lib: mkOfflineFlakeLock init

copied from the clan-core-flake module

I plan to re-use this function to improve the clan test framework to allow cli execution

lib/default.nix

@@ -51,6 +51,11 @@ lib.fix (clanLib: {
   jsonschema = import ./jsonschema { inherit lib; };
   facts = import ./facts.nix { inherit lib; };
 
+  # flakes
+  flakes = clanLib.callLib ./flakes.nix {
+    clan-core = self;
+  };
+
   # deprecated
   # remove when https://git.clan.lol/clan/clan-core/pulls/3212 is implemented
   inherit (self.inputs.nix-select.lib) select;

lib/flakes.nix

@@ -0,0 +1,40 @@
+{
+  lib,
+  clan-core,
+  ...
+}:
+{
+  # A flake lock for offline use.
+  # All flake inputs are locked to an existing store path
+  mkOfflineFlakeLock =
+    flakePath:
+    let
+      flakeLock = lib.importJSON (flakePath + "/flake.lock");
+      flakeInputs = builtins.removeAttrs clan-core.inputs [ "self" ];
+      inputsToPaths =
+        flakeLock:
+        flakeLock
+        // {
+          nodes =
+            flakeLock.nodes
+            // (lib.flip lib.mapAttrs flakeInputs (
+              name: _:
+              # remove follows and let 'nix flake lock' re-compute it later
+              # (lib.removeAttrs flakeLock.nodes.${name} ["inputs"])
+              flakeLock.nodes.${name}
+              // {
+                locked = {
+                  inherit (flakeLock.nodes.${name}.locked) narHash;
+                  lastModified =
+                    # lol, nixpkgs has a different timestamp on the fs???
+                    if name == "nixpkgs" then 0 else 1;
+                  path = "${clan-core.inputs.${name}}";
+                  type = "path";
+                };
+              }
+            ));
+        };
+      lockAttrs = inputsToPaths flakeLock;
+    in
+    builtins.toFile "offline-flake.lock" (builtins.toJSON lockAttrs);
+}