fix: all levels deep flat key match (minio#12996)

this addresses a regression from minio#12984
which only addresses flat key from single
level deep at bucket level.

added extra tests as well to cover all
these scenarios.
# Conflicts:
#	cmd/metacache-walk.go

cmd/metacache-walk.go

@@ -128,9 +128,15 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 		dirObjects := make(map[string]struct{})
 		for i, entry := range entries {
 			if len(prefix) > 0 && !strings.HasPrefix(entry, prefix) {
+				// Do do not retain the file, since it doesn't
+				// match the prefix.
+				entries[i] = ""
 				continue
 			}
 			if len(forward) > 0 && entry < forward {
+				// Do do not retain the file, since its
+				// lexially smaller than 'forward'
+				entries[i] = ""
 				continue
 			}
 			if strings.HasSuffix(entry, slashSeparator) {

cmd/object-api-listobjects_test.go

@@ -84,6 +84,10 @@ func testListObjects(obj ObjectLayer, instanceType string, t1 TestErrHandler) {
 		{testBuckets[4], "file1/guidSplunk-aaaa/file", "content", nil},
 		{testBuckets[5], "dir/day_id=2017-10-10/issue", "content", nil},
 		{testBuckets[5], "dir/day_id=2017-10-11/issue", "content", nil},
+		{testBuckets[5], "foo/201910/1122", "content", nil},
+		{testBuckets[5], "foo/201910/1112", "content", nil},
+		{testBuckets[5], "foo/201910/2112", "content", nil},
+		{testBuckets[5], "foo/201910_txt", "content", nil},
 	}
 	for _, object := range testObjects {
 		md5Bytes := md5.Sum([]byte(object.content))
@@ -477,6 +481,24 @@ func testListObjects(obj ObjectLayer, instanceType string, t1 TestErrHandler) {
 			IsTruncated: true,
 			Prefixes:    []string{"dir/day_id=2017-10-10/"},
 		},
+		// ListObjectsResult-37 list with prefix match 2 levels deep
+		{
+			IsTruncated: false,
+			Objects: []ObjectInfo{
+				{Name: "foo/201910/1112"},
+				{Name: "foo/201910/1122"},
+			},
+		},
+		// ListObjectsResult-38 list with prefix match 1 level deep
+		{
+			IsTruncated: false,
+			Objects: []ObjectInfo{
+				{Name: "foo/201910/1112"},
+				{Name: "foo/201910/1122"},
+				{Name: "foo/201910/2112"},
+				{Name: "foo/201910_txt"},
+			},
+		},
 	}
 
 	testCases := []struct {
@@ -602,6 +624,9 @@ func testListObjects(obj ObjectLayer, instanceType string, t1 TestErrHandler) {
 		{testBuckets[4], "file1/", "", "guidSplunk", 1000, resultCases[35], nil, true},
 		// Test listing at prefix with expected prefix markers
 		{testBuckets[5], "dir/", "", SlashSeparator, 1, resultCases[36], nil, true},
+		// Test listing with prefix match
+		{testBuckets[5], "foo/201910/11", "", "", 1000, resultCases[37], nil, true},
+		{testBuckets[5], "foo/201910", "", "", 1000, resultCases[38], nil, true},
 	}
 
 	for i, testCase := range testCases {