Add flags and utils for logrus

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/log/log.go

@@ -0,0 +1,46 @@
+/*
+Copyright 2018 The Containerd Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package log
+
+import "github.com/sirupsen/logrus"
+
+// TODO(random-liu): Add trace support in containerd.
+
+// TraceLevel is the log level for trace.
+const TraceLevel = logrus.Level(uint32(logrus.DebugLevel + 1))
+
+// ParseLevel takes a string level and returns the Logrus log level constant.
+func ParseLevel(lvl string) (logrus.Level, error) {
+	if lvl == "trace" {
+		return TraceLevel, nil
+	}
+	return logrus.ParseLevel(lvl)
+}
+
+// Trace logs a message at level Trace on the standard logger.
+func Trace(args ...interface{}) {
+	if logrus.GetLevel() >= TraceLevel {
+		logrus.Debug(args...)
+	}
+}
+
+// Tracef logs a message at level Trace on the standard logger.
+func Tracef(format string, args ...interface{}) {
+	if logrus.GetLevel() >= TraceLevel {
+		logrus.Debugf(format, args...)
+	}
+}