BREAKING: Replace custom import() with normal Lua require(). (#5014)

* Use require() instead of a custom import()

* Also search relative to the current file

* Update documentation

src/controllers/plugins/LuaAPI.cpp

@@ -15,6 +15,7 @@
 #    include <QFileInfo>
 #    include <QLoggingCategory>
 #    include <QTextCodec>
+#    include <QUrl>
 
 namespace {
 using namespace chatterino;
@@ -282,69 +283,87 @@ int g_load(lua_State *L)
 #    endif
 }
 
-int g_import(lua_State *L)
+int loadfile(lua_State *L, const QString &str)
 {
-    auto countArgs = lua_gettop(L);
-    // Lua allows dofile() which loads from stdin, but this is very useless in our case
-    if (countArgs == 0)
-    {
-        lua_pushnil(L);
-        luaL_error(L, "it is not allowed to call import() without arguments");
-        return 1;
-    }
-
     auto *pl = getApp()->plugins->getPluginByStatePtr(L);
-    QString fname;
-    if (!lua::pop(L, &fname))
+    if (pl == nullptr)
     {
-        lua_pushnil(L);
-        luaL_error(L, "chatterino g_import: expected a string for a filename");
-        return 1;
+        return luaL_error(L, "loadfile: internal error: no plugin?");
     }
     auto dir = QUrl(pl->loadDirectory().canonicalPath() + "/");
-    auto file = dir.resolved(fname);
 
-    qCDebug(chatterinoLua) << "plugin" << pl->id << "is trying to load" << file
-                           << "(its dir is" << dir << ")";
-    if (!dir.isParentOf(file))
+    if (!dir.isParentOf(str))
     {
-        lua_pushnil(L);
-        luaL_error(L, "chatterino g_import: filename must be inside of the "
-                      "plugin directory");
+        // XXX: This intentionally hides the resolved path to not leak it
+        lua::push(
+            L, QString("requested module is outside of the plugin directory"));
+        return 1;
+    }
+    QFileInfo info(str);
+    if (!info.exists())
+    {
+        lua::push(L, QString("no file '%1'").arg(str));
         return 1;
     }
 
-    auto path = file.path(QUrl::FullyDecoded);
-    QFile qf(path);
-    qf.open(QIODevice::ReadOnly);
-    if (qf.size() > 10'000'000)
+    auto temp = str.toStdString();
+    const auto *filename = temp.c_str();
+
+    auto res = luaL_loadfilex(L, filename, "t");
+    // Yoinked from checkload lib/lua/src/loadlib.c
+    if (res == LUA_OK)
     {
-        lua_pushnil(L);
-        luaL_error(L, "chatterino g_import: size limit of 10MB exceeded, what "
-                      "the hell are you doing");
+        lua_pushstring(L, filename);
+        return 2;
+    }
+
+    return luaL_error(L, "error loading module '%s' from file '%s':\n\t%s",
+                      lua_tostring(L, 1), filename, lua_tostring(L, -1));
+}
+
+int searcherAbsolute(lua_State *L)
+{
+    auto name = QString::fromUtf8(luaL_checkstring(L, 1));
+    name = name.replace('.', QDir::separator());
+
+    QString filename;
+    auto *pl = getApp()->plugins->getPluginByStatePtr(L);
+    if (pl == nullptr)
+    {
+        return luaL_error(L, "searcherAbsolute: internal error: no plugin?");
+    }
+
+    QFileInfo file(pl->loadDirectory().filePath(name + ".lua"));
+    return loadfile(L, file.canonicalFilePath());
+}
+
+int searcherRelative(lua_State *L)
+{
+    lua_Debug dbg;
+    lua_getstack(L, 1, &dbg);
+    lua_getinfo(L, "S", &dbg);
+    auto currentFile = QString::fromUtf8(dbg.source, dbg.srclen);
+    if (currentFile.startsWith("@"))
+    {
+        currentFile = currentFile.mid(1);
+    }
+    if (currentFile == "=[C]" || currentFile == "")
+    {
+        lua::push(
+            L,
+            QString(
+                "Unable to load relative to file:caller has no source file"));
         return 1;
     }
 
-    // validate utf-8 to block bytecode exploits
-    auto data = qf.readAll();
-    auto *utf8 = QTextCodec::codecForName("UTF-8");
-    QTextCodec::ConverterState state;
-    utf8->toUnicode(data.constData(), data.size(), &state);
-    if (state.invalidChars != 0)
-    {
-        lua_pushnil(L);
-        luaL_error(L, "invalid utf-8 in import() target (%s) is not allowed",
-                   fname.toStdString().c_str());
-        return 1;
-    }
+    auto parent = QFileInfo(currentFile).dir();
 
-    // fetch dofile and call it
-    lua_getfield(L, LUA_REGISTRYINDEX, "real_dofile");
-    // maybe data race here if symlink was swapped?
-    lua::push(L, path);
-    lua_call(L, 1, LUA_MULTRET);
+    auto name = QString::fromUtf8(luaL_checkstring(L, 1));
+    name = name.replace('.', QDir::separator());
+    QString filename =
+        parent.canonicalPath() + QDir::separator() + name + ".lua";
 
-    return lua_gettop(L);
+    return loadfile(L, filename);
 }
 
 int g_print(lua_State *L)