harmony/opnsense-config/README.md

## Supporting a new field in OPNSense `config.xml`

Two steps:
- Supporting the field in `opnsense-config-xml`
- Enabling Harmony to control the field

We'll use the `filename` field in the `dhcpcd` section of the file as an example.

### Supporting the field

As type checking if enforced, every field from `config.xml` must be known by the code. Each subsection of `config.xml` has its `.rs` file. For the `dhcpcd` section, we'll modify `opnsense-config-xml/src/data/dhcpd.rs`.

When a new field appears in the xml file, an error like this will be thrown and Harmony will panic :
```
     Running `/home/stremblay/nt/dir/harmony/target/debug/example-nanodc`
Found unauthorized element filename
thread 'main' panicked at opnsense-config-xml/src/data/opnsense.rs:54:14:
OPNSense received invalid string, should be full XML: ()

```

Define the missing field (`filename`) in the `DhcpInterface` struct of `opnsense-config-xml/src/data/dhcpd.rs`:

```rust
pub struct DhcpInterface {
    ...
    pub filename: Option<String>,
```

Harmony should now be fixed, build and run.

### Controlling the field

Define the `xml field setter` in `opnsense-config/src/modules/dhcpd.rs`.

```rust
impl<'a> DhcpConfig<'a> {
    ...
    pub fn set_filename(&mut self, filename: &str) {
        self.enable_netboot();
        self.get_lan_dhcpd().filename = Some(filename.to_string());
    }
    ...
```

Define the `value setter` in the `DhcpServer trait`  in `domain/topology/network.rs`

```rust
#[async_trait]
pub trait DhcpServer: Send + Sync {
    ...
    async fn set_filename(&self, filename: &str) -> Result<(), ExecutorError>;
    ...
```

Implement the `value setter` in each `DhcpServer` implementation.
`infra/opnsense/dhcp.rs`:

```rust
#[async_trait]
impl DhcpServer for OPNSenseFirewall {
    ...
    async fn set_filename(&self, filename: &str) -> Result<(), ExecutorError> {
        {
            let mut writable_opnsense = self.opnsense_config.write().await;
            writable_opnsense.dhcp().set_filename(filename);
            debug!("OPNsense dhcp server set filename {filename}");
        }

        Ok(())
    }
    ...
```

`domain/topology/ha_cluster.rs`
```rust
#[async_trait]
impl DhcpServer for DummyInfra {
    ...
    async fn set_filename(&self, _filename: &str) -> Result<(), ExecutorError> {
        unimplemented!("{}", UNIMPLEMENTED_DUMMY_INFRA)
    }
    ...
```

Add the new field to the DhcpScore in `modules/dhcp.rs`

```rust
pub struct DhcpScore {
    ...
    pub filename: Option<String>,
```

Define it in its implementation in `modules/okd/dhcp.rs`

```rust
impl OKDDhcpScore {
        ...
        Self {
            dhcp_score: DhcpScore {
                ...
                filename: Some("undionly.kpxe".to_string()),
```

Define it in its implementation in `modules/okd/bootstrap_dhcp.rs`

```rust
impl OKDDhcpScore {
        ...
        Self {
            dhcp_score: DhcpScore::new(
                ...
                Some("undionly.kpxe".to_string()),
```

Update the interpret (function called by the `execute` fn of the interpret) so it now updates the `filename` field value in `modules/dhcp.rs`

```rust
impl DhcpInterpret {
        ...
        let filename_outcome = match &self.score.filename {
            Some(filename) => {
                let dhcp_server = Arc::new(topology.dhcp_server.clone());
                dhcp_server.set_filename(&filename).await?;
                Outcome::new(
                    InterpretStatus::SUCCESS,
                    format!("Dhcp Interpret Set filename to {filename}"),
                )
            }
            None => Outcome::noop(),
        };

        if next_server_outcome.status == InterpretStatus::NOOP
            && boot_filename_outcome.status == InterpretStatus::NOOP
            && filename_outcome.status == InterpretStatus::NOOP

            ...

            Ok(Outcome::new(
            InterpretStatus::SUCCESS,
            format!(
                "Dhcp Interpret Set next boot to [{:?}], boot_filename to [{:?}], filename to [{:?}]",
                self.score.boot_filename, self.score.boot_filename, self.score.filename
            )
            ...
```