Debugger stops on hot reload for Flask in debugger

[melvinkokxw]

Issue Type: Bug

Steps to reproduce problem:

1. launch.json configuration as given below:

{
            "name": "Docker: Python - Flask",
            "type": "docker",
            "request": "launch",
            "preLaunchTask": "docker-run: debug",
            "python": {
                "pathMappings": [
                    {
                        "localRoot": "${workspaceFolder}/web/",
                        "remoteRoot": "/app"
                    }
                ],
                "host": "localhost",
                "port": 5000,
                "projectType": "flask",
            },
            "removeContainerAfterDebug": true}
        }

task.json given below:

{
			"type": "docker-build",
			"label": "docker-build",
			"platform": "python",
			"dockerBuild": {
				"tag": "<tag name removed>",
				"dockerfile": "${workspaceFolder}/web/Dockerfile",
				"context": "${workspaceFolder}/web/",
				"pull": true
			}
		},
		{
			"type": "docker-run",
			"label": "docker-run: debug",
			"dependsOn": [
				"docker-build"
			],
			"dockerRun": {
				"containerName": "<container name removed>",
				"env": {
					"FLASK_APP": "<app name removed>",
					"FLASK_ENV": "development",
					"FLASK_DEBUG": "1"
				},
				"volumes": [
					{
					  "containerPath": "/app", "localPath": "${workspaceFolder}/web/"
					},
				],
				"envFiles": [
					"${workspaceFolder}/web/.env"
				],
				"ports": [
					{
						"containerPort": 5000, "hostPort": 5000
					}
				]
			},
			"python": {
				"args": [
					"run",
					"--host",
					"0.0.0.0",
					"--port",
					"5000",
				],
				"module": "flask",
			}
		}

2. Run debugger
3. Make edit anywhere on a python file and save

Results

Expected results:
Flask hot reloads.

Actual results:
Last line in debug console is:

* Detected change in '/<python file name>.py', reloading

and then the debugger simply stops.

Apologies but I do not have a code snippet available for copying!

[bwateratmsft]

@melvinkokxw Thank you for the detailed information! I was able to reproduce your issue. When I set "removeContainerAfterDebug": false in launch.json, I was able to do hot reloading, however, it does require manually stopping the container after debugging is done (or else the next docker-run task fails because the container name is already in use).

I'll look to see what can be done in the code to mitigate the above shortcomings.

[melvinkokxw]

@bwateratmsft Thank you for investigating the issue and identifying the cause. Setting "removeContainerAfterDebug": false in launch.json works for me.

Not setting that explicitly doesn't remove the container after debug, yet hot reload doesn't work. Is false the default value for removeContainerAfterDebug?

[bwateratmsft]

Hmm, it's supposed to be true by default, so if it's not specified it's still treated as true. I'll check on that as well.

[bwateratmsft]

@melvinkokxw I think I've found what went wrong and how to fix it, with hot reloading. The Python extension somewhat transparently makes two debug sessions--the outer "launch" session, and the inner "attach" session. When you save a file, the outer session is supposed to persist but the inner one is discarded. Our code for removing the container after debug was not able to tell the difference between those two--so when the inner one ended, it would remove the container.

I made a change so that we can tell them apart, and will not remove the container when the inner session ends.

As for the container not being removed when removeContainerAfterDebug was not explicitly set, I was not able to reproduce this. However, there have been some VSCode issues lately where changes in launch.json or tasks.json don't get picked up, so if it was false and then you removed it, it's possible you hit that bug--and VSCode thought it was still false.