tests: Fix rare test crash (#4175)

Fix rare test crash by improving the randomness logic.

cmd/server_test.go

@@ -1833,25 +1833,23 @@ func (s *TestSuiteCommon) TestGetPartialObjectMisAligned(c *C) {
 	c.Assert(response.StatusCode, Equals, http.StatusOK)
 
 	var buffer bytes.Buffer
-	line := `1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,1234567890,
-	1234567890,1234567890,1234567890,123`
-
-	rand.Seed(UTCNow().UnixNano())
-	// Create a misalgined data.
-	for i := 0; i < 13*rand.Intn(1<<16); i++ {
-		buffer.WriteString(fmt.Sprintf("[%05d] %s\n", i, line[:rand.Intn(1<<8)]))
+	// data to be written into buffer.
+	data := "1234567890"
+	// seed the random number generator once.
+	rand.Seed(3)
+	// generate a random number between 13 and 200.
+	randInt := getRandomRange(13, 200, -1)
+	// write into buffer till length of the buffer is greater than the generated random number.
+	for i := 0; i <= randInt; i += 10 {
+		buffer.WriteString(data)
 	}
-	putContent := buffer.String()
-	buf := bytes.NewReader([]byte(putContent))
-
+	// String content which is used for put object range test.
+	putBytes := buffer.Bytes()
+	putBytes = putBytes[:randInt]
+	// randomize the order of bytes in the byte array and create a reader.
+	putBytes = randomizeBytes(putBytes, -1)
+	buf := bytes.NewReader(putBytes)
+	putContent := string(putBytes)
 	objectName := "test-big-file"
 	// HTTP request to upload the object.
 	request, err = newTestSignedRequest("PUT", getPutObjectURL(s.endPoint, bucketName, objectName),
@@ -1882,6 +1880,7 @@ func (s *TestSuiteCommon) TestGetPartialObjectMisAligned(c *C) {
 		// request for last 7 bytes of the object.
 		{"-7", putContent[len(putContent)-7:]},
 	}
+
 	for _, t := range testCases {
 		// HTTP request to download the object.
 		request, err = newTestSignedRequest("GET", getGetObjectURL(s.endPoint, bucketName, objectName),

cmd/test-utils_test.go

@@ -2361,3 +2361,28 @@ func getEndpointsLocalAddr(endpoints EndpointList) string {
 
 	return globalMinioHost + ":" + globalMinioPort
 }
+
+// fetches a random number between range min-max.
+func getRandomRange(min, max int, seed int64) int {
+	// special value -1 means no explicit seeding.
+	if seed != -1 {
+		rand.Seed(seed)
+	}
+	return rand.Intn(max-min) + min
+}
+
+// Randomizes the order of bytes in the byte array
+// using Knuth Fisher-Yates shuffle algorithm.
+func randomizeBytes(s []byte, seed int64) []byte {
+	// special value -1 means no explicit seeding.
+	if seed != -1 {
+		rand.Seed(seed)
+	}
+	n := len(s)
+	var j int
+	for i := 0; i < n-1; i++ {
+		j = i + rand.Intn(n-i)
+		s[i], s[j] = s[j], s[i]
+	}
+	return s
+}