Adds more generic item-allocation queries (#1055)

* Adds more generic item-allocation queries

- Creates the `omicron_nexus::db::queries` submodule, for custom /
  complex queries, organized around specific resources
- Adds a general-purpose `NextItem` query, which looks for the first
  available item in a database column, with a defined starting point and
  maximum offset / scan size. This can be used to implement a linear
  search over a defined space. One of the benefits is that for
  randomly-selected items, we can avoid a retry loop, by instead
  starting from the random base item and inserting the first successful
  value. There are several places where we use a similar query, or use a
  retry loop that could be replaced by this query.
- Use the `NextGuestIpv4Address` query when creating NICs.
- Use the `NextNicSlot` query when creating guest NICs

* Review feedback

Author: bnaecker

nexus/src/app/instance.rs

@@ -13,7 +13,7 @@ use crate::db;
 use crate::db::identity::Resource;
 use crate::db::lookup::LookupPath;
 use crate::db::model::Name;
-use crate::db::subnet_allocation::NetworkInterfaceError;
+use crate::db::queries::network_interface::NetworkInterfaceError;
 use crate::external_api::params;
 use omicron_common::api::external;
 use omicron_common::api::external::CreateResult;

nexus/src/app/sagas/instance_create.rs

@@ -9,6 +9,7 @@ use crate::app::{MAX_DISKS_PER_INSTANCE, MAX_NICS_PER_INSTANCE};
 use crate::context::OpContext;
 use crate::db::identity::Resource;
 use crate::db::lookup::LookupPath;
+use crate::db::queries::network_interface::NetworkInterfaceError;
 use crate::external_api::params;
 use crate::saga_interface::SagaContext;
 use crate::{authn, authz, db};
@@ -331,7 +332,6 @@ async fn sic_create_custom_network_interfaces(
             )
             .await;
 
-        use crate::db::subnet_allocation::NetworkInterfaceError;
         match result {
             Ok(_) => Ok(()),
 
@@ -432,7 +432,7 @@ async fn sic_create_default_network_interface(
             interface,
         )
         .await
-        .map_err(db::subnet_allocation::NetworkInterfaceError::into_external)
+        .map_err(NetworkInterfaceError::into_external)
         .map_err(ActionError::action_failed)?;
     Ok(())
 }

nexus/src/app/vpc.rs

@@ -10,7 +10,7 @@ use crate::db;
 use crate::db::lookup::LookupPath;
 use crate::db::model::Name;
 use crate::db::model::VpcRouterKind;
-use crate::db::subnet_allocation::SubnetError;
+use crate::db::queries::vpc_subnet::SubnetError;
 use crate::defaults;
 use crate::external_api::params;
 use omicron_common::api::external;

nexus/src/app/vpc_subnet.rs

@@ -11,7 +11,7 @@ use crate::db::identity::Resource;
 use crate::db::lookup::LookupPath;
 use crate::db::model::Name;
 use crate::db::model::VpcSubnet;
-use crate::db::subnet_allocation::SubnetError;
+use crate::db::queries::vpc_subnet::SubnetError;
 use crate::defaults;
 use crate::external_api::params;
 use omicron_common::api::external;

nexus/src/db/datastore.rs

@@ -33,6 +33,10 @@ use crate::db::fixed_data::role_builtin::BUILTIN_ROLES;
 use crate::db::fixed_data::silo::{DEFAULT_SILO, SILO_ID};
 use crate::db::lookup::LookupPath;
 use crate::db::model::DatabaseString;
+use crate::db::queries::network_interface::InsertNetworkInterfaceQuery;
+use crate::db::queries::network_interface::NetworkInterfaceError;
+use crate::db::queries::vpc_subnet::FilterConflictingVpcSubnetRangesQuery;
+use crate::db::queries::vpc_subnet::SubnetError;
 use crate::db::{
     self,
     error::{public_error_from_diesel_pool, ErrorHandler, TransactionError},
@@ -49,10 +53,6 @@ use crate::db::{
     },
     pagination::paginated,
     pagination::paginated_multicolumn,
-    subnet_allocation::FilterConflictingVpcSubnetRangesQuery,
-    subnet_allocation::InsertNetworkInterfaceQuery,
-    subnet_allocation::NetworkInterfaceError,
-    subnet_allocation::SubnetError,
     update_and_check::{UpdateAndCheck, UpdateStatus},
 };
 use crate::external_api::{params, shared};

nexus/src/db/mod.rs

@@ -18,10 +18,12 @@ pub mod ipv6;
 pub mod lookup;
 mod pagination;
 mod pool;
+// This is marked public because the error types are used elsewhere, e.g., in
+// sagas.
+pub(crate) mod queries;
 mod saga_recovery;
 mod saga_types;
 mod sec_store;
-pub(crate) mod subnet_allocation;
 mod update_and_check;
 
 #[cfg(test)]

nexus/src/db/queries/mod.rs

@@ -0,0 +1,12 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+//! Specialized queries for inserting database records, usually to maintain
+//! complex invariants that are most accurately expressed in a single query.
+
+#[macro_use]
+mod next_item;
+pub mod network_interface;
+pub mod vni;
+pub mod vpc_subnet;