Python timer trigger does not run locally when runonstartup is set to true

[paulbatum]

Repro Steps:

1. In VS Code, create a new python function app, add a timer trigger.
2. Modify function.json, add this line: "runOnStartup": true,
3. Add a breakpoint to the function code
4. F5

Expected result: host starts, function runs, breakpoint is hit.
Actual result: host hangs, eventually shows this error:

[11/18/2019 9:59:11 PM] Executed 'Functions.TimerTrigger' (Failed, Id=9a4b4a95-f436-4502-bad9-6d72f5090339)
[11/18/2019 9:59:11 PM] System.Private.CoreLib: Exception while executing function: Functions.TimerTrigger. Microsoft.Azure.WebJobs.Script: Did not find any initialized language workers.

[maiqbal11]

Synced with @pragnagopa offline. She said that she would be able to take a look at this from the Host side.

[pragnagopa]

@yojagad - please verify fix #5458 fixes this issue as well.

[yojagad]

Took a closer look at this and following are the observations

• Does not repro with node (both from command line and from vs code)
• Does not repro when running python function from command line
• When running from VS code, it does and the following are the additional arguments noticed while starting up the worker process
  Starting worker process:python c:/Users/yojagad/.vscode/extensions/ms-python.python-2020.1.58038/pythonFiles/ptvsd_launcher.py --default --host 127.0.0.1 --port 9091 --wait "C:\ProgramData\chocolatey\lib\azure-functions-core-tools\tools\workers\python\3.7/WINDOWS/X64/worker.py" --host 127.0.0.1 --port 60099 --workerId 3088ded6-8f53-4fac-a166-b0f19631bdbd --requestId f64bf3c7-9070-4b4d-af96-7f093d15266b --grpcMaxMessageLength 134217728

Following the last observation, I tried to launch the runtime with debug arguments like
"languageWorkers:python:arguments": "-m ptvsd --host localhost --port 9091 --wait" and observed the same behavior as originally described.

I wonder if it's trying to complete attachment before runtime initialization is complete...?

[pragnagopa]

Thanks @yojagad for the investigation.

@EricJizbaMSFT - We have narrowed the issue to command line options --wait passed by VS Code for python functions. Can you please help us understand what is the requirement for adding '--wait'?

[ejizba]

The command line options come from the Python extension (related to microsoft/vscode-python#3121) and pretty sure we just went with the default. @DonJayamanne @brettcannon what exactly does that --wait flag do?

[ejizba]

I tried removing the --wait flag and it seemed to work ¯\_(ツ)_/¯. I'm fine to remove it, but would prefer to get confirmation from the python extension folks first.

Also this gets the timer trigger to run, but I still can't hit the breakpoint for the first execution. We have an issue on that here microsoft/vscode-azurefunctions#1492 and it's blocked by #4384