Add a system-wide flake registry /etc/nix/registry.json

One application for this is pinning the 'nixpkgs' flake to the exact
revision used to build the NixOS system, e.g.

  {
      "flakes": [
          {
              "from": {
                  "id": "nixpkgs",
                  "type": "indirect"
              },
              "to": {
                  "owner": "NixOS",
                  "repo": "nixpkgs",
                  "type": "github",
                  "rev": "b0c285807d6a9f1b7562ec417c24fa1a30ecc31a"
              }
          }
      ],
      "version": 2
  }

src/libfetchers/registry.cc

@@ -96,6 +96,18 @@ void Registry::remove(const std::shared_ptr<const Input> & input)
             ++i;
 }
 
+static Path getSystemRegistryPath()
+{
+    return settings.nixConfDir + "/registry.json";
+}
+
+static std::shared_ptr<Registry> getSystemRegistry()
+{
+    static auto systemRegistry =
+        Registry::read(getSystemRegistryPath(), Registry::System);
+    return systemRegistry;
+}
+
 Path getUserRegistryPath()
 {
     return getHome() + "/.config/nix/registry.json";
@@ -103,7 +115,9 @@ Path getUserRegistryPath()
 
 std::shared_ptr<Registry> getUserRegistry()
 {
-    return Registry::read(getUserRegistryPath(), Registry::User);
+    static auto userRegistry =
+        Registry::read(getUserRegistryPath(), Registry::User);
+    return userRegistry;
 }
 
 static std::shared_ptr<Registry> flagRegistry =
@@ -145,6 +159,7 @@ Registries getRegistries(ref<Store> store)
     Registries registries;
     registries.push_back(getFlagRegistry());
     registries.push_back(getUserRegistry());
+    registries.push_back(getSystemRegistry());
     registries.push_back(getGlobalRegistry(store));
     return registries;
 }

src/libfetchers/registry.hh

@@ -12,7 +12,8 @@ struct Registry
     enum RegistryType {
         Flag = 0,
         User = 1,
-        Global = 2,
+        System = 2,
+        Global = 3,
     };
 
     RegistryType type;

src/nix/flake.cc

@@ -69,6 +69,7 @@ struct CmdFlakeList : EvalCommand
                 std::cout << fmt("%s %s %s\n",
                     registry->type == Registry::Flag   ? "flags " :
                     registry->type == Registry::User   ? "user  " :
+                    registry->type == Registry::System ? "system" :
                     "global",
                     std::get<0>(entry)->to_string(),
                     std::get<1>(entry)->to_string());