[dns-server] use the Generation type to represent generations (#6847)

While working on reconfigurator simulation, the fact that generations were u64s
here was getting in the way. I could either work around it or simply fix it, so
here's the fix.

common/src/api/external/mod.rs

@@ -727,8 +727,15 @@ impl From<ByteCount> for i64 {
 
 /// Generation numbers stored in the database, used for optimistic concurrency
 /// control
-// Because generation numbers are stored in the database, we represent them as
-// i64.
+//
+// A generation is a value between 0 and 2**63-1, i.e. equivalent to a u63.
+// The reason is that we store it as an i64 in the database, and we want to
+// disallow negative values. (We could potentially use two's complement to
+// store values greater than that as negative values, but surely 2**63 is
+// enough.)
+//
+// TODO: This allows deserialization into a value that's out of range. That's
+// not correct. See <https://github.com/oxidecomputer/omicron/issues/6865>.
 #[derive(
     Copy,
     Clone,

dev-tools/reconfigurator-cli/src/main.rs

@@ -1392,15 +1392,15 @@ fn cmd_load_example(
     sim.internal_dns.insert(
         blueprint.internal_dns_version,
         DnsConfigParams {
-            generation: blueprint.internal_dns_version.into(),
+            generation: blueprint.internal_dns_version,
             time_created: Utc::now(),
             zones: vec![internal_dns],
         },
     );
     sim.external_dns.insert(
         blueprint.external_dns_version,
         DnsConfigParams {
-            generation: blueprint.external_dns_version.into(),
+            generation: blueprint.external_dns_version,
             time_created: Utc::now(),
             zones: vec![external_dns],
         },

dns-server/Cargo.toml

@@ -21,6 +21,7 @@ hickory-resolver.workspace = true
 hickory-server.workspace = true
 http.workspace = true
 internal-dns-types.workspace = true
+omicron-common.workspace = true
 pretty-hex.workspace = true
 schemars.workspace = true
 serde.workspace = true

dns-server/src/bin/dnsadm.rs

@@ -220,7 +220,7 @@ async fn main() -> Result<()> {
                 .collect();
 
             let new_config = DnsConfigParams {
-                generation: old_config.generation + 1,
+                generation: old_config.generation.next(),
                 time_created: chrono::Utc::now(),
                 zones,
             };
@@ -275,7 +275,7 @@ fn add_record(
     our_kv.1.push(record);
 
     Ok(DnsConfigParams {
-        generation: generation + 1,
+        generation: generation.next(),
         time_created: chrono::Utc::now(),
         zones: other_zones
             .into_iter()