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>

src/libcmd/installables.cc

@@ -102,6 +102,28 @@ MixFlakeOptions::MixFlakeOptions()
         }}
     });
 
+    addFlag({
+        .longName = "reference-lock-file",
+        .description = "Read the given lock file instead of `flake.lock` within the top-level flake",
+        .category = category,
+        .labels = {"flake-lock-path"},
+        .handler = {[&](std::string lockFilePath) {
+            lockFlags.referenceLockFilePath = lockFilePath;
+        }},
+        .completer = completePath
+    });
+
+    addFlag({
+        .longName = "output-lock-file",
+        .description = "Write the given lock file instead of `flake.lock` within the top-level flake",
+        .category = category,
+        .labels = {"flake-lock-path"},
+        .handler = {[&](std::string lockFilePath) {
+            lockFlags.outputLockFilePath = lockFilePath;
+        }},
+        .completer = completePath
+    });
+
     addFlag({
         .longName = "inputs-from",
         .description = "Use the inputs of the specified flake as registry entries.",

src/libexpr/flake/flake.cc

@@ -337,7 +337,8 @@ LockedFlake lockFlake(
 
         // FIXME: symlink attack
         auto oldLockFile = LockFile::read(
-            flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir + "/flake.lock");
+            lockFlags.referenceLockFilePath.value_or(
+                flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir + "/flake.lock"));
 
         debug("old lock file: %s", oldLockFile);
 
@@ -619,13 +620,20 @@ LockedFlake lockFlake(
 
         debug("new lock file: %s", newLockFile);
 
+        auto relPath = (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock";
+        auto sourcePath = topRef.input.getSourcePath();
+        auto outputLockFilePath = sourcePath ? std::optional{*sourcePath + "/" + relPath} : std::nullopt;
+        if (lockFlags.outputLockFilePath) {
+            outputLockFilePath = lockFlags.outputLockFilePath;
+        }
+
         /* Check whether we need to / can write the new lock file. */
-        if (!(newLockFile == oldLockFile)) {
+        if (newLockFile != oldLockFile || lockFlags.outputLockFilePath) {
 
             auto diff = LockFile::diff(oldLockFile, newLockFile);
 
             if (lockFlags.writeLockFile) {
-                if (auto sourcePath = topRef.input.getSourcePath()) {
+                if (outputLockFilePath) {
                     if (auto unlockedInput = newLockFile.isUnlocked()) {
                         if (fetchSettings.warnDirty)
                             warn("will not write lock file of flake '%s' because it has an unlocked input ('%s')", topRef, *unlockedInput);
@@ -633,25 +641,24 @@ LockedFlake lockFlake(
                         if (!lockFlags.updateLockFile)
                             throw Error("flake '%s' requires lock file changes but they're not allowed due to '--no-update-lock-file'", topRef);
 
-                        auto relPath = (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock";
+                        bool lockFileExists = pathExists(*outputLockFilePath);
-
-                        auto path = *sourcePath + "/" + relPath;
-
-                        bool lockFileExists = pathExists(path);
 
                         if (lockFileExists) {
                             auto s = chomp(diff);
                             if (s.empty())
-                                warn("updating lock file '%s'", path);
+                                warn("updating lock file '%s'", *outputLockFilePath);
                             else
-                                warn("updating lock file '%s':\n%s", path, s);
+                                warn("updating lock file '%s':\n%s", *outputLockFilePath, s);
                         } else
-                            warn("creating lock file '%s'", path);
+                            warn("creating lock file '%s'", *outputLockFilePath);
 
-                        newLockFile.write(path);
+                        newLockFile.write(*outputLockFilePath);
 
                         std::optional<std::string> commitMessage = std::nullopt;
                         if (lockFlags.commitLockFile) {
+                            if (lockFlags.outputLockFilePath) {
+                                throw Error("--commit-lock-file and --output-lock-file are currently incompatible");
+                            }
                             std::string cm;
 
                             cm = fetchSettings.commitLockFileSummary.get();

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;