Application droplet can be started with a different stack than the buildpack used to produce the droplet supports

Issue

Using the CF APIs it is possible to launch an application droplet with a CF stack which is different than the stack supported by the buildpack used to produce the droplet.

Steps to Reproduce

Prerequisite is a CF offering two different stacks. E.g. cflinuxfs3 and cflinuxfs4. For the steps below we want to switch an application from the cflinuxfs3 to cflinuxfs4 stack.

1. Get the app latest package guid with: cf curl "/v3/packages?app_guids=<app-guid>&order_by=-created_at&states=READY"
2. Create a new droplet with the cflinuxfs4 stack and the corresponding buildpack supporting the cflinuxfs4 stack:

  curl curl "/v3/builds" \
      -X POST \
      -H "Content-type: application/json" \
      -d '{
        "package": {
          "guid": "[package-guid]"
         },
         lifecycle: {
           type: "buildpack",
           data: { buildpacks: [<cflinuxfs4-buildpack>], stack: cflinuxfs4}
        }
      }'

3. Wait until the build is done and get the new droplet guid with: cf curl "/v3/builds/<build-guid>"
4. Stop the app with: cf curl "/v3/apps/<guid>/actions/stop" -X POST"
5. Set the app's current droplet:

cf curl "/v3/apps/[guid]/relationships/current_droplet" \
  -X PATCH \
  -H "Content-type: application/json" \
  -d '{ "data": { "guid": "[<droplet_guid>]" } }'

6. Restart the app: cf curl "/v3/apps/[guid]/actions/restart" -X POST"

Expected result

CF restart request fails because the current stack of the application configured in its lifecycle is cflinuxfs3 but the current droplet is built for the cflinuxfs4 stack.

Current result

The application is restarted with its configured stack which is cflinuxfs3 and e.g. node apps are crashing with strange errors like:

 [APP/PROC/WEB/0] STDERR node: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by node)
 [APP/PROC/WEB/0] STDERR node: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by node)

This is happening because the droplet has dependencies to the CF stack and they can't be resolved.

Possible Fix

Possible fix could be to reject such a start/restart request when the droplet stack is different than the app lifecycle stack. Such an error is happening when you try to push an app with a buildpack which doesn't support the current lifecycle stack of the app:

 [STG/0] OUT **ERROR** Stack not supported by buildpack: required stack cflinuxfs3 was not found
 [STG/0] ERR Failed to compile droplet: Failed to run all supply scripts: exit status 11

Trace the v3 builds flow, the app current_droplet relationship, and the app restart action described in the reproduction steps. Check how the app lifecycle stack and droplet stack are compared, then identify the relevant regression-test entry point. Done means a mismatched stack is rejected before the app starts, with coverage for the API path.