assert: handle TokenTooLong error scenario while formatting assertion…

… messages

As pointed out in stretchr#1525, when the assertion message is too long, it gets
completely truncated in the final output. This is because,
`bufio.Scanner.Scan()` has a default `MaxScanTokenSize` set to `65536`
characters (64 * 1024). The `Scan()` function return false whenever the
line being scanned exceeds that max limit. This leads to the final
assertion message being truncated.

This commit fixes that by manually setting the internal scan buffer size
to `len(message) + 1` to make sure that above scenario never occurs.

Fixes stretchr#1525

assert/assertions.go

@@ -301,13 +301,17 @@ 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))
+	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 - extreamly long",
+			msg:             strings.Repeat("hello ", 20000),
+			longestLabelLen: 11,
+			expected:        strings.Repeat("hello ", 20000),
+		},
+		{
+			name:            "multi line - extreamly 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))
+		})
+	}
+}