assert: handle TokenTooLong error scenario

assert/assertions.go

@@ -300,14 +300,25 @@ func messageFromMsgAndArgs(msgAndArgs ...interface{}) string {
 // basis on which the alignment occurs).
 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
-			outBuf.WriteString("\n\t" + strings.Repeat(" ", longestLabelLen+1) + "\t")
+	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 maximum buffer length to 1 more
+	// than the length of the message to avoid exceeding the default
+	// MaxScanTokenSize while scanning lines. This can happen when there is a
+	// single very long line. Refer to issue #1525
+	msgScanner.Buffer(nil, len(message)+1)
+
+	isFirstLine := true
+	indent := fmt.Sprintf("\n\t%*s\t", longestLabelLen+1, "")
+	for msgScanner.Scan() {
+		if !isFirstLine {
+			outBuf.WriteString(indent)
 		}
-		outBuf.WriteString(scanner.Text())
+
+		outBuf.Write(msgScanner.Bytes())
+		isFirstLine = false
 	}
 
 	return outBuf.String()

assert/assertions_priv_test.go

@@ -0,0 +1,87 @@
+package assert
+
+import (
+	"bufio"
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func Test_indentMessageLines(t *testing.T) {
+	tt := []struct {
+		name            string
+		longestLabelLen int
+
+		// the input is constructed based on the below parameters
+		bytesPerLine int
+		lineCount    int
+	}{
+		{
+			name:            "single line - over the bufio default limit",
+			longestLabelLen: 11,
+			bytesPerLine:    bufio.MaxScanTokenSize + 10,
+			lineCount:       1,
+		},
+		{
+			name:            "multi line - over the bufio default limit",
+			longestLabelLen: 11,
+			bytesPerLine:    bufio.MaxScanTokenSize + 10,
+			lineCount:       3,
+		},
+		{
+			name:            "single line - just under the bufio default limit",
+			longestLabelLen: 11,
+			bytesPerLine:    bufio.MaxScanTokenSize - 10,
+			lineCount:       1,
+		},
+		{
+			name:            "single line - just under the bufio default limit",
+			longestLabelLen: 11,
+			bytesPerLine:    bufio.MaxScanTokenSize - 10,
+			lineCount:       1,
+		},
+		{
+			name:            "single line - equal to the bufio default limit",
+			longestLabelLen: 11,
+			bytesPerLine:    bufio.MaxScanTokenSize,
+			lineCount:       1,
+		},
+		{
+			name:            "multi line - equal to the bufio default limit",
+			longestLabelLen: 11,
+			bytesPerLine:    bufio.MaxScanTokenSize,
+			lineCount:       3,
+		},
+		{
+			name:            "longest label length is zero",
+			longestLabelLen: 0,
+			bytesPerLine:    10,
+			lineCount:       1,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			var input bytes.Buffer
+			for i := 0; i < tc.lineCount; i++ {
+				input.WriteString(strings.Repeat("#", tc.bytesPerLine))
+				input.WriteRune('\n')
+			}
+
+			output := indentMessageLines(
+				strings.TrimSpace(input.String()), tc.longestLabelLen,
+			)
+			outputLines := strings.Split(output, "\n")
+			for i, line := range outputLines {
+				if i > 0 {
+					// count the leading white spaces. It should be equal to the longest
+					// label length + 3. The +3 is to account for the 2 '\t' and 1 extra
+					// space. Read the comment in the function for more context
+					Equal(t, tc.longestLabelLen+3, strings.Index(line, "#"))
+				}
+
+				Len(t, strings.TrimSpace(line), tc.bytesPerLine)
+			}
+		})
+	}
+}