[IMP] warn when using logrotate in multiprocess

The built-in log rotation is *very* unreliable in a multiprocess
context, which can lead to significant data loss (up to hours of log
missing) right after rotation[0][1].

Technically, according to the cookbook even filehandler does not
support multiprocess logging[2].

Either way, we should at the very least warn that logrotate is a Bad
Idea in multi-workers scenario. Ideally we'd just fall back on regular
file/stream logging but that likely could & would break existing
deployments, so maybe implement that for master instead?

Alternatively: implement it properly in multi-worker scenario with
only the "root" process actually logging to whatever output was set
up, and all workers sending stuff to log to it via a queue or network
socket or w/e.

[0]
https://stackoverflow.com/questions/34186774/why-doesnt-timedrotatingfilehandler-work-properly-and-how-to-solve-this-issuer
[1] https://github.com/odoo/odoo/issues/16566
[2] https://docs.python.org/3/howto/logging-cookbook.html#logging-to-a-single-file-from-multiple-processes

openerp/netsvc.py

@@ -144,6 +144,13 @@ def init_logger():
             if dirname and not os.path.isdir(dirname):
                 os.makedirs(dirname)
             if tools.config['logrotate'] is not False:
+                if tools.config['workers'] > 1:
+                    # TODO: fallback to regular file logging in master for safe(r) defaults?
+                    #
+                    # Doing so here would be a good idea but also might break
+                    # situations were people do log-shipping of rotated data?
+                    _logger.warn("WARNING: built-in log rotation is not reliable in multi-worker scenarios and may incur significant data loss. "
+                                 "It is strongly recommended to use an external log rotation utility or use system loggers (--syslog) instead.")
                 handler = logging.handlers.TimedRotatingFileHandler(filename=logf, when='D', interval=1, backupCount=30)
             elif os.name == 'posix':
                 handler = logging.handlers.WatchedFileHandler(logf)