feat(proxy): hand a bare /region on a lobby to the lobby's world map (#75)

/region <code> still transfers. Without a code, a player on a lobby now gets the
lobby's map dialog for picking the default region: hasPermission declines that
one case, and Velocity forwards a declined command to the current server. On any
other server the bare command still lists the regions. Lobbies are recognised by
GROUNDS_LOBBY_SERVER_PREFIX (default "lobby"), the prefix plugin-lobby uses.

Author: hbrombeer

velocity/src/main/kotlin/gg/grounds/proxy/velocity/GroundsProxyPlugin.kt

@@ -166,9 +166,20 @@ constructor(private val proxy: ProxyServer, private val logger: Logger) {
         // Looked up per invocation rather than captured: the registry is written during startup and
         // a reference taken now could be the one from before another plugin registered.
         val service = { ProxyServiceRegistry.get(ProxyService::class.java) }
+        // The same prefix plugin-lobby uses to tell lobbies from other backends.
+        val lobbyServerPrefix =
+            System.getenv("GROUNDS_LOBBY_SERVER_PREFIX")?.trim()?.takeIf(String::isNotEmpty)
+                ?: "lobby"
         val region = { RegionCatalog.currentCode() }
         proxy.commandManager.register("online", OnlineCommand(service))
-        proxy.commandManager.register("region", RegionCommand(catalog, region, service))
+        proxy.commandManager.register(
+            "region",
+            RegionCommand(catalog, region, service) { player ->
+                player.currentServer
+                    .map { it.serverInfo.name.startsWith(lobbyServerPrefix, ignoreCase = true) }
+                    .orElse(false)
+            },
+        )
 
         // How many players each server holds, answered to a backend that asks. Registered here
         // rather than in the lobby's own plugin because the number only exists on this side:

velocity/src/main/kotlin/gg/grounds/proxy/velocity/command/RegionCommand.kt

@@ -17,13 +17,27 @@ import net.kyori.adventure.text.format.NamedTextColor
  *
  * Geo-steering already puts a player in the nearest region; this is the override — for playing with
  * someone on the other side of the world, or for testing a region deliberately.
+ *
+ * A bare `/region` from a player on a lobby is not answered here: the lobby draws a world map for
+ * picking the default region, and Velocity hands a command the proxy declines in [hasPermission] to
+ * the player's current server. Everywhere else the bare command still lists the regions.
  */
 class RegionCommand(
     private val catalog: RegionCatalog,
     private val currentRegion: () -> String?,
     private val proxyService: () -> ProxyService?,
+    private val isOnLobby: (Player) -> Boolean = { false },
 ) : SimpleCommand {
 
+    override fun hasPermission(invocation: SimpleCommand.Invocation): Boolean {
+        val player = invocation.source() as? Player
+        return regionAnswersHere(
+            isPlayer = player != null,
+            hasArguments = invocation.arguments().isNotEmpty(),
+            onLobby = player != null && isOnLobby(player),
+        )
+    }
+
     override fun execute(invocation: SimpleCommand.Invocation) {
         val source = invocation.source()
         val player = source as? Player
@@ -98,3 +112,14 @@ class RegionCommand(
         return catalog.codes.filter { it.lowercase().startsWith(typed) }
     }
 }
+
+/**
+ * Whether the proxy handles a `/region`. Only a player's bare command on a lobby is declined, which
+ * makes Velocity forward it to the lobby's world map. Top level so a test can reach it without the
+ * Velocity API, which is compile-only.
+ */
+internal fun regionAnswersHere(
+    isPlayer: Boolean,
+    hasArguments: Boolean,
+    onLobby: Boolean,
+): Boolean = !isPlayer || hasArguments || !onLobby

velocity/src/test/kotlin/gg/grounds/proxy/velocity/RegionCommandTest.kt

@@ -0,0 +1,22 @@
+package gg.grounds.proxy.velocity
+
+import gg.grounds.proxy.velocity.command.regionAnswersHere
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+class RegionCommandTest {
+
+    /** Declining is what makes Velocity forward the command to the lobby's world map. */
+    @Test
+    fun `a bare region on a lobby goes to the lobby`() {
+        assertFalse(regionAnswersHere(isPlayer = true, hasArguments = false, onLobby = true))
+    }
+
+    @Test
+    fun `a region code, another server or the console stays on the proxy`() {
+        assertTrue(regionAnswersHere(isPlayer = true, hasArguments = true, onLobby = true))
+        assertTrue(regionAnswersHere(isPlayer = true, hasArguments = false, onLobby = false))
+        assertTrue(regionAnswersHere(isPlayer = false, hasArguments = false, onLobby = false))
+    }
+}