nix_api_expr: always force values before giving them to the user

src/libexpr/nix_api_expr.cc

@@ -49,6 +49,7 @@ nix_err nix_expr_eval_from_string(nix_c_context *context, State *state,
     nix::Expr *parsedExpr = state->state.parseExprFromString(
         expr, state->state.rootPath(nix::CanonPath(path)));
     state->state.eval(parsedExpr, *(nix::Value *)value);
+    state->state.forceValue(*(nix::Value *)value, nix::noPos);
   }
   NIXC_CATCH_ERRS
 }
@@ -60,6 +61,7 @@ nix_err nix_value_call(nix_c_context *context, State *state, Value *fn,
   try {
     state->state.callFunction(*(nix::Value *)fn, *(nix::Value *)arg,
                               *(nix::Value *)value, nix::noPos);
+    state->state.forceValue(*(nix::Value *)value, nix::noPos);
   }
   NIXC_CATCH_ERRS
 }

src/libexpr/nix_api_expr.h

@@ -100,7 +100,11 @@ nix_err nix_value_call(nix_c_context *context, State *state, Value *fn,
  * The Nix interpreter is lazy, and not-yet-evaluated Values can be
  * of type NIX_TYPE_THUNK instead of their actual value.
  *
- * This function converts Values into their final type.
+ * This function converts these Values into their final type.
+ *
+ * @note You don't need this function for basic API usage, since all functions
+ * that return a value call it for you. The only place you will see a
+ * NIX_TYPE_THUNK is in the primop callback.
  *
  * @param[out] context Optional, stores error information
  * @param[in] state The state of the evaluation.

src/libexpr/nix_api_value.cc

@@ -201,7 +201,7 @@ ExternalValue *nix_get_external(nix_c_context *context, Value *value) {
 }
 
 Value *nix_get_list_byidx(nix_c_context *context, const Value *value,
-                          unsigned int ix) {
+                          State *state, unsigned int ix) {
   if (context)
     context->last_err_code = NIX_OK;
   try {
@@ -209,6 +209,7 @@ Value *nix_get_list_byidx(nix_c_context *context, const Value *value,
     assert(v.type() == nix::nList);
     auto *p = v.listElems()[ix];
     nix_gc_incref(nullptr, p);
+    state->state.forceValue(*p, nix::noPos);
     return (Value *)p;
   }
   NIXC_CATCH_ERRS_NULL
@@ -225,6 +226,7 @@ Value *nix_get_attr_byname(nix_c_context *context, const Value *value,
     auto attr = v.attrs->get(s);
     if (attr) {
       nix_gc_incref(nullptr, attr->value);
+      state->state.forceValue(*attr->value, nix::noPos);
       return attr->value;
     }
     nix_set_err_msg(context, NIX_ERR_KEY, "missing attribute");
@@ -258,6 +260,7 @@ Value *nix_get_attr_byidx(nix_c_context *context, const Value *value,
     const nix::Attr &a = (*v.attrs)[i];
     *name = ((const std::string &)(state->state.symbols[a.name])).c_str();
     nix_gc_incref(nullptr, a.value);
+    state->state.forceValue(*a.value, nix::noPos);
     return a.value;
   }
   NIXC_CATCH_ERRS_NULL

src/libexpr/nix_api_value.h

@@ -178,11 +178,12 @@ ExternalValue *nix_get_external(nix_c_context *context, Value *);
  * Owned by the GC. Use nix_gc_decref when you're done with the pointer
  * @param[out] context Optional, stores error information
  * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
  * @param[in] ix list element to get
  * @return value, NULL in case of errors
  */
 Value *nix_get_list_byidx(nix_c_context *context, const Value *value,
-                          unsigned int ix);
+                          State *state, unsigned int ix);
 /** @brief Get an attr by name
  *
  * Owned by the GC. Use nix_gc_decref when you're done with the pointer