libexpr-c: fix changing eval settings having no effect

Using the Rust bindings (which call into the C bindings) to set
eval-related settings was not working.

Since EvalSettings were defined in libcmd, we need to move them to
libexpr if libexpr-c wants to use them. Since libcmd enforces a
dependency on libflake, we also need to modify config-global to absorb
duplicate registrations and support callbacks for customizing behavior
that we only need in libcmd.

Note that we add back some of the global setting variables with this,
but this does fix the C API.

Author: numinit

src/libcmd/common-eval-args.cc

@@ -19,9 +19,8 @@
 
 namespace nix {
 
-EvalSettings evalSettings{
-    settings.readOnlyMode,
-    {
+static GlobalConfig::Register rEvalSettings(&evalSettings, [] {
+    evalSettings.lookupPathHooks = {
         {
             "flake",
             [](EvalState & state, std::string_view rest) {
@@ -40,10 +39,8 @@ EvalSettings evalSettings{
                 return state.storePath(storePath);
             },
         },
-    },
-};
-
-static GlobalConfig::Register rEvalSettings(&evalSettings);
+    };
+});
 
 flake::Settings flakeSettings;
 

src/libexpr-c/nix_api_expr.cc

@@ -6,6 +6,7 @@
 #include "nix/expr/eval-gc.hh"
 #include "nix/store/globals.hh"
 #include "nix/expr/eval-settings.hh"
+#include "nix/fetchers/fetch-settings.hh"
 #include "nix/util/ref.hh"
 
 #include "nix_api_expr.h"
@@ -137,8 +138,8 @@ nix_eval_state_builder * nix_eval_state_builder_new(nix_c_context * context, Sto
         return unsafe_new_with_self<nix_eval_state_builder>([&](auto * self) {
             return nix_eval_state_builder{
                 .store = nix::ref<nix::Store>(store->ptr),
-                .settings = nix::EvalSettings{/* &bool */ self->readOnlyMode},
-                .fetchSettings = nix::fetchers::Settings{},
+                .settings = &nix::evalSettings,
+                .fetchSettings = &nix::fetchSettings,
                 .readOnlyMode = true,
             };
         });
@@ -159,9 +160,9 @@ nix_err nix_eval_state_builder_load(nix_c_context * context, nix_eval_state_buil
         context->last_err_code = NIX_OK;
     try {
         // TODO: load in one go?
-        builder->settings.readOnlyMode = nix::settings.readOnlyMode;
-        loadConfFile(builder->settings);
-        loadConfFile(builder->fetchSettings);
+        builder->settings->readOnlyMode = nix::settings.readOnlyMode;
+        loadConfFile(*builder->settings);
+        loadConfFile(*builder->fetchSettings);
     }
     NIXC_CATCH_ERRS
 }
@@ -188,9 +189,9 @@ EvalState * nix_eval_state_build(nix_c_context * context, nix_eval_state_builder
     try {
         return unsafe_new_with_self<EvalState>([&](auto * self) {
             return EvalState{
-                .fetchSettings = std::move(builder->fetchSettings),
-                .settings = std::move(builder->settings),
-                .state = nix::EvalState(builder->lookupPath, builder->store, self->fetchSettings, self->settings),
+                .fetchSettings = builder->fetchSettings,
+                .settings = builder->settings,
+                .state = nix::EvalState(builder->lookupPath, builder->store, *self->fetchSettings, *self->settings),
             };
         });
     }

src/libexpr-c/nix_api_expr_internal.h

@@ -13,17 +13,17 @@ extern "C" {
 struct nix_eval_state_builder
 {
     nix::ref<nix::Store> store;
-    nix::EvalSettings settings;
-    nix::fetchers::Settings fetchSettings;
+    nix::EvalSettings * settings;
+    nix::fetchers::Settings * fetchSettings;
     nix::LookupPath lookupPath;
     // TODO: make an EvalSettings setting own this instead?
     bool readOnlyMode;
 };
 
 struct EvalState
 {
-    nix::fetchers::Settings fetchSettings;
-    nix::EvalSettings settings;
+    nix::fetchers::Settings * fetchSettings;
+    nix::EvalSettings * settings;
     nix::EvalState state;
 };
 

src/libexpr/eval-settings.cc

@@ -1,4 +1,5 @@
 #include "nix/util/users.hh"
+#include "nix/util/config-global.hh"
 #include "nix/store/globals.hh"
 #include "nix/store/profiles.hh"
 #include "nix/expr/eval.hh"
@@ -118,4 +119,8 @@ std::filesystem::path getNixDefExpr()
     return settings.useXDGBaseDirectories ? getStateDir() / "defexpr" : getHome() / ".nix-defexpr";
 }
 
+EvalSettings evalSettings{settings.readOnlyMode};
+
+static GlobalConfig::Register rEvalSettings(&evalSettings);
+
 } // namespace nix

src/libexpr/include/nix/expr/eval-settings.hh

@@ -412,4 +412,9 @@ std::filesystem::path getNixDefExpr();
  */
 constexpr size_t evalStackSize = 60 * 1024 * 1024;
 
+/**
+ * EvalSettings instance from libexpr.
+ */
+extern EvalSettings evalSettings;
+
 } // namespace nix

src/libflake-c/nix_api_flake.cc

@@ -32,7 +32,7 @@ nix_err nix_flake_settings_add_to_eval_state_builder(
 {
     nix_clear_err(context);
     try {
-        settings->settings->configureEvalSettings(builder->settings);
+        settings->settings->configureEvalSettings(*builder->settings);
     }
     NIXC_CATCH_ERRS
 }

src/libutil/config-global.cc

@@ -61,7 +61,16 @@ GlobalConfig globalConfig;
 
 GlobalConfig::Register::Register(Config * config)
 {
-    configRegistrations().emplace_back(config);
+    auto regs = configRegistrations();
+    if (std::find(regs.begin(), regs.end(), config) == regs.end()) {
+        configRegistrations().emplace_back(config);
+    }
+}
+
+GlobalConfig::Register::Register(Config * config, std::function<void()> && callback)
+    : Register(config)
+{
+    callback();
 }
 
 ExperimentalFeatureSettings experimentalFeatureSettings;

src/libutil/include/nix/util/config-global.hh

@@ -26,6 +26,7 @@ struct GlobalConfig : public AbstractConfig
     struct Register
     {
         Register(Config * config);
+        Register(Config * config, std::function<void()> && callback);
     };
 };