plan 9: the log restores a missing final newline

1 files changed, 25 insertions, 1 deletions

diff --git a/internal/journal/journal.go b/internal/journal/journal.go
index 465cd51..085c88a 100644
--- a/internal/journal/journal.go
+++ b/internal/journal/journal.go
@@ -54,13 +54,37 @@ func Open(path string) (*Writer, error) {
 	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return nil, fmt.Errorf("journal: %w", err)
 	}
-	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
+	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_RDWR, 0o644)
 	if err != nil {
 		return nil, fmt.Errorf("journal: %w", err)
 	}
+	if err := endWithNewline(f); err != nil {
+		f.Close()
+		return nil, fmt.Errorf("journal: %w", err)
+	}
 	return &Writer{f: f}, nil
 }
 
+// endWithNewline restores a missing final newline. A crash can cut the log's
+// last line short (spec §15.1: krino survives a truncated line); appending
+// straight after the fragment would glue the next run's first line onto it,
+// and that run could then never be undone.
+func endWithNewline(f *os.File) error {
+	fi, err := f.Stat()
+	if err != nil || fi.Size() == 0 {
+		return err
+	}
+	last := make([]byte, 1)
+	if _, err := f.ReadAt(last, fi.Size()-1); err != nil {
+		return err
+	}
+	if last[0] == '\n' {
+		return nil
+	}
+	_, err = f.Write([]byte{'\n'})
+	return err
+}
+
 // Append writes e as one line and flushes it before returning. The whole
 // line is written with a single Write call so that two concurrent runs
 // appending to the same file cannot interleave a partial line.