Enable error logging

[yongxiangng]

Related issue #506

Feature Request

To enable logging of errors only. Currently, setting SIMDJSON_VERBOSE_LOGGING will log errors and other info. It would be nice to differentiate the logs into different levels and configure the log levels to be logged via an environment variable / compile option.

Is this a feature that would be desired? Should we implement this with an environment variable or compile option?

Other notes

The motivation behind this is because I'd like to know the key passed in when we receive NO_SUCH_FIELD error code is returned. Enabling verbose logging would be too expensive, because I only need the logs when an error occurs.

Possible Implementation

I am willing to help and make the changes.

My proposal is to use a environment variable SIMDJSON_LOG_LEVEL, which defaults to INFO. To log error messages only, we compile with SIMDJSON_VERBOSE_LOGGING and set the env variable SIMDJSON_LOG_LEVEL to ERROR.

1. Add enum class log_level in the logger with 2 levels, INFO, ERROR
2. Add method should_log(log_level) in the logger class. It will fetch the env variable on first invocation.
3. Modify log_line to take in log_level as an additional param (should I default this to INFO?)
4. inside log_line do something like

if (LOG_ENABLED && simdjson_unlikely(should_log(log_level)))
{
  // logging impl ...
}

[lemire]

The PR, at a glance, looks reasonable.

[lemire]

I am going to have to remove the PR. It can have a large negative performance effect.

[lemire]

Fixed by #2030