[dnf5] Implement new argument "--dump-variables"

If dnf5 is run with this argument, variable values are added to standard
output.

The old DNF4 supports this argument only with the `config-manager` plugin.
If the user wants to know the values of the variables with which a certain
command will run, he must first use `config-manager` and then run
the desired command with the same settings (same environment and
configuration files) and hope that nothing changed in between
(e.g. thanks to a plugin).

This implements part of the functionality of the dnf4 config-manager plugin,
but for general use.

dnf5/include/dnf5/context.hpp

@@ -97,6 +97,11 @@ class Context : public libdnf5::cli::session::Session {
 
     bool get_quiet() const { return quiet; }
 
+    /// Set to true to print information about variables
+    void set_dump_variables(bool enable) { this->dump_variables = enable; }
+
+    bool get_dump_variables() const { return dump_variables; }
+
     Plugins & get_plugins() { return *plugins; }
 
     libdnf5::Goal * get_goal(bool new_if_not_exist = true);
@@ -125,6 +130,7 @@ class Context : public libdnf5::cli::session::Session {
     const char * comment{nullptr};
 
     bool quiet{false};
+    bool dump_variables{false};
 
     std::unique_ptr<Plugins> plugins;
     std::unique_ptr<libdnf5::Goal> goal;

dnf5/main.cpp

@@ -507,6 +507,20 @@ void RootCommand::set_argument_parser() {
         global_options_group->register_argument(debug_solver);
     }
 
+    {
+        auto dump_variables = parser.add_new_named_arg("dump-variables");
+        dump_variables->set_long_name("dump-variables");
+        dump_variables->set_description("Print variable values to stdout");
+        dump_variables->set_parse_hook_func([&ctx](
+                                                [[maybe_unused]] ArgumentParser::NamedArg * arg,
+                                                [[maybe_unused]] const char * option,
+                                                [[maybe_unused]] const char * value) {
+            ctx.set_dump_variables(true);
+            return true;
+        });
+        global_options_group->register_argument(dump_variables);
+    }
+
     {
         auto version = parser.add_new_named_arg("version");
         version->set_long_name("version");
@@ -695,6 +709,14 @@ static void print_transaction_size_stats(Context & context) {
     }
 }
 
+static void dump_variables(Context & context) {
+    std::cout << _("======== Variables: ========") << std::endl;
+    for (const auto & var : context.base.get_vars()->get_variables()) {
+        const auto & val = var.second;
+        std::cout << fmt::format("{} = {}", var.first, val.value) << std::endl;
+    }
+}
+
 }  // namespace dnf5
 
 
@@ -810,6 +832,10 @@ int main(int argc, char * argv[]) try {
             // Write messages from memory buffer logger to stream logger
             dynamic_cast<libdnf5::MemoryBufferLogger &>(*file_logger).write_to_logger(log_router);
 
+            if (context.get_dump_variables()) {
+                dump_variables(context);
+            }
+
             auto repo_sack = base.get_repo_sack();
             repo_sack->create_repos_from_system_configuration();
             any_repos_from_system_configuration = repo_sack->size() > 0;