+func TestLogCutsAnIncompleteLastLine(t *testing.T) {
+ root := t.TempDir()
+ clock := newTestClock()
+ log, err := openLog(t.Context(), root, clock.Now)
+ require.NoError(t, err)
+
+ _, err = log.Append("user", "whole")
+ require.NoError(t, err)
+ require.NoError(t, log.Close())
+
+ path := filepath.Join(root, "main", "2026-01-01.jsonl")
+ file, err := os.OpenFile(path, os.O_WRONLY|os.O_APPEND, 0)
+ require.NoError(t, err)
+
+ _, err = file.WriteString(`{"i":1,"kind":"user","te`)
+ require.NoError(t, err)
+ require.NoError(t, file.Close())
+
+ log, err = openLog(t.Context(), root, clock.Now)
+ require.NoError(t, err)
+ require.Equal(t, 1, log.Len())
+
+ message, err := log.Append("user", "after the cut")
+ require.NoError(t, err)
+ require.Equal(t, 1, message.ID)
+ require.NoError(t, log.Close())
+
+ log, err = openLog(t.Context(), root, clock.Now)
+ require.NoError(t, err)
+
+ defer func() {
+ require.NoError(t, log.Close())
+ }()
+
+ message, err = log.Message(1)
+ require.NoError(t, err)
+ require.Equal(t, "after the cut", message.Text)
+}
+
+func TestLogRejectsAnIncompleteLineBeforeTheLastDay(t *testing.T) {
+ root := t.TempDir()
+ require.NoError(t, os.MkdirAll(filepath.Join(root, "main"), 0o700))
+
+ first := `{"i":0,"kind":"user","text":"x","size":1,"date":"2026-01-01T12:00:00Z"}` + "\n" + `{"i":1,"kin`
+ second := `{"i":1,"kind":"user","text":"y","size":1,"date":"2026-01-02T12:00:00Z"}` + "\n"
+ require.NoError(t, os.WriteFile(filepath.Join(root, "main", "2026-01-01.jsonl"), []byte(first), 0o600))
+ require.NoError(t, os.WriteFile(filepath.Join(root, "main", "2026-01-02.jsonl"), []byte(second), 0o600))
+
+ _, err := openLog(t.Context(), root, newTestClock().Now)
+ require.ErrorContains(t, err, "ends in an incomplete line")
+}
+