[Dune-devel] A proper logging subsystem for Dune

[Steffen Müthing]

Hi everyone,

I’ve been recently been annoyed (again) by the state of console message output in Dune and fought with the
usual problems (setting verbosities, disabling output on rank > 0, long and complicated output formatting).

So I bit the bullet and wrote a little logging system that among others does the following:
- Adds cheap logger objects that do the output filtering (log levels are set at runtime).
- Uses Python-like format strings for output formatting (based on https://fmtlib.net, currently expected to
  become part of C++2a as P0645. Format strings are optionally syntax-checked at compile time. Together
  with the loggers, this makes log calls much less verbose than what we have now.
- Configurable routing for log messages: Each logger is attached to a backend singleton (of which there can
  be several), which is in turn attached to one or more sinks that actually write the log messages somewhere.
- Completely configurable via ParameterTree with usable defaults.
- Optional support for redirecting std::cout and std::cerr through the logging system for integration with legacy
  code.

The code is currently a MR in PDELab because it vendors fmtlib and uses a number of compiler features that
are C++17 only such as string_view, structured bindings, class template argument deduction, which make it a
difficult proposition for the core modules:

https://gitlab.dune-project.org/pdelab/dune-pdelab/merge_requests/393

That said, I would really like something like this in the core modules (maybe after ironing out the kinks in PDELab
for a release cycle), so if you are interested, please take a look at it!

Cheers
Steffen
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