document file: a clean stop marked, and `--keep-actor` writes on as the same actor

A node writes as a fresh Automerge actor at each start, so no change number of an actor
comes twice: one sent before a crash but not yet appended to the document file would
otherwise be made again, differently. An epoch thus gathers a writer per start, and
every transfer of its history carries a signature per writer.

The document file now says when it holds everything the node wrote:

* at a clean stop, after the last append, an empty `Closed` record, only where the
  file is up to the document's heads (ready, not rejoining);
* as a node starts on the file, before anything is written, an empty `Opened` record,
  so a crash from then on leaves a file without the mark;
* the header names the actor the node writes with (`current`).

With `--keep-actor` (off by default; needs `--document-file`), a node starting on a
file that ends with `Closed` writes on as that actor. A crash, a drop, a switch or a
rejoin still starts a fresh one, so an epoch's writers come to one per node plus one
per such event. lifecycle.md says so.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/swarm/document_file.rs b/src/swarm/document_file.rs
index e066e27..4c7843d 100644
--- a/src/swarm/document_file.rs
+++ b/src/swarm/document_file.rs
@@ -25,6 +25,20 @@ pub(crate) async fn run(state: AppState) {
     }
 }
 
+/// At a clean stop, after the last `flush`: the file marked as holding everything this
+/// node wrote (`DocumentFile::close`), where it does (ready, not rejoining, up to the
+/// document's heads), so a restart may write on as the same actor (`--keep-actor`).
+pub(crate) async fn close(state: &AppState) {
+    let Some(file) = state.document_file.clone() else {
+        return;
+    };
+    if state.remake.is_rejoining() || !state.is_ready().await {
+        return;
+    }
+    let heads = state.doc.read().await.doc.get_heads();
+    file.close(&heads);
+}
+
 /// Writes what is due now (`DocumentFile::due`). Also called at shutdown.
 pub(crate) async fn flush(state: &AppState) {
     let Some(file) = state.document_file.clone() else {