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, 4 insertions, 1 deletions

diff --git a/docs/access.md b/docs/access.md
index 782724a..9d38bd5 100644
--- a/docs/access.md
+++ b/docs/access.md
@@ -266,7 +266,10 @@ stays as it is, wherever it comes from later (a joining node catching up, say).
 
 A node judges each change it makes the same way before it leaves, its own writes and
 those through the API alike: one every node would refuse is rolled back, never
-written, and the request refused (`403`, saying why). So a write the API's checks let
+written, and the request refused (`403`, saying why). A rollback costs a copy of the
+document, so a user with more than ten such writes within a minute
+(`--refused-writes-per-minute`) has its writes answered `429`, untried, until the
+oldest leave the minute. So a write the API's checks let
 through (they look at the path asked for, the rules at everything the change does)
 can't cut its node off.