Avoid DDOS in PutObject() when objectName is '/' and size '0' (#4962)

It can happen that an incoming PutObject() request might
have inputs of following form eg:-

 - bucketName is 'testbucket'
 - objectName is '/'

bucketName exists and was previously created but there
are no other objects in this bucket. In a situation like
this parentDirIsObject() goes into an infinite loop.

Verifying that if '/' is an object fails on both backends
but the resulting `path.Dir('/')` returns `'/'` this causes
the closure to loop onto itself.

Fixes #4940

cmd/xl-v1-common_test.go

@@ -0,0 +1,99 @@
+/*
+ * Minio Cloud Storage, (C) 2017 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 (
+	"bytes"
+	"os"
+	"testing"
+)
+
+// Tests for if parent directory is object
+func TestXLParentDirIsObject(t *testing.T) {
+	rootPath, err := newTestConfig(globalMinioDefaultRegion)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(rootPath)
+
+	obj, fsDisks, err := prepareXL()
+	if err != nil {
+		t.Fatalf("Unable to initialize 'XL' object layer.")
+	}
+
+	// Remove all disks.
+	for _, disk := range fsDisks {
+		defer os.RemoveAll(disk)
+	}
+
+	bucketName := "testbucket"
+	objectName := "object"
+
+	if err = obj.MakeBucketWithLocation(bucketName, ""); err != nil {
+		t.Fatal(err)
+	}
+	objectContent := "12345"
+	objInfo, err := obj.PutObject(bucketName, objectName,
+		NewHashReader(bytes.NewReader([]byte(objectContent)), int64(len(objectContent)), "", ""), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if objInfo.Name != objectName {
+		t.Fatalf("Unexpected object name returned got %s, expected %s", objInfo.Name, objectName)
+	}
+
+	fs := obj.(*xlObjects)
+	testCases := []struct {
+		parentIsObject bool
+		objectName     string
+	}{
+		// parentIsObject is true if object is available.
+		{
+			parentIsObject: true,
+			objectName:     objectName,
+		},
+		{
+			parentIsObject: false,
+			objectName:     "",
+		},
+		{
+			parentIsObject: false,
+			objectName:     ".",
+		},
+		// Should not cause infinite loop.
+		{
+			parentIsObject: false,
+			objectName:     "/",
+		},
+		{
+			parentIsObject: false,
+			objectName:     "\\",
+		},
+		// Should not cause infinite loop with double forward slash.
+		{
+			parentIsObject: false,
+			objectName:     "//",
+		},
+	}
+
+	for i, testCase := range testCases {
+		gotValue := fs.parentDirIsObject(bucketName, testCase.objectName)
+		if testCase.parentIsObject != gotValue {
+			t.Errorf("Test %d: Unexpected value returned got %t, expected %t", i+1, gotValue, testCase.parentIsObject)
+		}
+	}
+}