1 files changed, 78 insertions, 1 deletions

diff --git a/src/api/access.rs b/src/api/access.rs
index ade6542..63aa87b 100644
--- a/src/api/access.rs
+++ b/src/api/access.rs
@@ -20,7 +20,7 @@ use crate::state::AppState;
 /// What the user of a request may do (a request extension, set by `authorize`).
 #[derive(Clone, Debug)]
 pub(crate) enum Access {
-    /// The swarm has no registry (one from before it, `access.md`): every user the
+    /// The swarm has no registry (`access.md`, A swarm without a registry): every user the
     /// node lets in may do what all could before; protocol data only those in
     /// `--protocol-writers` (`store::parse_write_path`).
     Everything,
@@ -138,6 +138,70 @@ pub(crate) fn prune_shallow(mut value: JsonValue, path: &[String], access: &Acce
     value
 }
 
+/// The registry as a write found it (`registry_kept`): whether it existed, and whether
+/// the writing user could manage users (`registry::managers`).
+#[derive(Clone, Copy)]
+pub(crate) struct Before {
+    existed: bool,
+    managed: bool,
+}
+
+impl Before {
+    pub(crate) fn of(doc: &impl crate::document::ReadDoc, user: &str) -> Self {
+        use crate::access::registry;
+        let existed = registry::exists(doc);
+        Self {
+            existed,
+            managed: existed
+                && registry::caps_of(doc, user)
+                    .allows(crate::access::Right::Write, &registry::users_dir()),
+        }
+    }
+}
+
+/// `409` for a write to `path` by `user` that would lock that user out of managing
+/// users: one that could and no longer may (deleting, disabling or narrowing its own
+/// entry), and the write that makes the registry, if it doesn't make `user` one who may
+/// (from then on only registered users are served). And for one that would remove
+/// the registry, back to a swarm without one. Checked in the write's transaction,
+/// after it: the write is dropped whole. Others are not kept from removing the last who
+/// may manage users: no node could keep two such writes at once on two nodes from
+/// leaving nobody once they merge; then the users in `--protocol-writers` may do
+/// everything again (`access::authorize`).
+pub(crate) fn registry_kept(
+    doc: &impl crate::document::ReadDoc,
+    path: &[String],
+    before: Before,
+    user: &str,
+) -> Result<(), super::ApiError> {
+    use crate::access::registry;
+    if !crate::document::store::paths_related(path, &registry::users_dir()) {
+        return Ok(());
+    }
+    let refuse = |why: &str| {
+        Err(super::ApiError(
+            StatusCode::CONFLICT,
+            Some(format!("{why} (docs/access.md)")),
+        ))
+    };
+    if !registry::exists(doc) {
+        return if before.existed {
+            refuse("this would remove the user registry")
+        } else {
+            Ok(())
+        };
+    }
+    let manages =
+        registry::caps_of(doc, user).allows(crate::access::Right::Write, &registry::users_dir());
+    if !before.existed && !manages {
+        return refuse("the registry's first users must include you, with write on _continuum");
+    }
+    if before.managed && !manages {
+        return refuse("this would keep you from managing users");
+    }
+    Ok(())
+}
+
 /// What an endpoint needs (`needed`).
 #[derive(Debug, PartialEq, Eq)]
 enum Need {
@@ -164,6 +228,14 @@ fn needed(method: &Method, path: &str) -> Need {
         Need::Observe
     } else if under("/peers") {
         protocol(Right::Write, &[meta::FORGOTTEN])
+    } else if under("/admin/users") {
+        // Changing them is checked by their handlers too (`users::may_manage`): in a
+        // swarm without a registry, `--protocol-writers`.
+        if looking {
+            protocol(Right::Read, &[meta::USERS])
+        } else {
+            protocol(Right::Write, &[meta::USERS])
+        }
     } else if under("/admin/remakes") {
         if looking {
             protocol(Right::Read, &[meta::REMAKES])
@@ -296,6 +368,11 @@ mod tests {
             write(&[meta::REMAKES])
         );
         assert_eq!(needed(&Method::GET, "/admin/config"), read(&[]));
+        assert_eq!(
+            needed(&Method::GET, "/admin/users/alice"),
+            read(&[meta::USERS])
+        );
+        assert_eq!(needed(&Method::POST, "/admin/users"), write(&[meta::USERS]));
         assert_eq!(needed(&Method::GET, "/admin/quarantine/q"), read(&[]));
         assert_eq!(
             needed(&Method::POST, "/admin/quarantine/q/replay"),