disable disk-usage when export is root mount path (#6091)

disk usage crawling is not needed when a tenant
is not sharing the same disk for multiple other
tenants. This PR adds an optimization when we
see a setup uses entire disk, we simply rely on
statvfs() to give us total usage.

This PR also additionally adds low priority
scheduling for usage check routine, such that
other go-routines blocked will be automatically
unblocked and prioritized before usage.

pkg/mountinfo/mountinfo_windows.go

@@ -0,0 +1,59 @@
+// +build windows
+
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * 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 mountinfo
+
+import (
+	"os"
+)
+
+// CheckCrossDevice - check if any input path has multiple sub-mounts.
+// this is a dummy function and returns nil for now.
+func CheckCrossDevice(paths []string) error {
+	return nil
+}
+
+// fileExists checks if specified file exists.
+func fileExists(filename string) (bool, error) {
+	if _, err := os.Stat(filename); os.IsNotExist(err) {
+		return false, nil
+	} else if err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+// IsLikelyMountPoint determines if a directory is a mountpoint.
+func IsLikelyMountPoint(file string) bool {
+	stat, err := os.Lstat(file)
+	if err != nil {
+		return false
+	}
+
+	// If current file is a symlink, then it is a mountpoint.
+	if stat.Mode()&os.ModeSymlink != 0 {
+		target, err := os.Readlink(file)
+		if err != nil {
+			return false
+		}
+		exists, _ := fileExists(target)
+		return exists
+	}
+
+	return false
+}