Fixed out of order entries allowed in a chunk on edge case

pkg/chunkenc/gzip.go

@@ -314,7 +314,15 @@ func (c *MemChunk) SpaceFor(*logproto.Entry) bool {
 
 // Append implements Chunk.
 func (c *MemChunk) Append(entry *logproto.Entry) error {
-	if err := c.head.append(entry.Timestamp.UnixNano(), entry.Line); err != nil {
+	entryTimestamp := entry.Timestamp.UnixNano()
+
+	// If the head block is empty but there are cut blocks, we have to make
+	// sure the new entry is not out of order compared to the previous block
+	if c.head.isEmpty() && len(c.blocks) > 0 && c.blocks[len(c.blocks)-1].maxt > entryTimestamp {
+		return ErrOutOfOrder
+	}
+
+	if err := c.head.append(entryTimestamp, entry.Line); err != nil {
 		return err
 	}
 

pkg/chunkenc/gzip_test.go

@@ -10,6 +10,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/assert"
+
 	"github.com/grafana/loki/pkg/logproto"
 	"github.com/stretchr/testify/require"
 )
@@ -216,6 +218,47 @@ func TestGZIPChunkFilling(t *testing.T) {
 	require.Equal(t, int64(lines), i)
 }
 
+func TestMemChunk_AppendOutOfOrder(t *testing.T) {
+	t.Parallel()
+
+	type tester func(t *testing.T, chk *MemChunk)
+
+	tests := map[string]tester{
+		"append out of order in the same block": func(t *testing.T, chk *MemChunk) {
+			assert.NoError(t, chk.Append(logprotoEntry(5, "test")))
+			assert.NoError(t, chk.Append(logprotoEntry(6, "test")))
+
+			assert.EqualError(t, chk.Append(logprotoEntry(1, "test")), ErrOutOfOrder.Error())
+		},
+		"append out of order in a new block right after cutting the previous one": func(t *testing.T, chk *MemChunk) {
+			assert.NoError(t, chk.Append(logprotoEntry(5, "test")))
+			assert.NoError(t, chk.Append(logprotoEntry(6, "test")))
+			assert.NoError(t, chk.cut())
+
+			assert.EqualError(t, chk.Append(logprotoEntry(1, "test")), ErrOutOfOrder.Error())
+		},
+		"append out of order in a new block after multiple cuts": func(t *testing.T, chk *MemChunk) {
+			assert.NoError(t, chk.Append(logprotoEntry(5, "test")))
+			assert.NoError(t, chk.cut())
+
+			assert.NoError(t, chk.Append(logprotoEntry(6, "test")))
+			assert.NoError(t, chk.cut())
+
+			assert.EqualError(t, chk.Append(logprotoEntry(1, "test")), ErrOutOfOrder.Error())
+		},
+	}
+
+	for testName, tester := range tests {
+		tester := tester
+
+		t.Run(testName, func(t *testing.T) {
+			t.Parallel()
+
+			tester(t, NewMemChunk(EncGZIP))
+		})
+	}
+}
+
 var result []Chunk
 
 func BenchmarkWriteGZIP(b *testing.B) {