Fix handling for json-file io.UnexpectedEOF

When the multireader hits EOF, we will always get EOF from it, so we
cannot store the multrireader fro later error handling, only for the
decoder.

Thanks @tobiasstadler for pointing this error out.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/logger/jsonfilelog/read.go

@@ -61,9 +61,10 @@ func decodeLogLine(dec *json.Decoder, l *jsonlog.JSONLog) (*logger.Message, erro
 }
 
 type decoder struct {
-	rdr io.Reader
-	dec *json.Decoder
-	jl  *jsonlog.JSONLog
+	rdr      io.Reader
+	dec      *json.Decoder
+	jl       *jsonlog.JSONLog
+	maxRetry int
 }
 
 func (d *decoder) Reset(rdr io.Reader) {
@@ -87,7 +88,11 @@ func (d *decoder) Decode() (msg *logger.Message, err error) {
 	if d.jl == nil {
 		d.jl = &jsonlog.JSONLog{}
 	}
-	for retries := 0; retries < maxJSONDecodeRetry; retries++ {
+	if d.maxRetry == 0 {
+		// We aren't using maxJSONDecodeRetry directly so we can give a custom value for testing.
+		d.maxRetry = maxJSONDecodeRetry
+	}
+	for retries := 0; retries < d.maxRetry; retries++ {
 		msg, err = decodeLogLine(d.dec, d.jl)
 		if err == nil || err == io.EOF {
 			break
@@ -105,8 +110,7 @@ func (d *decoder) Decode() (msg *logger.Message, err error) {
 		// If the json logger writes a partial json log entry to the disk
 		// while at the same time the decoder tries to decode it, the race condition happens.
 		if err == io.ErrUnexpectedEOF {
-			d.rdr = io.MultiReader(d.dec.Buffered(), d.rdr)
-			d.dec = json.NewDecoder(d.rdr)
+			d.dec = json.NewDecoder(io.MultiReader(d.dec.Buffered(), d.rdr))
 			continue
 		}
 	}

daemon/logger/jsonfilelog/read_test.go

@@ -2,6 +2,7 @@ package jsonfilelog // import "github.com/docker/docker/daemon/logger/jsonfilelo
 
 import (
 	"bytes"
+	"encoding/json"
 	"io"
 	"testing"
 	"time"
@@ -93,3 +94,61 @@ func TestEncodeDecode(t *testing.T) {
 	_, err = dec.Decode()
 	assert.Assert(t, err == io.EOF)
 }
+
+func TestUnexpectedEOF(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	msg1 := &logger.Message{Timestamp: time.Now(), Line: []byte("hello1")}
+	msg2 := &logger.Message{Timestamp: time.Now(), Line: []byte("hello2")}
+
+	err := marshalMessage(msg1, json.RawMessage{}, buf)
+	assert.NilError(t, err)
+	err = marshalMessage(msg2, json.RawMessage{}, buf)
+	assert.NilError(t, err)
+
+	r := &readerWithErr{
+		err:   io.EOF,
+		after: buf.Len() / 4,
+		r:     buf,
+	}
+	dec := &decoder{rdr: r, maxRetry: 1}
+
+	_, err = dec.Decode()
+	assert.Error(t, err, io.ErrUnexpectedEOF.Error())
+	// again just to check
+	_, err = dec.Decode()
+	assert.Error(t, err, io.ErrUnexpectedEOF.Error())
+
+	// reset the error
+	// from here all reads should succeed until we get EOF on the underlying reader
+	r.err = nil
+
+	msg, err := dec.Decode()
+	assert.NilError(t, err)
+	assert.Equal(t, string(msg1.Line)+"\n", string(msg.Line))
+
+	msg, err = dec.Decode()
+	assert.NilError(t, err)
+	assert.Equal(t, string(msg2.Line)+"\n", string(msg.Line))
+
+	_, err = dec.Decode()
+	assert.Error(t, err, io.EOF.Error())
+}
+
+type readerWithErr struct {
+	err   error
+	after int
+	r     io.Reader
+	read  int
+}
+
+func (r *readerWithErr) Read(p []byte) (int, error) {
+	if r.err != nil && r.read > r.after {
+		return 0, r.err
+	}
+
+	n, err := r.r.Read(p[:1])
+	if n > 0 {
+		r.read += n
+	}
+	return n, err
+}