Proposal: Support resizing multiple filesystems in the resizefs module

[jinkkkang]

Cloud-init currently has two modules related to partition and filesystem expansion:

• growpart expands partitions and supports multiple entries through growpart.devices.
• resizefs expands the filesystem, but it is controlled only by resize_rootfs, so it can resize only the root filesystem.

For example:

growpart:
  mode: auto
  devices: ["/", "/data"]

resize_rootfs: true

In this configuration, growpart expands the partitions backing both / and /data, but resizefs expands only the filesystem mounted at /. The filesystem mounted at /data remains unchanged, so users must resize it separately.

I would like to propose adding support for multiple filesystems to the resizefs module, aligned with the devices configuration supported by growpart.

One possible configuration could be:

growpart:
  mode: auto
  devices: ["/", "/data"]

resizefs:
  devices: ["/", "/data"]

The entries could use the same format and resolution rules as growpart.devices, such as mount points or device paths.

For backward compatibility, resize_rootfs should continue to work as it does today. If the new configuration is not specified, the current behavior would remain unchanged. We would also need to define the behavior when both resize_rootfs and the new configuration are present.

[jinkkkang]

@holmanb @blackboxsw Can you help me look at this idea? I haven't figured out how to design a good configuration file that is compatible with the previous resize_rootfs configuration，Do you have any good ideas