openSUSE / agama / 8634306511

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Fixed setting unlimited maximum partition size (#1142)

## Problem

- https://github.com/openSUSE/agama/issues/1065
- The maximum partition size cannot be set to "unlimited", using empty
value results in the default maximum size

**Example:** Changing the default 2GB max size is not possible, it is
always restored back:

![agama_max_size_broken](https://github.com/openSUSE/agama/assets/907998/04bb3f90-dbff-4a7e-99c2-32045162d64d)

## Details

- The problem is that the "unlimited" value is represented using the
`undefined` value in the web UI.
- But `undefined` cannot be sent over D-Bus because D-Bus does not have
concept for `undefined`, `nil` or `NULL` values, Iin that case the
`MaxSize` is simply missing the sent D-Bus data.
- When the D-Bus service creates a `Volume` object it first initializes
it with the default data from the YAML product configuration file. Then
it updates the attributes using the D-Bus data. Obviously when a value
is missing in the D-Bus then it keeps the default value from the config.
And that's the problem.

## Solution

- Set the "unlimited" value when the `MaxSize` attribute is missing in
the D-Bus data.

## Questions

- The fix has a drawback, the current code allows sending only partial
data set and ensures the defaults from the config file are used for
missing values.
- This won't work any more for the maximum size as a missing value won't
be considered as a "use the default" but as "use the unlimited size".
- Can this have some bad consequences?
- The code works, but is it safe in all scenarios?

## Alternative Solution

- If this fix is not correct then we should probably introduce a new key
in the D-Bus data, something like `MaxSizeUnlimited` with `true` /
`false` values which can be transferred via D-Bus...

## Testing

- Tested manually

Now it is possible to unset  the maximum size:

![agama_max_size_fixed](https://github.com/openSUSE/agama/assets/907998/067c8108-7... (continued)

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