[libvirt] [PATCH] logging: remove concept of default log priority

[Jiri Denemark]

On Wed, May 11, 2016 at 15:12:27 +0100, Daniel P. Berrange wrote:
> The logging framework has categories which can be selectively
> enabled/disabled by setting a suitable LIBVIRT_LOG_FILTERS
> environment variable or config file setting.
> 
> Along side that we also have the LIBVIRT_DEBUG environment
> variable which unconditionally enables every single category
> at DEBUG level. With the amount of logging produced by libvirt
> these days, the signal/noise ratio in the output from setting
> LIBVIRT_DEBUG is to poor for it to be useful.
> 
> Originally the LIBVIRT_DEBUG env variable had a very specific
> use case - it resulted in logging of anything in our public
> API entrypoints. eg it turned on src/libvirt.c debugging and
> nothing else. Additionally it would always result in log
> messages being sent to stderr.
> 
> When applied to any of the daemons, the log output no longers
> gets sent to stderr, but rather to whatever default log output
> has been configured by the daemon. If no log_outputs setting
> or LIBVIRT_LOG_OUTPUTS env is set, then messages will often
> go to the systemd journal or a /var/log/libvirt/libvirtd.log
> file rather than stderr.
> 
> These factors have conspired to make the LIBVIRT_DEBUG env
> and/or default log priority to be pretty useless in the real
> world.
> 
> This change attempts to take us back towards the original
> semantics of the LIBVIRT_DEBUG env variable as follows.
> 
> If LIBVIRT_DEBUG is set to a plain integer, or log level
> string, then it will turn on logging for the "libvirt" log
> category at that level. Any other string will be parsed in
> the same way as LIBVIRT_LOG_FILTERS would be. In all cases
> use of LIBVIRT_DEBUG will result in an explicit output being
> added for stderr. This ensures that messages always go to
> stderr, even if other outputs are already configured.
> 
> IOW,   LIBVIRT_DEBUG=1 virsh or LIBVIRT_DEBUG=1 libvirtd
> will both result in printing logs of libvirt public API
> calls to stderr. Meanwhile setting LIBVIRT_DEBUG="1:qemu"
> is equivalent to setting LIBVIRT_LOG_FILTERS="1:qemu" and
> LIBVIRT_LOG_OUTPUTS="1:stderr"

Hmm, this looks like it's going to break existing valid configurations,
such as

log_level = 1
log_filters="3:util.log 3:rpc.keepalive 3:rpc 3:remote 3:util.json"
log_outputs="1:file:/var/log/libvirt/libvirtd.log"

Shouldn't log_level = 1 add an all matching filter rather than the one
matching just libvirt?

Jirka