nixos/flake: options to set up registry and nixPath for flake configs

Currently there are a bunch of really wacky hacks required to get nixpkgs path correctly set up under flake configs such that `nix run nixpkgs#hello` and `nix run -f '<nixpkgs>' hello` hit the nixpkgs that the system was built with. In particular you have to use specialArgs or an anonymous module, and everyone has to include this hack in their own configs. We can do this for users automatically. I have tested these manually with a basic config; I don't know if it is even possible to write a nixos test for it since you can't really get a string-with-context to yourself unless you are in a flake context.
NixOS · Oct 1, 2023 · 3120b3e · 3120b3e
1 parent 366e586
commit 3120b3e
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diff --git a/flake.nix b/flake.nix
@@ -25,6 +25,7 @@
                 system.nixos.versionSuffix =
                   ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
                 system.nixos.revision = final.mkIf (self ? rev) self.rev;
+                nixpkgs.flake.source = final.mkDefault (toString self);
               }];
             } // lib.optionalAttrs (! args?system) {
               # Allow system to be set modularly in nixpkgs.system.

diff --git a/nixos/doc/manual/release-notes/rl-2311.section.md b/nixos/doc/manual/release-notes/rl-2311.section.md
@@ -8,6 +8,12 @@
 
 - LXD now supports virtual machine instances to complement the existing container support
 
+- NIX_PATH and the system flake registry will be automatically set up for
+  flake-based NixOS configurations if
+  [nixpkgs.flake.setNixPath](#opt-nixpkgs.flake.setNixPath) and
+  [nixpkgs.flake.setFlakeRegistry](#opt-nixpkgs.flake.setFlakeRegistry) are
+  set, respectively. These will be made true by default in a future release.
+
 ## New Services {#sec-release-23.11-new-services}
 
 - [MCHPRS](https://github.com/MCHPR/MCHPRS), a multithreaded Minecraft server built for redstone. Available as [services.mchprs](#opt-services.mchprs.enable).

diff --git a/nixos/modules/config/nix.nix b/nixos/modules/config/nix.nix
@@ -332,6 +332,16 @@ in
                 always allowed to connect.
               '';
             };
+
+            flake-registry = mkOption {
+              type = types.str;
+              default = "https://channels.nixos.org/flake-registry.json";
+              description = lib.mdDoc ''
+                Path or URI of the global flake registry.
+
+                When empty, disables the global flake registry.
+              '';
+            };
           };
         };
         default = { };

diff --git a/nixos/modules/misc/nixpkgs.nix b/nixos/modules/misc/nixpkgs.nix
@@ -328,64 +328,122 @@ in
         Ignored when `nixpkgs.pkgs`, `nixpkgs.localSystem` or `nixpkgs.hostPlatform` is set.
       '';
     };
-  };
 
-  config = {
-    _module.args = {
-      pkgs =
-        # We explicitly set the default override priority, so that we do not need
-        # to evaluate finalPkgs in case an override is placed on `_module.args.pkgs`.
-        # After all, to determine a definition priority, we need to evaluate `._type`,
-        # which is somewhat costly for Nixpkgs. With an explicit priority, we only
-        # evaluate the wrapper to find out that the priority is lower, and then we
-        # don't need to evaluate `finalPkgs`.
-        lib.mkOverride lib.modules.defaultOverridePriority
-          finalPkgs.__splicedPackages;
-    };
+    flake = {
+      source = mkOption {
+        type = types.nullOr types.str;
 
-    assertions = let
-      # Whether `pkgs` was constructed by this module. This is false when any of
-      # nixpkgs.pkgs or _module.args.pkgs is set.
-      constructedByMe =
-        # We set it with default priority and it can not be merged, so if the
-        # pkgs module argument has that priority, it's from us.
-        (lib.modules.mergeAttrDefinitionsWithPrio options._module.args).pkgs.highestPrio
-          == lib.modules.defaultOverridePriority
-        # Although, if nixpkgs.pkgs is set, we did forward it, but we did not construct it.
-          && !opt.pkgs.isDefined;
-    in [
-      (
-        let
-          nixosExpectedSystem =
-            if config.nixpkgs.crossSystem != null
-            then config.nixpkgs.crossSystem.system or (lib.systems.parse.doubleFromSystem (lib.systems.parse.mkSystemFromString config.nixpkgs.crossSystem.config))
-            else config.nixpkgs.localSystem.system or (lib.systems.parse.doubleFromSystem (lib.systems.parse.mkSystemFromString config.nixpkgs.localSystem.config));
-          nixosOption =
-            if config.nixpkgs.crossSystem != null
-            then "nixpkgs.crossSystem"
-            else "nixpkgs.localSystem";
-          pkgsSystem = finalPkgs.stdenv.targetPlatform.system;
-        in {
-          assertion = constructedByMe -> !hasPlatform -> nixosExpectedSystem == pkgsSystem;
-          message = "The NixOS nixpkgs.pkgs option was set to a Nixpkgs invocation that compiles to target system ${pkgsSystem} but NixOS was configured for system ${nixosExpectedSystem} via NixOS option ${nixosOption}. The NixOS system settings must match the Nixpkgs target system.";
-        }
-      )
-      {
-        assertion = constructedByMe -> hasPlatform -> legacyOptionsDefined == [];
-        message = ''
-          Your system configures nixpkgs with the platform parameter${optionalString hasBuildPlatform "s"}:
-          ${hostPlatformLine
-          }${buildPlatformLine
-          }
-          However, it also defines the legacy options:
-          ${concatMapStrings showOptionWithDefLocs legacyOptionsDefined}
-          For a future proof system configuration, we recommend to remove
-          the legacy definitions.
+        default = null;
+
+        description = mdDoc ''
+          The derivation to the nixpkgs sources, as a string with context. This
+          is set up by the nixpkgs flake.
+        '';
+      };
+
+      setNixPath = mkOption {
+        type = types.bool;
+
+        default = false;
+
+        description = mdDoc ''
+          Whether to create /etc/nix/inputs/nixpkgs as a symlink and set
+          NIX_PATH to include /etc/nix/inputs/nixpkgs.
+
+          This will be true by default for flake-based configurations in a
+          future release.
+        '';
+      };
+
+      setFlakeRegistry = mkOption {
+        type = types.bool;
+
+        default = false;
+
+        description = mdDoc ''
+          Whether to set nixpkgs in the local flake registry and disable the
+          external flake registry.
+
+          This will be true by default for flake-based configurations in a
+          future release.
         '';
-      }
-    ];
+      };
+    };
   };
 
+  config = mkMerge [
+    {
+      _module.args = {
+        pkgs =
+          # We explicitly set the default override priority, so that we do not need
+          # to evaluate finalPkgs in case an override is placed on `_module.args.pkgs`.
+          # After all, to determine a definition priority, we need to evaluate `._type`,
+          # which is somewhat costly for Nixpkgs. With an explicit priority, we only
+          # evaluate the wrapper to find out that the priority is lower, and then we
+          # don't need to evaluate `finalPkgs`.
+          lib.mkOverride lib.modules.defaultOverridePriority
+            finalPkgs.__splicedPackages;
+      };
+
+      assertions = let
+        # Whether `pkgs` was constructed by this module. This is false when any of
+        # nixpkgs.pkgs or _module.args.pkgs is set.
+        constructedByMe =
+          # We set it with default priority and it can not be merged, so if the
+          # pkgs module argument has that priority, it's from us.
+          (lib.modules.mergeAttrDefinitionsWithPrio options._module.args).pkgs.highestPrio
+            == lib.modules.defaultOverridePriority
+          # Although, if nixpkgs.pkgs is set, we did forward it, but we did not construct it.
+            && !opt.pkgs.isDefined;
+      in [
+        (
+          let
+            nixosExpectedSystem =
+              if config.nixpkgs.crossSystem != null
+              then config.nixpkgs.crossSystem.system or (lib.systems.parse.doubleFromSystem (lib.systems.parse.mkSystemFromString config.nixpkgs.crossSystem.config))
+              else config.nixpkgs.localSystem.system or (lib.systems.parse.doubleFromSystem (lib.systems.parse.mkSystemFromString config.nixpkgs.localSystem.config));
+            nixosOption =
+              if config.nixpkgs.crossSystem != null
+              then "nixpkgs.crossSystem"
+              else "nixpkgs.localSystem";
+            pkgsSystem = finalPkgs.stdenv.targetPlatform.system;
+          in {
+            assertion = constructedByMe -> !hasPlatform -> nixosExpectedSystem == pkgsSystem;
+            message = "The NixOS nixpkgs.pkgs option was set to a Nixpkgs invocation that compiles to target system ${pkgsSystem} but NixOS was configured for system ${nixosExpectedSystem} via NixOS option ${nixosOption}. The NixOS system settings must match the Nixpkgs target system.";
+          }
+        )
+        {
+          assertion = constructedByMe -> hasPlatform -> legacyOptionsDefined == [];
+          message = ''
+            Your system configures nixpkgs with the platform parameter${optionalString hasBuildPlatform "s"}:
+            ${hostPlatformLine
+            }${buildPlatformLine
+            }
+            However, it also defines the legacy options:
+            ${concatMapStrings showOptionWithDefLocs legacyOptionsDefined}
+            For a future proof system configuration, we recommend to remove
+            the legacy definitions.
+          '';
+        }
+      ];
+    }
+    (mkIf (cfg.flake.source != null) (mkMerge [
+      (mkIf cfg.flake.setNixPath {
+        environment.etc."nix/inputs/nixpkgs".source = mkDefault cfg.flake.source;
+        nix.nixPath = mkDefault [ "nixpkgs=/etc/nix/inputs/nixpkgs" ];
+      })
+      (mkIf cfg.flake.setFlakeRegistry {
+        nix.registry.nixpkgs.to = mkDefault {
+          type = "path";
+          path = cfg.flake.source;
+        };
+        # This is set due to a Nix bug where the registry json is fetched on
+        # every `nix` invocation even if the requested flake is entirely local.
+        nix.settings.flake-registry = mkDefault "";
+      })
+    ]))
+  ];
+
   # needs a full nixpkgs path to import nixpkgs
   meta.buildDocsInSandbox = false;
 }