Allow specifying alternative paths for reading/writing flake locks

This allows having multiple separate lockfiles for a single
project, which can be useful for testing against different versions of
nixpkgs; it also allows tracking custom input overrides for remote
flakes without requiring local clones of these flakes.

For example, if I want to build Nix against my locally pinned nixpkgs,
and have a lock file tracking this override independently of future
updates to said nixpkgs:

nix flake lock --output-lock-file /tmp/nix-flake.lock --override-input nixpkgs flake:nixpkgs
nix build --reference-lock-file /tmp/nix-flake.lock

Co-Authored-By: Will Fancher <elvishjerricco@gmail.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index 10301d8aa..b5db56312 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -117,6 +117,12 @@ struct LockFlags
     /* Whether to commit changes to flake.lock. */
     bool commitLockFile = false;
 
+    /* The path to a lock file to read instead of the `flake.lock` file in the top-level flake */
+    std::optional<std::string> referenceLockFilePath = std::nullopt;
+
+    /* The path to a lock file to write to instead of the `flake.lock` file in the top-level flake */
+    std::optional<Path> outputLockFilePath = std::nullopt;
+
     /* Flake inputs to be overridden. */
     std::map<InputPath, FlakeRef> inputOverrides;