Logging changes. Use --debug-file for a replaced log. Use --append to append to the log file if it exists. Also fixed a bug where cthulhu could try to restart without replacing the old running version.

internal/logging/async.go

@@ -359,11 +359,17 @@ type fileWriter struct {
 	closeOnce sync.Once
 }
 
-func OpenFile(path string, fallback io.Writer) (DiagnosticsWriter, error) {
+func OpenFile(path string, fallback io.Writer, appendLog bool) (DiagnosticsWriter, error) {
 	if path == "" || path == "." || path == ".." {
 		return nil, errors.New("invalid diagnostics file path")
 	}
-	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0o600)
+	// Keep writes at the end even after truncation: an outgoing reader may
+	// still append its shutdown records while --replace is in progress.
+	flags := os.O_WRONLY | os.O_CREATE | os.O_APPEND
+	if !appendLog {
+		flags |= os.O_TRUNC
+	}
+	file, err := os.OpenFile(path, flags, 0o600)
 	if err != nil {
 		return nil, fmt.Errorf("open diagnostics file: %w", err)
 	}

internal/logging/async_test.go

@@ -294,7 +294,7 @@ func TestAsyncRepeatedStopAfterSuccess(t *testing.T) {
 
 func TestOpenFileIsAppendOnlyOwnerRestrictedAndCancellable(t *testing.T) {
 	path := filepath.Join(t.TempDir(), "diagnostics.log")
-	writer, err := OpenFile(path, nil)
+	writer, err := OpenFile(path, nil, true)
 	if err != nil {
 		t.Fatalf("OpenFile: %v", err)
 	}
@@ -312,7 +312,7 @@ func TestOpenFileIsAppendOnlyOwnerRestrictedAndCancellable(t *testing.T) {
 	if info.Mode().Perm() != 0o600 {
 		t.Fatalf("file mode = %o, want 0600", info.Mode().Perm())
 	}
-	writer, err = OpenFile(path, nil)
+	writer, err = OpenFile(path, nil, true)
 	if err != nil {
 		t.Fatalf("reopen: %v", err)
 	}
@@ -332,3 +332,57 @@ func TestOpenFileIsAppendOnlyOwnerRestrictedAndCancellable(t *testing.T) {
 }
 
 var _ io.Writer = (*controlledWriter)(nil)
+
+func TestOpenFileReplacesPreviousContents(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "diagnostics.log")
+	if err := os.WriteFile(path, []byte("previous session with longer contents\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	writer, err := OpenFile(path, nil, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := writer.Write([]byte("new session\n")); err != nil {
+		t.Fatal(err)
+	}
+	if err := writer.Close(); err != nil {
+		t.Fatal(err)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != "new session\n" {
+		t.Fatalf("file contents = %q", data)
+	}
+}
+
+func TestFreshLogPreservesWritesFromOutgoingReader(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "diagnostics.log")
+	old, err := OpenFile(path, nil, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer old.Close()
+	if _, err := old.Write([]byte("old session\n")); err != nil {
+		t.Fatal(err)
+	}
+	next, err := OpenFile(path, nil, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer next.Close()
+	if _, err := old.Write([]byte("outgoing shutdown\n")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := next.Write([]byte("new session\n")); err != nil {
+		t.Fatal(err)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != "outgoing shutdown\nnew session\n" {
+		t.Fatalf("overlapping reader writes = %q", data)
+	}
+}