libexpr-c: Add nix_eval_state_builder

src/libexpr-c/nix_api_expr.cc

@@ -6,6 +6,7 @@
 #include "eval-gc.hh"
 #include "globals.hh"
 #include "eval-settings.hh"
+#include "ref.hh"
 
 #include "nix_api_expr.h"
 #include "nix_api_expr_internal.h"
@@ -93,7 +94,46 @@ nix_err nix_value_force_deep(nix_c_context * context, EvalState * state, nix_val
     NIXC_CATCH_ERRS
 }
 
-EvalState * nix_state_create(nix_c_context * context, const char ** lookupPath_c, Store * store)
+nix_eval_state_builder * nix_eval_state_builder_new(nix_c_context * context, Store * store)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        // Allocate ahead of time, because .settings needs self-reference
+        void * p = ::operator new(
+            sizeof(nix_eval_state_builder),
+            static_cast<std::align_val_t>(alignof(nix_eval_state_builder)));
+        auto * p2 = static_cast<nix_eval_state_builder *>(p);
+        new (p) nix_eval_state_builder{
+            .store = nix::ref<nix::Store>(store->ptr),
+            .settings = nix::EvalSettings{/* &bool */ p2->readOnlyMode},
+            .fetchSettings = nix::fetchers::Settings{},
+            .readOnlyMode = true,
+        };
+        return p2;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+void nix_eval_state_builder_free(nix_eval_state_builder * builder)
+{
+    delete builder;
+}
+
+nix_err nix_eval_state_builder_load(nix_c_context * context, nix_eval_state_builder * builder)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        // TODO: load in one go?
+        builder->settings.readOnlyMode = nix::settings.readOnlyMode;
+        loadConfFile(builder->settings);
+        loadConfFile(builder->fetchSettings);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_eval_state_builder_set_lookup_path(nix_c_context * context, nix_eval_state_builder * builder, const char ** lookupPath_c)
 {
     if (context)
         context->last_err_code = NIX_OK;
@@ -102,28 +142,51 @@ EvalState * nix_state_create(nix_c_context * context, const char ** lookupPath_c
         if (lookupPath_c != nullptr)
             for (size_t i = 0; lookupPath_c[i] != nullptr; i++)
                 lookupPath.push_back(lookupPath_c[i]);
+        builder->lookupPath = nix::LookupPath::parse(lookupPath);
+    }
+    NIXC_CATCH_ERRS
+}
 
+EvalState * nix_eval_state_build(nix_c_context * context, nix_eval_state_builder * builder)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        // Allocate ahead of time, because .state init needs self-reference
         void * p = ::operator new(
             sizeof(EvalState),
             static_cast<std::align_val_t>(alignof(EvalState)));
         auto * p2 = static_cast<EvalState *>(p);
         new (p) EvalState {
-            .fetchSettings = nix::fetchers::Settings{},
-            .settings = nix::EvalSettings{
-                nix::settings.readOnlyMode,
-            },
+            .fetchSettings = std::move(builder->fetchSettings),
+            .settings = std::move(builder->settings),
             .state = nix::EvalState(
-                nix::LookupPath::parse(lookupPath),
-                store->ptr,
+                builder->lookupPath,
+                builder->store,
                 p2->fetchSettings,
                 p2->settings),
         };
-        loadConfFile(p2->settings);
         return p2;
     }
     NIXC_CATCH_ERRS_NULL
 }
 
+EvalState * nix_state_create(nix_c_context * context, const char ** lookupPath_c, Store * store)
+{
+    auto builder = nix_eval_state_builder_new(context, store);
+    if (builder == nullptr)
+        return nullptr;
+
+    if (nix_eval_state_builder_load(context, builder) != NIX_OK)
+        return nullptr;
+
+    if (nix_eval_state_builder_set_lookup_path(context, builder, lookupPath_c)
+            != NIX_OK)
+        return nullptr;
+
+    return nix_eval_state_build(context, builder);
+}
+
 void nix_state_free(EvalState * state)
 {
     delete state;

src/libexpr-c/nix_api_expr.h

@@ -30,6 +30,11 @@ extern "C" {
 // cffi start
 
 // Type definitions
+/**
+ * @brief Builder for EvalState
+ */
+typedef struct nix_eval_state_builder nix_eval_state_builder;
+
 /**
  * @brief Represents a state of the Nix language evaluator.
  *
@@ -174,12 +179,70 @@ nix_err nix_value_force(nix_c_context * context, EvalState * state, nix_value *
 nix_err nix_value_force_deep(nix_c_context * context, EvalState * state, nix_value * value);
 
 /**
- * @brief Create a new Nix language evaluator state.
+ * @brief Create a new nix_eval_state_builder
+ *
+ * The settings are initialized to their default value.
+ * Values can be sourced elsewhere with nix_eval_state_builder_load.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] store The Nix store to use.
+ * @return A new nix_eval_state_builder or NULL on failure.
+ */
+nix_eval_state_builder * nix_eval_state_builder_new(nix_c_context * context, Store * store);
+
+/**
+ * @brief Read settings from the ambient environment
+ *
+ * Settings are sourced from environment variables and configuration files,
+ * as documented in the Nix manual.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[out] builder The builder to modify.
+ * @return NIX_OK if successful, an error code otherwise.
+ */
+nix_err nix_eval_state_builder_load(nix_c_context * context, nix_eval_state_builder * builder);
+
+/**
+ * @brief Set the lookup path for `<...>` expressions
+ *
+ * @param[in] context Optional, stores error information
+ * @param[in] builder The builder to modify.
+ * @param[in] lookupPath Null-terminated array of strings corresponding to entries in NIX_PATH.
+ */
+nix_err nix_eval_state_builder_set_lookup_path(
+    nix_c_context * context, nix_eval_state_builder * builder, const char ** lookupPath);
+
+/**
+ * @brief Create a new Nix language evaluator state
+ *
+ * Remember to nix_eval_state_builder_free after building the state.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] builder The builder to use and free
+ * @return A new Nix state or NULL on failure.
+ * @see nix_eval_state_builder_new, nix_eval_state_builder_free
+ */
+EvalState * nix_eval_state_build(nix_c_context * context, nix_eval_state_builder * builder);
+
+/**
+ * @brief Free a nix_eval_state_builder
+ *
+ * Does not fail.
+ *
+ * @param[in] builder The builder to free.
+ */
+void nix_eval_state_builder_free(nix_eval_state_builder * builder);
+
+/**
+ * @brief Create a new Nix language evaluator state
+ *
+ * For more control, use nix_eval_state_builder
  *
  * @param[out] context Optional, stores error information
  * @param[in] lookupPath Null-terminated array of strings corresponding to entries in NIX_PATH.
  * @param[in] store The Nix store to use.
  * @return A new Nix state or NULL on failure.
+ * @see nix_state_builder_new
  */
 EvalState * nix_state_create(nix_c_context * context, const char ** lookupPath, Store * store);
 

src/libexpr-c/nix_api_expr_internal.h

@@ -6,6 +6,17 @@
 #include "eval-settings.hh"
 #include "attr-set.hh"
 #include "nix_api_value.h"
+#include "search-path.hh"
+
+struct nix_eval_state_builder
+{
+    nix::ref<nix::Store> store;
+    nix::EvalSettings settings;
+    nix::fetchers::Settings fetchSettings;
+    nix::LookupPath lookupPath;
+    // TODO: make an EvalSettings setting own this instead?
+    bool readOnlyMode;
+};
 
 struct EvalState
 {