commit b72867f0e68c96ca25e1f9929ce92be0a802db6b
author Harald Welte <laforge@osmocom.org> Sat Sep 26 21:45:16 2020 +0200
committer Harald Welte <laforge@osmocom.org> Tue Oct 26 17:16:47 2021 +0200
tree 2f1200f645ee47d49bf39ebdf802a82fb10d9d5a
parent 8db64ee81019d046fc61c61db77dacfadfe5165b

logging: Change stderr + file target to use non-blocking write

So far, we used blocking, buffered fwrite() to write to stderr
and file targets.  This causes problems if there are [slow] consumers
causing delays, such as gnome-terminal (when the program is started
interactively) or systemd/journald (where we observe 64..128ms blocks on
stderr).

This patch introduces stderr/file based logging via write_queue
and osmo_select_main(), i.e. switch from glibc-buffered, blocking
to internally buffered, non-blocking writes.

* when osmo_stderr_target is created via application.c, we create it
  in blocking stream mode for backwards compatibility, particularly
  for [smaller] programs that don't use osmo_select_main()

* when the VTY code encounters 'log stderr' or 'log file FILENAME',
  we switch that respective target to non-blocking write-queue mode,
  as this means the application is in fact using osmo_select_main()

* The config file can now state 'log stderr blocking-io' or
  'log file FILENAME blocking-io' to explicitly enforce using blocking
  stream based I/O

* The application can at any time use API functions to switch either way

Closes: OS#4311
Change-Id: Ia58fd78535c41b3da3aeb7733aadc785ace610da