Coverage   91.719%   91.754%   +0.036%     
=============================================
  Files           35        35               
  Lines         3200      3226       +26     
  Branches       555       557        +2     
=============================================
+ Hits          2935      2960       +25     
  Misses         181       181               
- Partials        84        85        +1     

Impacted Files	Coverage Δ
sanic/server.py	87.958% <94.737%> (+0.287%)	⬆️
sanic/app.py	91.793% <100.000%> (+0.072%)	⬆️
sanic/mixins/listeners.py	100.000% <100.000%> (ø)
sanic/worker.py	84.800% <100.000%> (+0.374%)	⬆️

---

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[sjsadowski]

Approved.

[sjsadowski]

I appreciate this given the number of create_server issues that come up.