1 files changed, 75 insertions, 3 deletions

diff --git a/src/swarm/members.rs b/src/swarm/members.rs
index a534c04..1cb4062 100644
--- a/src/swarm/members.rs
+++ b/src/swarm/members.rs
@@ -12,11 +12,12 @@
 //! for people reading /peers and metrics (not unique and never an identity: the public
 //! key stays that), where people reach its dashboard (`--dashboard-url`, `dashboard`),
 //! where it can be reached (`addresses`, as its transport gives them:
-//! hints for the others' transports), and an `alive` stamp the node renews now and
+//! hints for the others' transports), an `alive` stamp the node renews now and
 //! then: the members renewed within `--electorate-window` are the electorate of a
-//! remake.
+//! remake, and where its links differ from the member list's graph (`links`,
+//! `swarm::working_links`).
 
-use std::collections::HashMap;
+use std::collections::{BTreeSet, HashMap};
 use std::time::Duration;
 
 use serde_json::{Value as JsonValue, json};
@@ -426,6 +427,77 @@ pub(crate) async fn ensure_registered(state: &AppState) {
     }
 }
 
+fn links_path(pk: &str) -> Vec<String> {
+    let mut path = member_path(pk);
+    path.push("links".to_owned());
+    path
+}
+
+/// The most members a member may record in each of `RecordedLinks`' sets: more than it
+/// could hold sessions with.
+pub(crate) const MAX_RECORDED_LINKS: usize = 256;
+
+/// Where a member's links differ from the member list's graph, as it recorded them
+/// (`links`, `swarm::working_links`): its neighbours by the graph it doesn't reach
+/// (`unreached`), and the members it reaches the graph doesn't link it to (`reached`).
+/// Both empty while every link the graph gives works, and no other does.
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub(crate) struct RecordedLinks {
+    pub(crate) unreached: BTreeSet<String>,
+    pub(crate) reached: BTreeSet<String>,
+}
+
+impl RecordedLinks {
+    fn is_empty(&self) -> bool {
+        self.unreached.is_empty() && self.reached.is_empty()
+    }
+}
+
+/// `pk`'s recorded links; none if its entry doesn't say (at most `MAX_RECORDED_LINKS`
+/// in each set).
+pub(crate) fn links(doc: &impl ReadDoc, pk: &str) -> RecordedLinks {
+    let Ok(Some(JsonValue::Object(links))) = store::read_path(doc, &links_path(pk)) else {
+        return RecordedLinks::default();
+    };
+    let set = |name: &str| -> BTreeSet<String> {
+        match links.get(name) {
+            Some(JsonValue::Array(items)) => items
+                .iter()
+                .filter_map(JsonValue::as_str)
+                .take(MAX_RECORDED_LINKS)
+                .map(str::to_owned)
+                .collect(),
+            _ => BTreeSet::new(),
+        }
+    };
+    RecordedLinks {
+        unreached: set("unreached"),
+        reached: set("reached"),
+    }
+}
+
+/// Records `pk`'s `links` (at most `MAX_RECORDED_LINKS` in each set); none removes them.
+pub(crate) fn set_links(
+    doc: &mut impl Transactable,
+    pk: &str,
+    links: &RecordedLinks,
+) -> Result<(), StoreError> {
+    if links.is_empty() {
+        return match store::delete_path(doc, &links_path(pk), false) {
+            Err(StoreError::NotFound) => Ok(()),
+            result => result,
+        };
+    }
+    let list = |set: &BTreeSet<String>| -> Vec<String> {
+        set.iter().take(MAX_RECORDED_LINKS).cloned().collect()
+    };
+    let value = json!({
+        "unreached": list(&links.unreached),
+        "reached": list(&links.reached),
+    });
+    store::write_path(doc, &links_path(pk), &value)
+}
+
 /// Removes `pk`'s entry (an operator forgot it). Absent is fine.
 pub(crate) fn unregister(doc: &mut impl Transactable, pk: &str) -> Result<(), StoreError> {
     match store::delete_path(doc, &member_path(pk), false) {