log: update Logger interface, so that Log is usable even as a Logger instance; add a test to ensure it (#13376)

vlib/log/log.v

@@ -63,6 +63,7 @@ pub fn level_from_tag(tag string) ?Level {
 
 // Logger is an interface that describes a generic Logger
 pub interface Logger {
+mut:
 	fatal(s string)
 	error(s string)
 	warn(s string)

vlib/log/log_test.v

@@ -0,0 +1,92 @@
+module log
+
+import time
+
+fn log_mutable_statements(mut log Log) {
+	println(@FN + ' start')
+	log.info('info')
+	log.warn('warn')
+	log.error('error')
+	log.debug('no output for debug')
+	log.set_level(.debug)
+	log.debug('debug now')
+	log.set_level(level_from_tag('INFO') or { Level.disabled })
+	log.info('info again')
+	log.set_level(level_from_tag('') or { Level.disabled })
+	log.error('no output anymore')
+	println(@FN + ' end')
+}
+
+fn logger_mutable_statements(mut log Logger) {
+	println(@FN + ' start')
+	log.info('info')
+	log.warn('warn')
+	log.error('error')
+	log.debug('no output for debug')
+	// log.set_level(.debug) // not usable here because not part of Logger interface
+	println(@FN + ' end')
+}
+
+fn delay() {
+	time.sleep(1 * time.second)
+}
+
+// new_log create and return a new Log reference
+pub fn new_log() &Log {
+	return &Log{
+		level: .info
+	} // reference needed for its parallel usage
+}
+
+// new_log_as_logger create and return a new Log reference as a generic Logger
+pub fn new_log_as_logger() &Logger {
+	return &Log{
+		level: .info
+	} // reference needed for its parallel usage
+}
+
+fn test_log_mutable() {
+	println(@FN + ' start')
+	mut log := Log{}
+	log.set_level(.info)
+	log_mutable_statements(mut log)
+	assert true
+	println(@FN + ' end')
+}
+
+/*
+// TODO: with Logger methods requiring a mutable instance, now I get a compilation  error: `l` is immutable, declare it with `mut` to make it mutable ... check if it's good the same now and/or what to do ... wip
+fn test_log_not_mutable() {
+	println(@FN + ' start')
+	l := log.Log{}
+
+	l.info('info')
+	l.warn('warn')
+	l.error('error')
+	l.debug('no output for debug')
+
+	assert true
+	println(@FN + ' end')
+}
+*/
+
+fn test_log_mutable_reference() {
+	println(@FN + ' start')
+	mut log := new_log()
+	assert typeof(log).name == '&log.Log'
+	go log_mutable_statements(mut log)
+	delay() // wait to finish
+	assert true
+	println(@FN + ' end')
+}
+
+fn test_logger_mutable_reference() {
+	println(@FN + ' start')
+	// get log as Logger and use it
+	mut logger := new_log_as_logger()
+	assert typeof(logger).name == '&log.Logger'
+	go logger_mutable_statements(mut logger)
+	delay() // wait to finish
+	assert true
+	println(@FN + ' end')
+}