[nexus] instance create: make hostname param optional

[david-crespo]

When hostname is None in the create request body, use the instance name.

Related to #68. There's pretty extensive discussion on there, but it's over two years old — what are hostnames for, could we just get rid of the param entirely, etc. But the change I've made here is a superficial one that we could make right away without much downside that I can see. I was going to make this change on the CLI layer, but then I realized it could just be done here and the CLI would get it for free.

---

Note that in #4938 we started validating hostname:

omicron/common/src/api/external/mod.rs

Lines 822 to 872 in 2dcf896

	/// An RFC-1035-compliant hostname.
	#[derive(
	Clone, Debug, Deserialize, Display, Eq, PartialEq, SerializeDisplay,
	)]
	#[display("{0}")]
	#[serde(try_from = "String", into = "String")]
	pub struct Hostname(String);
	impl Hostname {
	/// Return the hostname as a string slice.
	pub fn as_str(&self) -> &str {
	self.0.as_str()
	}
	}
	// Regular expression for hostnames.
	//
	// Each name is a dot-separated sequence of labels. Each label is supposed to
	// be an "LDH": letter, dash, or hyphen. Hostnames can consist of one label, or
	// many, separated by a `.`. While _domain_ names are allowed to end in a `.`,
	// making them fully-qualified, hostnames are not.
	//
	// Note that labels are allowed to contain a hyphen, but may not start or end
	// with one. See RFC 952, "Lexical grammar" section.
	//
	// Note that we need to use a regex engine capable of lookbehind to support
	// this, since we need to check that labels don't end with a `-`.
	const HOSTNAME_REGEX: &str = r#"^([a-zA-Z0-9]+[a-zA-Z0-9\-]*(?<!-))(\.[a-zA-Z0-9]+[a-zA-Z0-9\-]*(?<!-))*$"#;
	// Labels need to be encoded on the wire, and prefixed with a signel length
	// octet. They also need to end with a length octet of 0 when encoded. So the
	// longest name is a single label of 253 characters, which will be encoded as
	// `\xfd<the label>\x00`.
	const HOSTNAME_MAX_LEN: u32 = 253;
	impl FromStr for Hostname {
	type Err = anyhow::Error;
	fn from_str(s: &str) -> Result<Self, Self::Err> {
	anyhow::ensure!(
	s.len() <= HOSTNAME_MAX_LEN as usize,
	"Max hostname length is {HOSTNAME_MAX_LEN}"
	);
	let re = regress::Regex::new(HOSTNAME_REGEX).unwrap();
	if re.find(s).is_some() {
	Ok(Hostname(s.to_string()))
	} else {
	anyhow::bail!("Hostnames must comply with RFC 1035")
	}
	}
	}

but it doesn't follow the same rules as Name. It seems to be a little broader: allowing up to 253 characters, not disallowing UUIDs, allowing .. So this can work as long as we are confident that any valid name is also a valid hostname. It seems true, but I am planning on adding some tests to help ensure it.

[david-crespo]

There aren't that many examples in the list. It would be cool to do a proptest type thing here but I didn't see any tests like that in omicron.

[ahl]

This looks good and I like the idea, but I don't know that I'm the best person to confirm the validity of this approach. Perhaps @davepacheco or @smklein can validate the concept?