spdlog::initialize_logger undesired effect on patterns

[afshinpir]

Hi all

I have a small code with 1 logger and 2 sinkers with different pattern on sinkers:

auto create_logger() {
    spdlog::cfg::load_env_levels();

    std::vector<spdlog::sink_ptr> sinks;

    auto console1_sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt>();
    console1_sink->set_level(spdlog::level::info);
    console1_sink->set_pattern("[%n] %v");
    sinks.push_back(console1_sink);

    auto console2_sink = std::make_shared<spdlog::sinks::stderr_color_sink_mt>();
    console2_sink->set_level(spdlog::level::trace);
    sinks.push_back(console2_sink);

    auto logger = std::make_shared<spdlog::logger>(GGL_PROVIDER_LOG_NAME, std::begin(sinks), std::end(sinks));
    spdlog::initialize_logger(logger);
    return logger;
}

now if you log anything with logger like this:

logger->info("test");

You will see default pattern on output of both sinkers. Why? Because spdlog::initialize_logger applies pattern on loggers(which is default) and these patterns will be applied to sinkers overriding their patterns. I used spdlog::initialize_logger only in order to apply SPDLOG_LEVEL values from command line, but this undesired behavior is very bad. I think I have a workaround by setting patterns after spdlog::initialize_logger(logger), but I don't think this is a desired approach.

[tt4g]

The description of spdlog::initialize_logger() clearly states that it changes the formatter. Its behavior cannot be easily changed.

spdlog/include/spdlog/spdlog.h

Lines 39 to 47 in da14258

	// Initialize and register a logger,
	// formatter and flush level will be set according the global settings.
	//
	// Useful for initializing manually created loggers with the global settings.
	//
	// Example:
	// auto mylogger = std::make_shared<spdlog::logger>("mylogger", ...);
	// spdlog::initialize_logger(mylogger);
	SPDLOG_API void initialize_logger(std::shared_ptr<logger> logger);

If you are calling spdlog::initialize_logger() to apply the log levels loaded by spdlog::cfg::load_env_levels(), the formatter can only be changed after spdlog::initialize_logger().

[afshinpir]

@tt4g Since you normally create spdlog::logger after creating sinkers and setting up their configuration, I think this behavior is somehow not great.
I think it is better to have another method that only applies levels to loggers directly based on SPDLOG_LEVEL environment variable too.

[tt4g]

If you think there is demand, please open a PR to add an API that allows only log levels to be set.
The following code sets the log level loaded with spdlog::cfg::load_env_levels() in spdlog::initialize_logger().

spdlog/include/spdlog/details/registry-inl.h

Lines 70 to 73 in 7d805c2

	// set new level according to previously configured level or default level
	auto it = log_levels_.find(new_logger->name());
	auto new_level = it != log_levels_.end() ? it->second : global_log_level_;
	new_logger->set_level(new_level);

[afshinpir]

Already created a PR #2649 . I hope I have not made any mistakes there.

[afshinpir]

My PR already merged. Now everyone can use apply_logger_env_levels to apply SPDLOG_LEVEL to their manually created loggers without changing sinkers' pattern.