Logging configuration and installation

[c42f]

Here's an issue about two related problems:

Logger configuration

Several people have asked on slack about logger configuration, with the number 1 question being how to enable debug logging.

For the moment the answer is something like using Logging; global_logger(SimpleLogger(STDERR, Logging.Debug)), but this is pretty unsatisfactory:

• It's verbose and unfriendly
• When ConsoleLogger or something like it is merged (see ConsoleLogger for more fully featured log printing #25370), they'll be baking in the use of SimpleLogger rather than configuring the current logger.

We need a simple way to configure the current logger. Ideally to support a few basic desires:

• Easily enable debug logging
• Log level filtering in general
• Per-module log level filtering as would by typical in logging systems like python's.

Because stdlib Logging currently has no opinion about the internal structure of a Logger, I think we just need a neat way of applying some configuration options to either the current, global or (task) local logger. Perhaps just configure_logging(logger::AbstractLogger; kwargs) with configure_logging(:global; ...) applying to the global logger, configure_logging(::Task; ...) applying to a task local logger, and configure_logging(; ...) = configure_logging(current_logger(); ...)?

Logger installation

The current with_logger interface for installing a task-local logger doesn't allow us to install or remove a task local logger permanently. To do that, one needs to dig into the internals which is clearly nasty. To fix this global_logger probably needs a companion which applies to tasks. Or perhaps we unify things to have Logging.logger!(context, logger) set the logger for context to logger:

Logging.logger!(:global, logger) # set global logger
Logging.logger!(some_task, logger) # set task local logger

Similarly, Logging.logger(context) would get the logger installed for context.

[c42f]

This was quickly becoming a slack FAQ, so to mention a few people who have asked or might care about this - @quinnj @tshort @iamed2 @rofinn @StefanKarpinski

[rofinn]

I haven't been following the 0.7 changes changes that much, but the API in Memento would be setlevel!(getlogger(), "debug") (there's also Memento.config("debug")). In this case, the getlogger() returns the current root/global logger and then we're just mutating that logger reference.

I'm not sure I follow the Task logger situation. I would have assumed that a Task could access a logger via some kind of getlogger(...) interface or via an appropriately managed constant.