listing: Fix result when prefix is an object with a slash (#10267)

In a non recursive mode, issuing a list request where prefix
is an existing object with a slash and delimiter is a slash will
return entries in the object directory (data dir IDs)

```
$ aws s3api --profile minioadmin --endpoint-url http://localhost:9000 \
        list-objects-v2 --bucket testbucket --prefix code_of_conduct.md/ --delimiter '/'
{
    "CommonPrefixes": [
        {
            "Prefix":
"code_of_conduct.md/ec750fe0-ea7e-4b87-bbec-1e32407e5e47/"
        }
    ]
}
```

This commit adds a fast exit track in Walk() in this specific case.

cmd/object-api-listobjects_test.go

@@ -1,5 +1,5 @@
 /*
- * MinIO Cloud Storage, (C) 2015-2016, 2017 MinIO, Inc.
+ * MinIO Cloud Storage, (C) 2015-2020 MinIO, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -448,7 +448,9 @@ func testListObjects(obj ObjectLayer, instanceType string, t1 TestErrHandler) {
 				{Name: "temporary/0/"},
 			},
 		},
-		// ListObjectsResult-34 Listing with marker > last object should return empty
+		// ListObjectsResult-34:
+		//    * Listing with marker > last object should return empty
+		//    * Listing an object with a trailing slash and '/' delimiter
 		{
 			IsTruncated: false,
 			Objects:     []ObjectInfo{},
@@ -569,6 +571,8 @@ func testListObjects(obj ObjectLayer, instanceType string, t1 TestErrHandler) {
 		{"test-bucket-empty-dir", "", "temporary/", "", 10, resultCases[33], nil, true},
 		// Test listing with marker > last object such that response should be empty (65)
 		{"test-bucket-single-object", "", "A/C", "", 1000, resultCases[34], nil, true},
+		// Test listing an object with a trailing slash and a slash delimiter (66)
+		{"test-bucket-list-object", "Asia-maps.png/", "", "/", 1000, resultCases[34], nil, true},
 	}
 
 	for i, testCase := range testCases {

cmd/xl-storage.go

@@ -949,6 +949,14 @@ func (s *xlStorage) Walk(volume, dirPath, marker string, recursive bool, endWalk
 		return nil, err
 	}
 
+	// Fast exit track to check if we are listing an object with
+	// a trailing slash, this will avoid to list the object content.
+	if HasSuffix(dirPath, SlashSeparator) {
+		if st, err := os.Stat(pathJoin(volumeDir, dirPath, xlStorageFormatFile)); err == nil && st.Mode().IsRegular() {
+			return nil, errFileNotFound
+		}
+	}
+
 	// buffer channel matches the S3 ListObjects implementation
 	ch = make(chan FileInfo, maxObjectList)
 	go func() {