Comment 0 for bug 2066985

[ Impact ]

Cloud-init recently added policy-based routing for netplan-only systems on EC2. In order to gate the netplan-specific code, it checked to see in the netplan activator was being used. However, if the datasource is fetched in init-local timeframe (such as on EC2), it is possible to specify a different renderer without also changing the activator. This means the netplan-gating no longer works, even when rendering to a non-netplan system.

The fix is to instead check if we're using the netplan renderer before rendering the netplan-specific code.

[ Test Plan ]

Launch an Ubuntu instance on EC2 having two NICs attached.
Run "apt update; apt install ifupdown"
In /etc/cloud/cloud.cfg, update the `renderers` line to start with `eni` rather than `netplan`
`cloud-init clean --logs --reboot`
Connect to instance
Verify no networking related tracebacks exist in logs
Verify /etc/network/interfaces.d/ contains valid networking information for both NICs.

Run tests/integration_tests/modules/test_hotplug.py and
tests/integration_tests/test_networking.py
to ensure no regression to existing multinic rendering behavior.

[ Where problems could occur ]

If the renderer is still somehow misdetecting a netplan or non-netplan based system, we could still attempt to render netplan config where we shouldn't be, or instead skip rendering the config where we shouldn't be.

[ Other Info ]

Upstream bug:
Upstream fix: https://github.com/canonical/cloud-init/pull/5321