fix(distributed): a server with no parent figures leaves our cap alone

The speed-derived child limit read a missing ratio as a refusal, so on any
server that never announces ParentSpeedRatio — soulfind among them — the
client carried no children at all, however it was configured. A ratio we
were never told is not a decision: the server's figures set the limit only
when it has given them, and otherwise the configured cap stands, which is
how slskd runs throughout. A speed below the announced minimum still means
none, which is the safety property the rule exists for.

Also stops repeating AcceptChildren: it is a standing state on the server,
so it goes out when the answer changes rather than on every message that
happens to prompt a recount.

soulseek-rs-lib/src/client/children.rs

@@ -32,17 +32,24 @@ use crate::{debug, trace};
 pub const MAX_CHILDREN: usize = 10;
 
 /// The child limit the server's figures imply, following Nicotine+:
-/// `speed / ratio / 100`, capped, and none at all below the minimum speed the
-/// server set. `None` when the server has not reported our speed yet, which
-/// leaves the caller's own cap standing.
+/// `speed / ratio / 100`, capped, and none at all when our recorded speed is
+/// below the minimum the server set.
+///
+/// `None` means the server's figures do not decide it — either it has not
+/// reported our speed yet, or it never announced a ratio at all (soulfind
+/// does not) — and the caller's own cap stands, which is how slskd runs the
+/// whole time.
 #[must_use]
 pub fn limit_from_speed(
     own_speed: Option<u32>,
     min_speed: u32,
     ratio: u32,
 ) -> Option<usize> {
+    if ratio == 0 {
+        return None;
+    }
     let speed = own_speed?;
-    if ratio == 0 || speed < min_speed {
+    if speed < min_speed {
         return Some(0);
     }
     Some((speed / ratio / 100) as usize)
@@ -336,8 +343,8 @@ mod tests {
         );
         assert_eq!(
             limit_from_speed(Some(10_000), 1_024, 0),
-            Some(0),
-            "a zero ratio means the server is not allowing children"
+            None,
+            "a server that never announced a ratio does not decide the limit"
         );
         assert_eq!(
             limit_from_speed(None, 1_024, 50),
@@ -364,6 +371,12 @@ mod tests {
 
         children.set_limits(Some(100), 1_024, 50);
         assert!(!children.has_room(), "a slow client carries none");
+
+        children.set_limits(Some(100), 1_024, 0);
+        assert!(
+            children.has_room(),
+            "a server with no figures leaves our own cap standing"
+        );
         assert_eq!(ends.len(), 2);
     }
 

soulseek-rs-lib/src/client/distributed.rs

@@ -321,8 +321,11 @@ pub fn announce_move(
 ) {
     let server = ctx.server_sender.clone();
     // What the server needs to know is whether it may offer us to another
-    // peer right now, not whether we serve children in principle.
+    // peer right now, not whether we serve children in principle. The stance
+    // carries it, so record it as told: a separate AcceptChildren right
+    // behind this one would say nothing new.
     let has_room = ctx.children.has_room();
+    let _ = ctx.accept_children_change();
     announce(server.as_ref(), branch, has_parent, has_room);
     ctx.children.broadcast_stance(&branch.root, branch.level);
 }

soulseek-rs-lib/src/client/mod.rs

@@ -466,6 +466,9 @@ pub struct ClientContext {
     leaf: distributed::Leaf,
     /// The children hanging from us, when the client serves any.
     pub(crate) children: children::Children,
+    /// The last `AcceptChildren` answer we gave the server, so the same
+    /// answer is not repeated: it is a standing state, not a heartbeat.
+    announced_accept_children: Option<bool>,
     /// The upload speed the server records for us, once it has told us. The
     /// child limit is derived from it.
     own_average_speed: Option<u32>,
@@ -642,6 +645,7 @@ impl ClientContext {
             operations: None,
             leaf: distributed::Leaf::new(""),
             children: children::Children::default(),
+            announced_accept_children: None,
             own_average_speed: None,
             parent_min_speed: 0,
             parent_speed_ratio: 0,
@@ -829,6 +833,16 @@ impl ClientContext {
         self.own_interests.clone()
     }
 
+    /// Take the `AcceptChildren` answer to send, or `None` when the server
+    /// already has this one.
+    pub fn accept_children_change(&mut self) -> Option<bool> {
+        let has_room = self.children.has_room();
+        (self.announced_accept_children != Some(has_room)).then(|| {
+            self.announced_accept_children = Some(has_room);
+            has_room
+        })
+    }
+
     /// The upload speed the server records for us, if it has said.
     #[must_use]
     pub const fn own_average_speed(&self) -> Option<u32> {

soulseek-rs-lib/src/client/operations.rs

@@ -1253,8 +1253,16 @@ impl Client {
     /// (`AcceptChildren`, code 100). Sent whenever that answer changes: the
     /// server only offers us to peers while it is true.
     fn announce_child_capacity(client_context: &Arc<RwLock<ClientContext>>) {
-        let (sender, has_room) = match client_context.read_safe() {
-            Ok(ctx) => (ctx.server_sender.clone(), ctx.children.has_room()),
+        let (sender, has_room) = match client_context.write_safe() {
+            Ok(mut ctx) => {
+                // Only on a change: the server holds this as a standing
+                // state, and repeating it on every message we happen to
+                // receive would be noise.
+                let Some(has_room) = ctx.accept_children_change() else {
+                    return;
+                };
+                (ctx.server_sender.clone(), has_room)
+            }
             Err(_) => return,
         };
         let Some(sender) = sender else { return };

soulseek-rs-lib/src/client/tests.rs

@@ -1126,3 +1126,15 @@ fn leaving_a_room_drops_its_ticker_board() {
         "a board for a room we left is stale"
     );
 }
+
+#[test]
+fn the_server_is_told_our_child_capacity_only_when_it_changes() {
+    // AcceptChildren is a standing state on the server, not a heartbeat.
+    let mut ctx = ClientContext::new();
+    assert_eq!(
+        ctx.accept_children_change(),
+        Some(false),
+        "the first answer is always worth sending"
+    );
+    assert_eq!(ctx.accept_children_change(), None, "unchanged, so silent");
+}