Allow SyncProcessor to be called from appenders

[eagletmt]

SyncProcessor#log could be called recursively from an appender like Bugsnag and Sentry.

Issue # (if available)

#281

Changelog

Allow SyncProcessor to be called from appenders

Description of changes

SyncProcessor#log could be called recursively from an appender like Bugsnag and Sentry. However, SyncProcessor doesn't allow it because it uses Mutex to protect SyncProcessor#log. Here are code snippets to reproduce the Mutex error.

require 'semantic_logger/sync'
require 'sentry-ruby'

SemanticLogger.add_appender(appender: :sentry_ruby)

Sentry.init do |config|
  config.logger = SemanticLogger[Sentry]
  config.background_worker_threads = 0
  config.dsn = 'http://foo@localhost/bar'
end

logger = SemanticLogger['MyClass']
begin
  raise 'something went wrong'
rescue => e
  logger.error('oops', e)
end

require 'semantic_logger/sync'
require 'bugsnag'

ENV['BUGSNAG_API_KEY'] = '0123456789abcdef0123456789abcdef'

SemanticLogger.add_appender(appender: :bugsnag)

logger = SemanticLogger['MyClass']
begin
  raise 'something went wrong'
rescue => e
  logger.error('oops', e)
end

This PR resolves the issue by replacing Mutex with Monitor. Monitor allows to enter the critical section if the Mutex's owner is the current thread.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.