Add gossip replication between nodes

Nodes run anti-entropy over a TCP link: each advertises its version
vector, the peer replies with the log entries it lacks, and ingest is
idempotent so re-delivery and reconnect after a split both converge.
The engine is shared behind a mutex; config gains [gossip] and [[peer]].

src/engine/mod.rs

@@ -9,7 +9,7 @@ use std::time::{Duration, Instant};
 use base64::{engine::general_purpose::STANDARD, Engine as _};
 
 use crate::proto::{NetAction, NetEvent, RegReply};
-use db::{Db, RegError};
+use db::{Db, LogEntry, RegError};
 use scram::Verifier;
 use service::{Sender, Service, ServiceCtx};
 use state::Network;
@@ -91,6 +91,29 @@ impl Engine {
         self.db.scram_iterations
     }
 
+    // Gossip pass-throughs to the account store, used by the replication layer.
+    pub fn gossip_digest(&self) -> HashMap<String, u64> {
+        self.db.version_vector()
+    }
+
+    pub fn gossip_missing(&self, peer: &HashMap<String, u64>) -> Vec<LogEntry> {
+        self.db.missing_for(peer)
+    }
+
+    pub fn gossip_ingest(&mut self, entry: LogEntry) -> std::io::Result<()> {
+        self.db.ingest(entry)
+    }
+
+    #[cfg(test)]
+    pub(crate) fn test_register(&mut self, name: &str) {
+        self.db.register(name, "pw", None).unwrap();
+    }
+
+    #[cfg(test)]
+    pub(crate) fn test_has_account(&self, name: &str) -> bool {
+        self.db.exists(name)
+    }
+
     // Insert or refresh a client's in-progress SASL session, stamped now.
     fn stash_sasl(&mut self, client: String, session: SaslSession) {
         self.sasl_sessions.insert(client, TimedSession { touched: Instant::now(), session });