libexpr-c: expose essential evaluator functions for derivation tree inspection

src/libexpr-c/meson.build

@@ -30,6 +30,7 @@ subdir('nix-meson-build-support/subprojects')
 subdir('nix-meson-build-support/common')
 
 sources = files(
+  'nix_api_eval.cc',
   'nix_api_expr.cc',
   'nix_api_external.cc',
   'nix_api_value.cc',

src/libexpr-c/nix_api_eval.cc

@@ -0,0 +1,55 @@
+#include "nix/expr/eval.hh"
+#include "nix/expr/get-drvs.hh"
+
+#include "nix_api_expr.h"
+#include "nix_api_expr_internal.h"
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+
+static const nix::Bindings & get_bindings_or_empty(nix::EvalState & state, nix_value * autoArgs)
+{
+    if (!autoArgs) {
+        return nix::Bindings::emptyBindings;
+    }
+    auto & v = check_value_in(autoArgs);
+    state.forceAttrs(v, nix::noPos, "while evaluating automatic function arguments");
+    return *v.attrs();
+}
+
+extern "C" {
+
+StorePath *
+nix_get_derivation(nix_c_context * context, EvalState * state, nix_value * value, bool ignoreAssertionFailures)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_in(value);
+        auto maybePkg = nix::getDerivation(state->state, v, ignoreAssertionFailures);
+        if (!maybePkg) {
+            return nullptr;
+        }
+        nix::StorePath sp = maybePkg->requireDrvPath();
+        return new StorePath{std::move(sp)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_err nix_value_auto_call_function(
+    nix_c_context * context, EvalState * state, nix_value * auto_args, nix_value * fn_val, nix_value * result)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & fn = check_value_in(fn_val);
+        auto & res = check_value_not_null(result);
+
+        auto & b = get_bindings_or_empty(state->state, auto_args);
+        state->state.autoCallFunction(b, fn, res);
+    }
+    NIXC_CATCH_ERRS
+}
+
+} // extern "C"

src/libexpr-c/nix_api_expr.h

@@ -342,6 +342,69 @@ void nix_gc_register_finalizer(void * obj, void * cd, void (*finalizer)(void * o
 
 /** @} */ // doxygen group GC
 
+/** @defgroup libexpr_eval Evaluation
+ * @ingroup libexpr
+ * @brief Higher-level evaluation helpers
+ * @{
+ */
+
+/**
+ * @brief Determine whether a Nix value is a derivation and, if so, return its
+ *        store derivation path.
+ *
+ * Forces @p value and inspects it. The value is considered a derivation when it
+ * is an attribute set whose `type` attribute is the string `"derivation"`; in
+ * that case its `drvPath` attribute is parsed and returned. Otherwise NULL is
+ * returned without recording an error.
+ *
+ * Only the derivation path is returned. Other metadata (`name`, `system`,
+ * outputs, `meta`, ...) lives on @p value itself and can be read with the
+ * attribute-set accessors such as nix_get_attr_byname() and nix_get_string().
+ *
+ * @param[out] context Optional, stores error information. On a NULL return,
+ *  inspect the error code via nix_err_code() to tell the two NULL cases apart:
+ *  NIX_OK means @p value is simply not a derivation, any other code means
+ *  inspection failed. See @ref errors.
+ * @param[in] state The evaluation state.
+ * @param[in] value The value to inspect. It is forced by this call.
+ * @param[in] ignoreAssertionFailures If true, an assertion failure raised while
+ *  forcing @p value is treated as "not a derivation" (NULL is returned without
+ *  an error) rather than being reported as an error.
+ * @return A newly allocated StorePath holding the derivation path, or NULL.
+ *  Free a non-NULL result with nix_store_path_free().
+ */
+StorePath *
+nix_get_derivation(nix_c_context * context, EvalState * state, nix_value * value, bool ignoreAssertionFailures);
+
+/**
+ * @brief Call a function, drawing its arguments from an attribute set.
+ *
+ * Forces @p fn_val and writes the application result into @p result. The result
+ * is not forced; call nix_value_force() to evaluate it before inspecting the
+ * final value.
+ *
+ * - If @p fn_val is a function that takes a set of named arguments
+ *   (e.g. `{ a, b ? 1 }: ...`), it is called with an attribute set assembled
+ *   from @p auto_args: each named argument is taken from @p auto_args when
+ *   present; an argument absent from @p auto_args falls back to its default;
+ *   an argument that is both absent and has no default is an error.
+ * - Otherwise @p fn_val is copied into @p result unchanged. This includes any
+ *   non-function value as well as a function that takes a single unnamed
+ *   argument (e.g. `x: ...`), since there are no named arguments to supply.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] state The evaluation state.
+ * @param[in] auto_args Attribute set value supplying the named arguments, or
+ *  NULL to supply none.
+ * @param[in] fn_val The value to call.
+ * @param[out] result Pre-allocated nix_value that receives the result.
+ * @return NIX_OK if the call was successful, an error code otherwise.
+ */
+nix_err nix_value_auto_call_function(
+    nix_c_context * context, EvalState * state, nix_value * auto_args, nix_value * fn_val, nix_value * result);
+
+/** @} */ // doxygen group libexpr_eval
+
 // cffi end
 #ifdef __cplusplus
 }

src/libexpr-c/nix_api_expr_internal.h

@@ -2,6 +2,7 @@
 #define NIX_API_EXPR_INTERNAL_H
 
 #include <memory>
+#include <stdexcept>
 
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/expr/eval.hh"
@@ -74,4 +75,35 @@ struct nix_realised_string
 
 } // extern "C"
 
+// Shared helpers for validating nix_value [in] parameters across libexpr-c translation units.
+inline const nix::Value & check_value_not_null(const nix_value * value)
+{
+    if (!value || !value->value)
+        throw std::runtime_error("nix_value is null");
+    return *value->value;
+}
+
+inline nix::Value & check_value_not_null(nix_value * value)
+{
+    if (!value || !value->value)
+        throw std::runtime_error("nix_value is null");
+    return *value->value;
+}
+
+inline const nix::Value & check_value_in(const nix_value * value)
+{
+    auto & v = check_value_not_null(value);
+    if (!v.isValid())
+        throw std::runtime_error("Uninitialized nix_value");
+    return v;
+}
+
+inline nix::Value & check_value_in(nix_value * value)
+{
+    auto & v = check_value_not_null(value);
+    if (!v.isValid())
+        throw std::runtime_error("Uninitialized nix_value");
+    return v;
+}
+
 #endif // NIX_API_EXPR_INTERNAL_H

src/libexpr-c/nix_api_value.cc

@@ -12,41 +12,6 @@
 #include "nix_api_store_internal.h"
 #include "nix_api_value.h"
 
-// Internal helper functions to check [in] and [out] `Value *` parameters
-static const nix::Value & check_value_not_null(const nix_value * value)
-{
-    if (!value) {
-        throw std::runtime_error("nix_value is null");
-    }
-    return *value->value;
-}
-
-static nix::Value & check_value_not_null(nix_value * value)
-{
-    if (!value) {
-        throw std::runtime_error("nix_value is null");
-    }
-    return *value->value;
-}
-
-static const nix::Value & check_value_in(const nix_value * value)
-{
-    auto & v = check_value_not_null(value);
-    if (!v.isValid()) {
-        throw std::runtime_error("Uninitialized nix_value");
-    }
-    return v;
-}
-
-static nix::Value & check_value_in(nix_value * value)
-{
-    auto & v = check_value_not_null(value);
-    if (!v.isValid()) {
-        throw std::runtime_error("Uninitialized nix_value");
-    }
-    return v;
-}
-
 static nix::Value & check_value_out(nix_value * value)
 {
     auto & v = check_value_not_null(value);

src/libexpr-tests/meson.build

@@ -53,6 +53,7 @@ sources = files(
   'json.cc',
   'lazy-fetcher-attr.cc',
   'main.cc',
+  'nix_api_eval.cc',
   'nix_api_expr.cc',
   'nix_api_external.cc',
   'nix_api_value.cc',