assert: handle TokenTooLong error scenario

assert/assertions.go

@@ -301,13 +301,23 @@ func messageFromMsgAndArgs(msgAndArgs ...interface{}) string {
 func indentMessageLines(message string, longestLabelLen int) string {
 	outBuf := new(bytes.Buffer)
 
-	for i, scanner := 0, bufio.NewScanner(strings.NewReader(message)); scanner.Scan(); i++ {
-		// no need to align first line because it starts at the correct location (after the label)
-		if i != 0 {
-			// append alignLen+1 spaces to align with "{{longestLabel}}:" before adding tab
+	msgScanner := bufio.NewScanner(strings.NewReader(message))
+
+	// a buffer is set manually to store the tokenization of the message string
+	// so that we can re-use the same buffer for each line of the message without
+	// any additional allocations. We set the buffer length to 1 more than the
+	// length of the message to avoid exceeding the default MaxScanTokenSize
+	// while scanning lines. This CAN happen. Refer to issue #1525
+	msgScanner.Buffer([]byte{}, len(message)+1)
+
+	first := true
+	for msgScanner.Scan() {
+		if !first {
 			outBuf.WriteString("\n\t" + strings.Repeat(" ", longestLabelLen+1) + "\t")
 		}
-		outBuf.WriteString(scanner.Text())
+
+		outBuf.WriteString(msgScanner.Text())
+		first = false
 	}
 
 	return outBuf.String()

assert/assertions_test.go

@@ -3192,3 +3192,45 @@ func TestErrorAs(t *testing.T) {
 		})
 	}
 }
+
+func Test_indentMessageLines(t *testing.T) {
+	tt := []struct {
+		name            string
+		msg             string
+		longestLabelLen int
+		expected        string
+	}{
+		{
+			name:            "single line",
+			msg:             "Hello World\n",
+			longestLabelLen: 11,
+			expected:        "Hello World",
+		},
+		{
+			name:            "multi line",
+			msg:             "Hello\nWorld\n",
+			longestLabelLen: 11,
+			expected:        "Hello\n\t            \tWorld",
+		},
+		{
+			name:            "single line - extremely long",
+			msg:             strings.Repeat("hello ", 20000),
+			longestLabelLen: 11,
+			expected:        strings.Repeat("hello ", 20000),
+		},
+		{
+			name:            "multi line - extremely long",
+			msg:             strings.Repeat("hello\n", 20000),
+			longestLabelLen: 3,
+			expected: strings.TrimSpace(
+				strings.TrimPrefix(strings.Repeat("\thello\n\t    ", 20000), "\t"),
+			),
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			Equal(t, tc.expected, indentMessageLines(tc.msg, tc.longestLabelLen))
+		})
+	}
+}