4 files changed, 29 insertions, 11 deletions
diff --git a/src/libexpr/common-eval-args.cc b/src/libexpr/common-eval-args.cc
index 3e5400052..8e94d358e 100644
--- a/src/libexpr/common-eval-args.cc
+++ b/src/libexpr/common-eval-args.cc
@@ -33,6 +33,15 @@ MixEvalArgs::MixEvalArgs()
         .handler([&](std::vector<std::string> ss) {
             evalSettings.pureEval = false;
         });
+
+    mkFlag()
+      .longName("override-flake")
+      .labels({"original-ref", "resolved-ref"})
+      .description("override a flake registry value")
+      .arity(2)
+      .handler([&](std::vector<std::string> ss) {
+          registryOverrides.push_back(std::make_pair(ss[0], ss[1]));
+      });
 }
 
 Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
diff --git a/src/libexpr/common-eval-args.hh b/src/libexpr/common-eval-args.hh
index be7fda783..54fb731de 100644
--- a/src/libexpr/common-eval-args.hh
+++ b/src/libexpr/common-eval-args.hh
@@ -16,6 +16,8 @@ struct MixEvalArgs : virtual Args
 
     Strings searchPath;
 
+    std::vector<std::pair<std::string, std::string>> registryOverrides;
+
 private:
 
     std::map<std::string, std::string> autoArgs;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 3f537da89..44988cd70 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -63,6 +63,8 @@ typedef std::list<SearchPathElem> SearchPath;
 /* Initialise the Boehm GC, if applicable. */
 void initGC();
 
+typedef std::vector<std::pair<std::string, std::string>> RegistryOverrides;
+
 
 class EvalState
 {
@@ -89,6 +91,9 @@ public:
 
     const ref<Store> store;
 
+    RegistryOverrides registryOverrides;
+
+
 private:
     SrcToStore srcToStore;
 
@@ -211,6 +216,8 @@ public:
        path.  Nothing is copied to the store. */
     Path coerceToPath(const Pos & pos, Value & v, PathSet & context);
 
+    void addRegistryOverrides(RegistryOverrides overrides) { registryOverrides = overrides; }
+
 public:
 
     /* The base environment, containing the builtin functions and
diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/primops/flake.cc
index 84e878f75..88a0293e3 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/primops/flake.cc
@@ -140,10 +140,13 @@ std::shared_ptr<FlakeRegistry> getUserRegistry()
     return readRegistry(getUserRegistryPath());
 }
 
-std::shared_ptr<FlakeRegistry> getFlagRegistry()
+std::shared_ptr<FlakeRegistry> getFlagRegistry(RegistryOverrides registryOverrides)
 {
-    // TODO (Nick): Implement this.
-    return std::make_shared<FlakeRegistry>();
+    auto flagRegistry = std::make_shared<FlakeRegistry>();
+    for (auto const & x : registryOverrides) {
+        flagRegistry->entries.insert_or_assign(FlakeRef(x.first), FlakeRef(x.second));
+    }
+    return flagRegistry;
 }
 
 // This always returns a vector with flakeReg, userReg, globalReg.
@@ -151,7 +154,7 @@ std::shared_ptr<FlakeRegistry> getFlagRegistry()
 const Registries EvalState::getFlakeRegistries()
 {
     Registries registries;
-    registries.push_back(getFlagRegistry());
+    registries.push_back(getFlagRegistry(registryOverrides));
     registries.push_back(getUserRegistry());
     registries.push_back(getGlobalRegistry());
     return registries;
@@ -357,9 +360,8 @@ NonFlake getNonFlake(EvalState & state, const FlakeRef & flakeRef, FlakeAlias al
 ResolvedFlake resolveFlake(EvalState & state, const FlakeRef & topRef,
     RegistryAccess registryAccess, bool isTopFlake)
 {
-    Flake flake = getFlake(state, topRef,
-        registryAccess == AllowRegistry || (registryAccess == AllowRegistryAtTop && isTopFlake));
-
+    bool allowRegistries = registryAccess == AllowRegistry || (registryAccess == AllowRegistryAtTop && isTopFlake);
+    Flake flake = getFlake(state, topRef, allowRegistries);
     LockFile lockFile;
 
     if (isTopFlake)
@@ -405,10 +407,8 @@ static LockFile makeLockFile(EvalState & evalState, FlakeRef & flakeRef)
 
 void updateLockFile(EvalState & state, const Path & path)
 {
-    // FIXME: don't copy 'path' to the store (especially since we
-    // dirty it immediately afterwards).
-
-    FlakeRef flakeRef = FlakeRef(path); // FIXME: ugly
+    // FIXME: We are writing the lockfile to the store here! Very bad practice!
+    FlakeRef flakeRef = FlakeRef(path);
     auto lockFile = makeLockFile(state, flakeRef);
     writeLockFile(lockFile, path + "/flake.lock");