Use bufio.Reader instead of bufio.Scanner for logger.Copier

When using a scanner, log lines over 64K will crash the Copier with
bufio.ErrTooLong. Subsequently, the ioutils.bufReader will grow without
bound as the logs are no longer being flushed to disk.

Signed-off-by: Burke Libbey <burke.libbey@shopify.com>

daemon/logger/copier.go

@@ -2,6 +2,7 @@ package logger
 
 import (
 	"bufio"
+	"bytes"
 	"io"
 	"sync"
 	"time"
@@ -40,14 +41,27 @@ func (c *Copier) Run() {
 
 func (c *Copier) copySrc(name string, src io.Reader) {
 	defer c.copyJobs.Done()
-	scanner := bufio.NewScanner(src)
-	for scanner.Scan() {
-		if err := c.dst.Log(&Message{ContainerID: c.cid, Line: scanner.Bytes(), Source: name, Timestamp: time.Now().UTC()}); err != nil {
-			logrus.Errorf("Failed to log msg %q for logger %s: %s", scanner.Bytes(), c.dst.Name(), err)
+	reader := bufio.NewReader(src)
+
+	for {
+		line, err := reader.ReadBytes('\n')
+		line = bytes.TrimSuffix(line, []byte{'\n'})
+
+		// ReadBytes can return full or partial output even when it failed.
+		// e.g. it can return a full entry and EOF.
+		if err == nil || len(line) > 0 {
+			if logErr := c.dst.Log(&Message{ContainerID: c.cid, Line: line, Source: name, Timestamp: time.Now().UTC()}); logErr != nil {
+				logrus.Errorf("Failed to log msg %q for logger %s: %s", line, c.dst.Name(), logErr)
+			}
 		}
-	}
-	if err := scanner.Err(); err != nil {
-		logrus.Errorf("Error scanning log stream: %s", err)
+
+		if err != nil {
+			if err != io.EOF {
+				logrus.Errorf("Error scanning log stream: %s", err)
+			}
+			return
+		}
+
 	}
 }