Fixes a bug where unpack would mutate log line. (#3502)

* Fixes a bug where unpack would mutate log line.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* add reset.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
(cherry picked from commit 6148794)

pkg/logql/log/parser.go

@@ -358,11 +358,10 @@ func (u *UnpackParser) unpack(it *jsoniter.Iterator, entry []byte, lbs *LabelsBu
 		case jsoniter.StringValue:
 			// we only unpack map[string]string. Anything else is skipped.
 			if field == PackedEntryKey {
-				s := iter.ReadString()
 				// todo(ctovena): we should just reslice the original line since the property is contiguous
 				// but jsoniter doesn't allow us to do this right now.
 				// https://github.com/buger/jsonparser might do a better job at this.
-				entry = append(entry[:0], []byte(s)...)
+				entry = []byte(iter.ReadString())
 				isPacked = true
 				return true
 			}

pkg/logql/log/parser_test.go

@@ -721,10 +721,13 @@ func Test_unpackParser_Parse(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			b := NewBaseLabelsBuilder().ForLabels(tt.lbs, tt.lbs.Hash())
 			b.Reset()
+			copy := string(tt.line)
 			l, _ := j.Process(tt.line, b)
 			sort.Sort(tt.wantLbs)
 			require.Equal(t, tt.wantLbs, b.Labels())
 			require.Equal(t, tt.wantLine, l)
+			require.Equal(t, string(tt.wantLine), string(l))
+			require.Equal(t, copy, string(tt.line), "the original log line should not be mutated")
 		})
 	}
 }