stats: the writers of the document

How many writers (Automerge actors) the document's changes have matters: a writer is a
node's run (each start, rejoin or drop makes one, a start keeping the one of before with
`--keep-actor` after a clean stop), and every transfer of the whole history carries a
signature for each. `/admin/stats` now says it (`document.writers`, and
`document_writers` for Prometheus), counted from the changes' metadata alongside the
saved size's measurement in the background (at least a minute apart), as that takes
time proportional to the document too; an export, which measures the size alone, keeps
the count of before. The dashboard's figures show it with the rest.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/document/mod.rs b/src/document/mod.rs
index c183537..a546d63 100644
--- a/src/document/mod.rs
+++ b/src/document/mod.rs
@@ -13,6 +13,16 @@ pub(crate) use automerge::{
     ScalarValue,
 };
 
+/// How many writers (actors) `doc`'s changes have: from the changes' metadata, in
+/// time proportional to the document.
+pub(crate) fn writers(doc: &Document) -> usize {
+    doc.get_changes_meta(&[])
+        .into_iter()
+        .map(|meta| meta.actor.into_owned())
+        .collect::<std::collections::HashSet<_>>()
+        .len()
+}
+
 /// The whole document as bytes, as the engine saves it: a backup, the document
 /// file's full save, the size a joining node downloads.
 pub(crate) fn save(doc: &Document) -> Vec<u8> {