fix: deliver last log line when it has no trailing newline (#4846)

Co-authored-by: Claude <noreply@anthropic.com>

internal/docker/log_reader.go

@@ -41,9 +41,6 @@ func NewLogReader(r io.Reader, tty bool) *LogReader {
 
 func (d *LogReader) Read() (string, container.StdType, error) {
 	message, stdType, err := d.readEvent()
-	if err != nil {
-		return "", 0, err
-	}
 
 	var std container.StdType
 	switch stdType {
@@ -53,10 +50,18 @@ func (d *LogReader) Read() (string, container.StdType, error) {
 		std = container.STDERR
 	}
 
+	// A final line without a trailing newline (e.g. a container's last write
+	// before exiting) arrives together with EOF. Return the partial message
+	// with the error instead of dropping it; the event generator emits the
+	// message before handling the error.
+	if err != nil {
+		return message, std, err
+	}
+
 	for !strings.HasSuffix(message, "\n") {
 		tail, _, err := d.readEvent()
 		if err != nil {
-			return "", std, err
+			return message, std, err
 		}
 
 		_, after, _ := strings.Cut(tail, " ")

internal/docker/log_reader_test.go

@@ -0,0 +1,84 @@
+package docker
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/amir20/dozzle/internal/container"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func writeFrame(buf *bytes.Buffer, streamType byte, payload string) {
+	header := make([]byte, 8)
+	header[0] = streamType
+	binary.BigEndian.PutUint32(header[4:], uint32(len(payload)))
+	buf.Write(header)
+	buf.WriteString(payload)
+}
+
+func TestRead_multiplexed(t *testing.T) {
+	buf := &bytes.Buffer{}
+	writeFrame(buf, 1, "2024-01-01T00:00:00.000000000Z out\n")
+	writeFrame(buf, 2, "2024-01-01T00:00:01.000000000Z err\n")
+
+	reader := NewLogReader(buf, false)
+
+	message, std, err := reader.Read()
+	require.NoError(t, err)
+	assert.Equal(t, "2024-01-01T00:00:00.000000000Z out\n", message)
+	assert.Equal(t, container.STDOUT, std)
+
+	message, std, err = reader.Read()
+	require.NoError(t, err)
+	assert.Equal(t, "2024-01-01T00:00:01.000000000Z err\n", message)
+	assert.Equal(t, container.STDERR, std)
+
+	_, _, err = reader.Read()
+	assert.Equal(t, io.EOF, err)
+}
+
+func TestRead_lastLineWithoutNewline(t *testing.T) {
+	buf := &bytes.Buffer{}
+	writeFrame(buf, 1, "2024-01-01T00:00:00.000000000Z first\n")
+	writeFrame(buf, 1, "2024-01-01T00:00:01.000000000Z last line without newline")
+
+	reader := NewLogReader(buf, false)
+
+	message, _, err := reader.Read()
+	require.NoError(t, err)
+	assert.Equal(t, "2024-01-01T00:00:00.000000000Z first\n", message)
+
+	message, std, err := reader.Read()
+	assert.Equal(t, io.EOF, err)
+	assert.Equal(t, container.STDOUT, std)
+	assert.Equal(t, "2024-01-01T00:00:01.000000000Z last line without newline", message)
+}
+
+func TestRead_lastLineWithoutNewlineTTY(t *testing.T) {
+	input := "2024-01-01T00:00:00.000000000Z first\n2024-01-01T00:00:01.000000000Z last line without newline"
+	reader := NewLogReader(strings.NewReader(input), true)
+
+	message, _, err := reader.Read()
+	require.NoError(t, err)
+	assert.Equal(t, "2024-01-01T00:00:00.000000000Z first\n", message)
+
+	message, _, err = reader.Read()
+	assert.Equal(t, io.EOF, err)
+	assert.Equal(t, "2024-01-01T00:00:01.000000000Z last line without newline", message)
+}
+
+func TestRead_continuedFrames(t *testing.T) {
+	buf := &bytes.Buffer{}
+	writeFrame(buf, 1, "2024-01-01T00:00:00.000000000Z part one ")
+	writeFrame(buf, 1, "2024-01-01T00:00:00.000000000Z and part two\n")
+
+	reader := NewLogReader(buf, false)
+
+	message, _, err := reader.Read()
+	require.NoError(t, err)
+	assert.Equal(t, "2024-01-01T00:00:00.000000000Z part one and part two\n", message)
+}

internal/k8s/log_reader.go

@@ -18,10 +18,9 @@ func NewLogReader(reader io.ReadCloser) *LogReader {
 }
 
 func (r *LogReader) Read() (string, container.StdType, error) {
+	// A final line without a trailing newline arrives together with EOF.
+	// Return the partial line with the error instead of dropping it; the
+	// event generator emits the message before handling the error.
 	line, err := r.reader.ReadString('\n')
-	if err != nil {
-		return "", 0, err
-	}
-
-	return line, container.STDOUT, nil
+	return line, container.STDOUT, err
 }

internal/k8s/log_reader_test.go

@@ -0,0 +1,23 @@
+package k8s
+
+import (
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRead_lastLineWithoutNewline(t *testing.T) {
+	input := "2024-01-01T00:00:00.000000000Z first\n2024-01-01T00:00:01.000000000Z last line without newline"
+	reader := NewLogReader(io.NopCloser(strings.NewReader(input)))
+
+	message, _, err := reader.Read()
+	require.NoError(t, err)
+	assert.Equal(t, "2024-01-01T00:00:00.000000000Z first\n", message)
+
+	message, _, err = reader.Read()
+	assert.Equal(t, io.EOF, err)
+	assert.Equal(t, "2024-01-01T00:00:01.000000000Z last line without newline", message)
+}