server: Sort disk arguments for consistent ordering (#3469)

This is important in a distributed setup, where the server hosting the
first disk formats a fresh setup. Sorting ensures that all servers
arrive at the same 'first' server.

Note: This change doesn't protect against different disk arguments
with some disks being same across servers.

cmd/url-sort.go

@@ -0,0 +1,34 @@
+/*
+ * Minio Cloud Storage, (C) 2016 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 cmd
+
+import "net/url"
+
+type byHostPath []*url.URL
+
+func (s byHostPath) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+func (s byHostPath) Len() int {
+	return len(s)
+}
+
+// Note: Host in url.URL includes the port too.
+func (s byHostPath) Less(i, j int) bool {
+	return (s[i].Host + s[i].Path) < (s[j].Host + s[j].Path)
+}