[Filebeat] keep track of bytes read when max_bytes exceeded (#28352)

* [Filebeat] keep track of bytes read when max_bytes exceeded in LineReader

Closes #28317

(cherry picked from commit dad462c)

CHANGELOG.next.asciidoc

@@ -199,6 +199,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Tolerate faults when Windows Event Log session is interrupted {issue}27947[27947] {pull}28191[28191]
 - Add support for username in cisco asa security negotiation logs {pull}26975[26975]
 - Relax time parsing and capture group and session type in Cisco ASA module {issue}24710[24710] {pull}28325[28325]
+- Correctly track bytes read when max_bytes is exceeded. {issue}28317[28317] {pull}28352[28352]
 
 *Heartbeat*
 

libbeat/reader/readfile/line.go

@@ -75,8 +75,13 @@ func NewLineReader(input io.ReadCloser, config Config) (*LineReader, error) {
 	}, nil
 }
 
-// Next reads the next line until the new line character
-func (r *LineReader) Next() ([]byte, int, error) {
+// Next reads the next line until the new line character.  The return
+// value b is the byte slice that contains the next line.  The return
+// value n is the number of bytes that were consumed from the
+// underlying reader to read the next line.  If the LineReader is
+// configured with maxBytes n may be larger than the length of b due
+// to skipped lines.
+func (r *LineReader) Next() (b []byte, n int, err error) {
 	// This loop is need in case advance detects an line ending which turns out
 	// not to be one when decoded. If that is the case, reading continues.
 	for {
@@ -162,6 +167,7 @@ func (r *LineReader) advance() error {
 			for idx != -1 && idx > r.maxBytes {
 				r.logger.Warnf("Exceeded %d max bytes in line limit, skipped %d bytes line", r.maxBytes, idx)
 				err = r.inBuffer.Advance(idx + len(r.nl))
+				r.byteCount += idx + len(r.nl)
 				r.inBuffer.Reset()
 				r.inOffset = 0
 				idx = r.inBuffer.IndexFrom(r.inOffset, r.nl)
@@ -175,6 +181,7 @@ func (r *LineReader) advance() error {
 					return err
 				}
 				r.logger.Warnf("Exceeded %d max bytes in line limit, skipped %d bytes line", r.maxBytes, skipped)
+				r.byteCount += skipped
 				idx = r.inBuffer.IndexFrom(r.inOffset, r.nl)
 			}
 		}

libbeat/reader/readfile/line_test.go

@@ -365,7 +365,7 @@ func TestMaxBytesLimit(t *testing.T) {
 	// Read decodec lines and test
 	var idx int
 	for i := 0; ; i++ {
-		b, n, err := reader.Next()
+		b, _, err := reader.Next()
 		if err != nil {
 			if err == io.EOF {
 				break
@@ -386,12 +386,7 @@ func TestMaxBytesLimit(t *testing.T) {
 			break
 		}
 
-		gotLen := n - len(nl)
 		s := string(b[:len(b)-len(nl)])
-		if len(line) != gotLen {
-			t.Fatalf("invalid line length, expected: %d got: %d", len(line), gotLen)
-		}
-
 		if line != s {
 			t.Fatalf("lines do not match, expected: %s got: %s", line, s)
 		}