hive-matrix: prefill the chat client with the server name, not the hive domain

docs/2-handbook/swarm/matrix.md

@@ -104,6 +104,12 @@ at the server name, which names `<gatewayHost>:443`
 ([`gateway.md::Discovery flow`](../../4-implementation/networking/gateway.md#discovery-flow-matrix)).
 Port 8448 isn't used.
 
+The web chat client at `chat.<swarm-domain>` signs in the same way: its
+login dialog preselects the server name and looks the homeserver up through
+that discovery answer. That's the swarm domain by default, and the hive
+domain on a homeserver that pinned `serverName` to it. Typing the server name
+into any other Matrix client uses the same value.
+
 Outbound, the container sits on the hive bridge like the CI runner: it
 leases an address from the gateway's DHCP, resolves through its dnsmasq,
 and leaves through the bridge's NAT. It reaches the secret store,

nix/host-modules/hive-matrix.nix

@@ -9,10 +9,6 @@ let
   networkCfg = config.services.hyperhive.network;
   tlsCfg = config.services.hyperhive.deploy.hive-controller.tls;
   gatewayCfg = config.services.hyperhive.gateway;
-  # The hive's own domain; null on a homeserver host that runs no hive, where
-  # the client is pointed at the server name instead.
-  hyperhiveDomain = config.services.hyperhive.domain;
-
   rootfsCheck = import ./lib/container-rootfs-check.nix { inherit config; };
 
   # Same runtime→build-time bridge hive-forge and swarm-ci-runner cross:
@@ -554,14 +550,13 @@ in
           };
           # FluffyChat boot-config pre-fill so the client's
           # `.well-known/matrix/client` lookup hits the right delegation
-          # endpoint. A hive host names its hive domain; a homeserver host
+          # endpoint. That lookup reads the server name, which the swarm apex
-          # without a hive has none and names the server name.
+          # (or the hive vhost a pinned server name equals) answers for, not
+          # the hive domain.
           "= /config.json" = {
             extraConfig = ''
               default_type application/json;
-              return 200 '{"defaultHomeserver":"${
+              return 200 '{"defaultHomeserver":"${effectiveServerName}"}';
-                if hyperhiveDomain != null then hyperhiveDomain else effectiveServerName
-              }"}';
             '';
           };
         };

nix/module-eval/matrix-core.nix

@@ -63,6 +63,17 @@ let
         in
         lib.hasInfix "client_max_body_size 100048576;" c && !(lib.hasInfix "client_max_body_size 50M" c);
     }
+    {
+      # The chat client signs in against the server name, which for a default
+      # swarm is the swarm domain; the hive domain (`h1.t.local`) is a vhost
+      # that answers no discovery there, so a prefill of it is a dead end.
+      name = "the chat client's homeserver prefill is the server name, not the hive domain";
+      ok =
+        let
+          c = matrixBodyCap.services.nginx.virtualHosts."chat.t.local".locations."= /config.json".extraConfig;
+        in
+        lib.hasInfix ''{"defaultHomeserver":"t.local"}'' c && !(lib.hasInfix "h1.t.local" c);
+    }
     {
       # Absence arm. A hive with no client identity gets no
       # store environment at all — the daemon reports a queue it cannot serve