state: limit refused writes per user

A write every node would refuse is rolled back by forking the document
at the heads before it, under the write lock, at a cost that grows with
the document's history. A user could ask for such writes in a loop.

Count each user's refused writes over the last minute. Past
--refused-writes-per-minute (default 10), its writes are answered 429
without being tried, until the oldest refusal leaves the minute.

1 files changed, 59 insertions, 0 deletions

diff --git a/src/state/refused.rs b/src/state/refused.rs
new file mode 100644
index 0000000..bb37af0
--- /dev/null
+++ b/src/state/refused.rs
@@ -0,0 +1,59 @@
+//! Writes every node would refuse, counted by user (`AppState::mutate_doc_as`): each is
+//! rolled back by forking the document at the heads before it, which costs as much as
+//! the document's history, under the write lock. A user may have so many refused within
+//! a minute (`--refused-writes-per-minute`); past it, its writes are answered `429`
+//! untried until the oldest leave the minute.
+
+use std::collections::{HashMap, VecDeque};
+use std::time::{Duration, Instant};
+
+/// How long a refused write counts against its user.
+const WINDOW: Duration = Duration::from_secs(60);
+
+/// Each user's writes refused within the last `WINDOW`, oldest first.
+#[derive(Debug, Default)]
+pub(crate) struct RefusedWrites(HashMap<String, VecDeque<Instant>>);
+
+impl RefusedWrites {
+    /// Whether `user` may try another write at `now`: fewer than `limit` of its writes
+    /// refused within the last minute.
+    pub(crate) fn allows(&mut self, user: &str, limit: usize, now: Instant) -> bool {
+        let Some(refused) = self.0.get_mut(user) else {
+            return true;
+        };
+        while refused
+            .front()
+            .is_some_and(|at| now.duration_since(*at) >= WINDOW)
+        {
+            refused.pop_front();
+        }
+        if refused.is_empty() {
+            self.0.remove(user);
+            return true;
+        }
+        refused.len() < limit
+    }
+
+    /// One of `user`'s writes refused at `now`.
+    pub(crate) fn note(&mut self, user: &str, now: Instant) {
+        self.0.entry(user.to_owned()).or_default().push_back(now);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Past the limit within a minute, a user waits; others don't; a minute on, it may.
+    #[test]
+    fn refused_writes_per_user() {
+        let mut refused = RefusedWrites::default();
+        let start = Instant::now();
+        assert!(refused.allows("alice", 2, start));
+        refused.note("alice", start);
+        refused.note("alice", start);
+        assert!(!refused.allows("alice", 2, start));
+        assert!(refused.allows("bob", 2, start));
+        assert!(refused.allows("alice", 2, start + WINDOW));
+    }
+}