Add a local YAML config file for persistent settings

Reads $XDG_CONFIG_HOME/slocker-lite/config.yaml (falling back to
$HOME/.config/slocker-lite/config.yaml), organized into sections. Only
the "global" section's log-level is supported for now -- other options
are one-shot flags, not standing preferences. An explicit --log-level
on the command line always overrides the config file, the same way
SPDLOG_LEVEL already does.

Uses libyaml directly (yaml_dep was already declared in meson.build but
unused). A missing config file isn't an error; unknown sections/keys
are ignored for forward-compatibility; malformed YAML syntax is a hard
error.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

src/config_file.cpp

@@ -0,0 +1,95 @@
+// Copyright (C) 2026 Viorel Munteanu
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, write to the Free Software Foundation, Inc.,
+// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+#include "config_file.h"
+
+#include <cstdlib>
+#include <string_view>
+
+#include <spdlog/spdlog.h>
+#include <yaml.h>
+
+namespace {
+
+std::string_view scalar_value(const yaml_node_t& node) {
+    return {reinterpret_cast<const char*>(node.data.scalar.value), node.data.scalar.length};
+}
+
+// Finds `key` in a YAML_MAPPING_NODE and returns its value node, or nullptr if
+// `node` isn't a mapping or has no such (scalar) key.
+const yaml_node_t* find_in_mapping(yaml_document_t& document, const yaml_node_t& node,
+                                    std::string_view key) {
+    if (node.type != YAML_MAPPING_NODE) {
+        return nullptr;
+    }
+    for (auto* pair = node.data.mapping.pairs.start; pair < node.data.mapping.pairs.top; ++pair) {
+        yaml_node_t* key_node = yaml_document_get_node(&document, pair->key);
+        if (key_node && key_node->type == YAML_SCALAR_NODE && scalar_value(*key_node) == key) {
+            return yaml_document_get_node(&document, pair->value);
+        }
+    }
+    return nullptr;
+}
+
+}  // namespace
+
+std::filesystem::path config_file_path() {
+    const char* xdg_config_home = std::getenv("XDG_CONFIG_HOME");
+    std::filesystem::path config_home;
+    if (xdg_config_home && *xdg_config_home) {
+        config_home = xdg_config_home;
+    } else {
+        const char* home = std::getenv("HOME");
+        config_home = std::filesystem::path(home ? home : "") / ".config";
+    }
+    return config_home / "slocker-lite" / "config.yaml";
+}
+
+std::optional<AppConfig> load_config_file(const std::filesystem::path& path) {
+    FILE* file = std::fopen(path.c_str(), "r");
+    if (!file) {
+        return AppConfig{};
+    }
+
+    yaml_parser_t parser;
+    yaml_parser_initialize(&parser);
+    yaml_parser_set_input_file(&parser, file);
+
+    yaml_document_t document;
+    bool loaded = yaml_parser_load(&parser, &document) != 0;
+    yaml_parser_delete(&parser);
+    std::fclose(file);
+
+    if (!loaded) {
+        spdlog::error("failed to parse config file {}", path.string());
+        return std::nullopt;
+    }
+
+    AppConfig config;
+    yaml_node_t* root = yaml_document_get_root_node(&document);
+    if (root) {
+        if (const yaml_node_t* global = find_in_mapping(document, *root, "global")) {
+            if (const yaml_node_t* log_level = find_in_mapping(document, *global, "log-level")) {
+                if (log_level->type == YAML_SCALAR_NODE) {
+                    config.log_level = std::string(scalar_value(*log_level));
+                }
+            }
+        }
+    }
+
+    yaml_document_delete(&document);
+    return config;
+}

src/config_file.h

@@ -0,0 +1,39 @@
+// Copyright (C) 2026 Viorel Munteanu
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, write to the Free Software Foundation, Inc.,
+// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+#pragma once
+
+#include <filesystem>
+#include <optional>
+#include <string>
+
+// Fields that make sense to persist across invocations (one-shot flags like
+// -m/-r/--user don't belong here). Only the "global" section's log-level is
+// supported today; add more optional fields as more long options gain config-file
+// support.
+struct AppConfig {
+    std::optional<std::string> log_level;  // global.log-level
+};
+
+// $XDG_CONFIG_HOME/slocker-lite/config.yaml, or $HOME/.config/slocker-lite/config.yaml
+// if XDG_CONFIG_HOME is unset/empty.
+std::filesystem::path config_file_path();
+
+// Loads and parses `path`'s "global" section. A missing file is not an error --
+// returns a default-constructed AppConfig (nothing set). Unknown sections/keys are
+// ignored, so the format stays forward-compatible. Malformed YAML syntax logs a
+// specific error and returns nullopt.
+std::optional<AppConfig> load_config_file(const std::filesystem::path& path);

src/main.cpp

@@ -31,6 +31,7 @@
 
 #include "bwrap.h"
 #include "config.h"
+#include "config_file.h"
 #include "containers_storage.h"
 #include "oci_image.h"
 #include "process.h"
@@ -311,6 +312,14 @@ int run_container(const std::filesystem::path& image_tar,
 int main(int argc, char* argv[]) {
     spdlog::cfg::load_env_levels();
 
+    auto config = load_config_file(config_file_path());
+    if (!config) {
+        return 1;
+    }
+    if (config->log_level) {
+        apply_log_level(*config->log_level);
+    }
+
     Mode mode = Mode::kNone;
     std::string mode_arg;
     bool disable_nsenter = false;