docs: Documenting how to debug Flask app

CONTRIBUTING.md

@@ -802,6 +802,130 @@ npm install
 npm run cypress open
 ```
 
+### Debugging Server App
+
+Follow these instructions to debug the Flask app running inside a docker container.
+
+First add the following to the ./docker-compose.yaml file
+
+```diff
+superset:
+    env_file: docker/.env
+    image: *superset-image
+    container_name: superset_app
+    command: ["/app/docker/docker-bootstrap.sh", "app"]
+    restart: unless-stopped
++   cap_add:
++     - SYS_PTRACE
+    ports:
+      - 8088:8088
++     - 5678:5678
+    user: "root"
+    depends_on: *superset-depends-on
+    volumes: *superset-volumes
+    environment:
+      CYPRESS_CONFIG: "${CYPRESS_CONFIG}"
+```
+
+Start Superset as usual
+```bash
+docker-compose up
+```
+
+Install the required libraries and packages to the docker container
+
+Enter the superset_app container
+```bash
+docker exec -it superset_app /bin/bash
+root@39ce8cf9d6ab:/app#
+```
+
+Run the following commands inside the container
+```bash
+apt update
+apt install -y gdb
+apt install -y net-tools
+pip install debugpy
+```
+
+Find the PID for the Flask process. Make sure to use the first PID. The Flask app will re-spawn a sub-process everytime you change any of the python code. So it's important to use the first PID.
+
+```bash
+ps -ef
+
+UID        PID  PPID  C STIME TTY          TIME CMD
+root         1     0  0 14:09 ?        00:00:00 bash /app/docker/docker-bootstrap.sh app
+root         6     1  4 14:09 ?        00:00:04 /usr/local/bin/python /usr/bin/flask run -p 8088 --with-threads --reload --debugger --host=0.0.0.0
+root        10     6  7 14:09 ?        00:00:07 /usr/local/bin/python /usr/bin/flask run -p 8088 --with-threads --reload --debugger --host=0.0.0.0
+```
+
+Inject debugpy into the running Flask process. In this case PID 6.
+```bash
+python3 -m debugpy --listen 0.0.0.0:5678 --pid 6
+```
+
+Verify that debugpy is listening on port 5678
+```bash
+netstat -tunap
+
+Active Internet connections (servers and established)
+Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
+tcp        0      0 0.0.0.0:5678            0.0.0.0:*               LISTEN      462/python
+tcp        0      0 0.0.0.0:8088            0.0.0.0:*               LISTEN      6/python
+```
+
+You are now ready to attach a debugger to the process. Using VSCode you can configure a launch configuration file .vscode/launch.json like so.
+```
+{
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Attach to Superset App in Docker Container",
+            "type": "python",
+            "request": "attach",
+            "connect": {
+                "host": "127.0.0.1",
+                "port": 5678
+            },
+            "pathMappings": [
+                {
+                    "localRoot": "${workspaceFolder}",
+                    "remoteRoot": "/app"
+                }
+            ]
+        },
+    ]
+}
+```
+
+VSCode will not stop on breakpoints right away. We've attached to PID 6 however it does not yet know of any sub-processes. In order to "wakeup" the debugger you need to modify a python file. This will trigger Flask to reload the code and create a new sub-process. This new sub-process will be detected by VSCode and breakpoints will be activated.
+
+
+### Debugging Server App in Kubernetes Environment
+
+To debug Flask running in POD inside kubernetes cluster. You'll need to make sure the pod runs as root and is granted the SYS_TRACE capability.These settings should not be used in production environments.
+
+```
+  securityContext:
+    capabilities:
+      add: ["SYS_PTRACE"]
+```
+
+See (set capabilities for a container)[https://kubernetes.io/docs/tasks/configure-pod-container/security-context/#set-capabilities-for-a-container] for more details.
+
+Once the pod is running as root and has the SYS_PTRACE capability it will be able to debug the Flask app.
+
+You can follow the same instructions as in the docker-compose. Enter the pod and install the required library and packages; gdb, netstat and debugpy.
+
+Often in a kuernetes environment nodes are not addressable from ouside the cluster. VSCode will thus be unable to remotely connect to port 5678 on a kubernetes node. In order to do this you need to create a tunnel that port forwards 5678 to your local machine.
+
+```
+kubectl port-forward  pod/superset-<some random id> 5678:5678
+```
+
+You can now launch your VSCode debugger with the same config as above. VSCode will connect to to 127.0.0.1:5678 which is forwarded by kubectl to your remote kubernetes POD.
+
+
 ### Storybook
 
 Superset includes a [Storybook](https://storybook.js.org/) to preview the layout/styling of various Superset components, and variations thereof. To open and view the Storybook: