Harden ImportBase: open read-only and fail fast on an unreadable file.

ImportBase opened the input with a default std::fstream (in|out), which had
two consequences: a missing file silently produced an empty module (no error),
and a present-but-read-only file failed to open and also loaded as empty. Open
the stream read-only (std::ios::in) instead, and expose is_open().

System::Load now builds the importer first, checks is_open(), and throws
"cannot open file: <path>" before creating the module — so a failed load
surfaces as `load failed: …` and leaves no empty module behind. A read-only
but present file now loads correctly.

Flip the test that pinned the old silent-empty behaviour to assert the clean
failure (error + no module created).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/domain/system.cpp

@@ -21,14 +21,8 @@ System::~System()
 
 void System::Load(std::string module_name, std::string file_name, ImportType type)
 {
+    // Build the importer first, based on the import type.
     ImportBase *imp;
-    Module *mod = nullptr;
-    Parts *prts = nullptr;
-
-    // Creation or retrieval of the module.
-    mod = mods->merge(module_name);
-
-    // Parsing of the file based on the import type.
     if (type == ImportType::IMPORT_MENTOR)
     {
         imp = new ImportMentor(file_name);
@@ -43,7 +37,17 @@ void System::Load(std::string module_name, std::string file_name, ImportType typ
     {
         throw std::runtime_error("Unknown import type");
     }
+
+    // Fail fast on a missing/unreadable file, before touching the module table,
+    // so a failed load never leaves behind an empty module.
+    if (!imp->is_open())
+    {
+        delete imp;
+        throw std::runtime_error("cannot open file: " + file_name);
+    }
+
+    // Creation or retrieval of the module, then parse into it.
+    Module *mod = mods->merge(module_name);
     imp->parse(mod->signals);
-    prts = imp->parts();
-    mod->add(prts);
+    mod->add(imp->parts());
 }

src/core/imports/import_base.hpp

@@ -27,11 +27,22 @@ public:
      *
      * @param file_name Name of the file to be imported.
      */
-    ImportBase(std::string file_name) : file_lines(std::fstream(file_name))
+    ImportBase(std::string file_name) : file_lines(file_name, std::ios::in)
     {
         prts = new Parts();
     };
 
+    /**
+     * @brief Whether the input file was opened successfully.
+     *
+     * Opened read-only, so this is false only when the file is genuinely
+     * missing or unreadable (a read-only but present file still opens).
+     * System::Load checks it to fail fast instead of producing an empty module.
+     *
+     * @return true if the file stream is open.
+     */
+    bool is_open() const { return file_lines.is_open(); }
+
     /**
      * @brief Pure virtual method for parsing the file.
      *

tests/test_load.cpp

@@ -1,6 +1,7 @@
 #include <doctest/doctest.h>
 
 #include "core/app/load.hpp"
+#include "core/domain/modules.hpp"
 #include "core/domain/system.hpp"
 
 #include <cstdio>
@@ -50,15 +51,13 @@ TEST_CASE("load_module imports, drops singletons and reports counts") {
     std::remove(path);
 }
 
-TEST_CASE("load_module on a missing file currently succeeds empty (no throw)") {
-    // Pre-existing behaviour, preserved by the extraction: ImportBase opens the
-    // stream without checking is_open(), so a missing file yields an empty
-    // module rather than an error. Pinned here so any future hardening of this
-    // path is a deliberate, visible change.
+TEST_CASE("load_module fails cleanly on a missing file") {
+    // ImportBase opens read-only and System::Load checks is_open(), so a missing
+    // file is a clean error — and no empty module is left in the system.
     System sys;
     app::LoadResult r = app::load_module(
         &sys, "M", "/nonexistent-dir-xyz/nope.net", ImportType::IMPORT_MENTOR);
-    CHECK(r.ok);
-    CHECK(r.parts == 0);
-    CHECK(r.signals == 0);
+    CHECK_FALSE(r.ok);
+    CHECK(r.error.find("cannot open") != std::string::npos);
+    CHECK(sys.modules()->size() == 0);
 }