feat: generic registry and custom evaluation

This PR bring custom evaluation, but does not offer yet custom
activation.

Therefore, you can evaluate your systems and refer to each of them, but
you cannot ask Colmena to build them for you.

Signed-off-by: Ryan Lahfa <ryan@dgnum.eu>

src/error.rs

@@ -64,6 +64,12 @@ pub enum ColmenaError {
     #[snafu(display("Don't know how to connect to the node"))]
     NoTargetHost,
 
+    #[snafu(display(
+        "Don't know how to deploy node: {} -- does your system type support deployment?",
+        node_name
+    ))]
+    UndeployableHost { node_name: String },
+
     #[snafu(display("Node name cannot be empty"))]
     EmptyNodeName,
 

src/nix/hive/eval.nix

@@ -38,19 +38,25 @@ let
     else if uncheckedHive ? network then uncheckedHive.network
     else {};
 
+  uncheckedRegistries = if uncheckedHive ? registry then uncheckedHive.registry else {};
+
   # The final hive will always have the meta key instead of network.
   hive = let
     userMeta = (lib.modules.evalModules {
       modules = [ colmenaOptions.metaOptions uncheckedUserMeta ];
     }).config;
 
+    registry = (lib.modules.evalModules {
+      modules = [ colmenaOptions.registryOptions  { registry = uncheckedRegistries; } ];
+    }).config.registry;
+
     mergedHive =
       assert lib.assertMsg (!(uncheckedHive ? __schema)) ''
         You cannot pass in an already-evaluated Hive into the evaluator.
 
         Hint: Use the `colmenaHive` output instead of `colmena`.
       '';
-      removeAttrs (defaultHive // uncheckedHive) [ "meta" "network" ];
+      removeAttrs (defaultHive // uncheckedHive) [ "meta" "network" "registry" ];
 
     meta = {
       meta =
@@ -58,7 +64,7 @@ let
         then userMeta // { nixpkgs = <nixpkgs>; }
         else userMeta;
     };
-  in mergedHive // meta;
+  in mergedHive // meta // { inherit registry; };
 
   configsFor = node: let
     nodeConfig = hive.${node};
@@ -112,14 +118,23 @@ let
   in mkNixpkgs "meta.nixpkgs" nixpkgsConf;
 
   lib = nixpkgs.lib;
-  reservedNames = [ "defaults" "network" "meta" ];
+  reservedNames = [ "defaults" "network" "meta" "registry" ];
 
-  evalNode = name: configs: let
+  evalNode = name: configs:
+  # Some help on error messages.
+  assert (lib.assertMsg (lib.hasAttrByPath [ "deployment" "systemType" ] hive.${name})
+  "${name} does not have a deployment system type!");
+  assert (lib.assertMsg (builtins.typeOf hive.registry == "set"))
+    "The hive's registry is not a set, but of type '${builtins.typeOf hive.registry}'";
+  assert (lib.assertMsg (lib.hasAttr hive.${name}.deployment.systemType hive.registry)
+    "${builtins.toJSON (hive.${name}.deployment.systemType)} does not exist in the registry of systems!");
+  let
+    # We cannot use `configs` because we need to access to the raw configuration fragment.
+    inherit (hive.registry.${hive.${name}.deployment.systemType}) evalConfig;
     npkgs =
       if hasAttr name hive.meta.nodeNixpkgs
       then mkNixpkgs "meta.nodeNixpkgs.${name}" hive.meta.nodeNixpkgs.${name}
       else nixpkgs;
-    evalConfig = import (npkgs.path + "/nixos/lib/eval-config.nix");
 
     # Here we need to merge the configurations in meta.nixpkgs
     # and in machine config.
@@ -139,15 +154,19 @@ let
       in
         lib.optional (!hasTypedConfig && length remainingKeys != 0)
         "The following Nixpkgs configuration keys set in meta.nixpkgs will be ignored: ${toString remainingKeys}";
+      } // lib.optionalAttrs (builtins.hasAttr "localSystem" npkgs || builtins.hasAttr "crossSystem" npkgs) {
+        nixpkgs.localSystem = lib.mkBefore npkgs.localSystem;
+        nixpkgs.crossSystem = lib.mkBefore npkgs.crossSystem;
       };
   in evalConfig {
-    inherit (npkgs) system;
+    # This doesn't exist for `evalModules` the generic way.
+    # inherit (npkgs) system;
 
     modules = [
       nixpkgsModule
       colmenaModules.assertionModule
       colmenaOptions.deploymentOptions
-      hive.defaults
+      (hive.registry.${hive.${name}.deployment.systemType}.defaults or hive.defaults)
     ] ++ configs;
     specialArgs = {
       inherit name;
@@ -177,6 +196,10 @@ let
     "allowApplyAll"
   ];
 
+  serializableSystemTypeConfigKeys = [
+    "supportsDeployment"
+  ];
+
 in rec {
   # Exported attributes
   __schema = "v0.20241006";
@@ -188,5 +211,9 @@ in rec {
   evalSelected =             names: lib.filterAttrs (name: _: elem name names) toplevel;
   evalSelectedDrvPaths =     names: lib.mapAttrs    (_: v: v.drvPath)          (evalSelected names);
   metaConfig = lib.filterAttrs (n: v: elem n metaConfigKeys) hive.meta;
-  introspect = f: f { inherit lib; pkgs = nixpkgs; nodes = uncheckedNodes; };
+  # We cannot perform a `metaConfigKeys`-style simple check here
+  # because registry is arbitrarily deep and may evaluate nixpkgs indirectly.
+  registryConfig = lib.mapAttrs (systemTypeName: systemType:
+    lib.filterAttrs (n: v: elem n serializableSystemTypeConfigKeys) systemType) hive.registry;
+  introspect = f: f { inherit lib; pkgs = nixpkgs; inherit nodes; };
 }

src/nix/hive/mod.rs

@@ -16,7 +16,7 @@ use validator::Validate;
 use super::deployment::TargetNode;
 use super::{
     Flake, MetaConfig, NixExpression, NixFlags, NodeConfig, NodeFilter, NodeName,
-    ProfileDerivation, SerializedNixExpression, StorePath,
+    ProfileDerivation, RegistryConfig, SerializedNixExpression, StorePath,
 };
 use crate::error::{ColmenaError, ColmenaResult};
 use crate::job::JobHandle;
@@ -125,6 +125,8 @@ pub struct Hive {
     nix_options: HashMap<String, String>,
 
     meta_config: OnceCell<MetaConfig>,
+
+    registry_config: OnceCell<RegistryConfig>,
 }
 
 struct NixInstantiate<'hive> {
@@ -179,6 +181,7 @@ impl Hive {
             impure: false,
             nix_options: HashMap::new(),
             meta_config: OnceCell::new(),
+            registry_config: OnceCell::new(),
         })
     }
 
@@ -205,6 +208,17 @@ impl Hive {
         self.evaluation_method = method;
     }
 
+    pub async fn get_registry_config(&self) -> ColmenaResult<&RegistryConfig> {
+        self.registry_config
+            .get_or_try_init(|| async {
+                self.nix_instantiate("hive.registryConfig")
+                    .eval()
+                    .capture_json()
+                    .await
+            })
+            .await
+    }
+
     pub fn set_show_trace(&mut self, value: bool) {
         self.show_trace = value;
     }
@@ -248,6 +262,9 @@ impl Hive {
     ) -> ColmenaResult<HashMap<NodeName, TargetNode>> {
         let mut node_configs = None;
 
+        log::info!("Enumerating systems...");
+        let registry = self.get_registry_config().await?;
+
         log::info!("Enumerating nodes...");
 
         let all_nodes = self.node_names().await?;
@@ -281,6 +298,24 @@ impl Hive {
             self.deployment_info_selected(&selected_nodes).await?
         };
 
+        for node_config in &node_configs {
+            if let Some(system_type) = node_config.1.system_type.as_ref() {
+                let Some(system_config) = registry.systems.get(system_type) else {
+                    // TODO: convert me to proper error?
+                    log::warn!("'{:?}' is not a known system type in the registry, double check your expressions!", system_type);
+                    return Err(ColmenaError::Unknown {
+                        message: "unknown system type".to_string(),
+                    });
+                };
+
+                if !system_config.supports_deployment {
+                    return Err(ColmenaError::UndeployableHost {
+                        node_name: node_config.0.to_string(),
+                    });
+                }
+            }
+        }
+
         let mut targets = HashMap::new();
         let mut n_ssh = 0;
         for node in selected_nodes.into_iter() {

src/nix/hive/options.nix

@@ -96,6 +96,14 @@ with builtins; rec {
   in {
     options = {
       deployment = {
+        systemType = lib.mkOption {
+          description = mdDoc ''
+            System type used for this node, e.g. NixOS.
+          '';
+          default = "nixos";
+          # TODO: enum among all registered systems?
+          type = types.str;
+        };
         targetHost = lib.mkOption {
           description = ''
             The target SSH node for deployment.
@@ -216,6 +224,52 @@ with builtins; rec {
       };
     };
   };
+  # Options for a registered system type
+  systemTypeOptions = { name, lib, ... }: let
+    inherit (lib) types;
+    mdDoc = lib.mdDoc or lib.id;
+  in
+  {
+    options = {
+      evalConfig = lib.mkOption {
+        description = mdDoc ''
+          Evaluation function which share the same interface as `nixos/lib/eval-config.nix`
+          which can be tailored to your own usecases or to target another type of system,
+          e.g. nix-darwin.
+        '';
+        type = types.functionTo types.unspecified;
+      };
+      supportsDeployment = lib.mkOption {
+        description = mdDoc ''
+          Whether this system type supports deployment or not.
+
+          If it supports deployment, it needs to have appropriate activation code,
+          refer to how to write custom activators.
+        '';
+        default = name == "nixos";
+        defaultText = "If a NixOS system, then true, otherwise false by default";
+      };
+      defaults = lib.mkOption {
+        description = mdDoc ''
+          Default configuration for that system type.
+        '';
+        type = types.functionTo types.unspecified;
+        default = _: {};
+      };
+    };
+  };
+  registryOptions = { lib, ... }: let
+    inherit (lib) types;
+    mdDoc = lib.mdDoc or lib.id;
+  in
+  {
+    options.registry = lib.mkOption {
+      description = mdDoc ''
+        A registry of all system types.
+      '';
+      type = types.attrsOf (types.submodule systemTypeOptions);
+    };
+  };
   # Hive-wide options
   metaOptions = { lib, ... }: let
     inherit (lib) types;

src/nix/mod.rs

@@ -55,6 +55,9 @@ pub struct NodeName(#[serde(deserialize_with = "NodeName::deserialize")] String)
 
 #[derive(Debug, Clone, Validate, Deserialize)]
 pub struct NodeConfig {
+    #[serde(rename = "systemType")]
+    system_type: Option<String>,
+
     #[serde(rename = "targetHost")]
     target_host: Option<String>,
 
@@ -94,6 +97,18 @@ pub struct MetaConfig {
     pub machines_file: Option<String>,
 }
 
+#[derive(Debug, Clone, Validate, Deserialize)]
+pub struct SystemTypeConfig {
+    #[serde(rename = "supportsDeployment")]
+    pub supports_deployment: bool,
+}
+
+#[derive(Debug, Clone, Validate, Deserialize)]
+pub struct RegistryConfig {
+    #[serde(flatten)]
+    pub systems: HashMap<String, SystemTypeConfig>,
+}
+
 /// Nix CLI flags.
 #[derive(Debug, Clone, Default)]
 pub struct NixFlags {