fix(federation): Queue PDUs during remote joins

src/core/config/check.rs

@@ -115,6 +115,13 @@ pub fn check(config: &Config) -> Result {
 		));
 	}
 
+	if config.remote_join_pdu_queue_capacity == 0 {
+		return Err!(Config(
+			"remote_join_pdu_queue_capacity",
+			"The remote join PDU queue capacity must be greater than zero"
+		));
+	}
+
 	// yeah, unless the user built a debug build hopefully for local testing only
 	if cfg!(not(debug_assertions)) && config.server_name == "your.server.name" {
 		return Err!(Config(

src/core/config/mod.rs

@@ -555,6 +555,12 @@ pub struct Config {
 	#[serde(default = "default_max_concurrent_inbound_transactions")]
 	pub max_concurrent_inbound_transactions: usize,
 
+	/// Maximum number of federation PDUs queued while completing a remote join.
+	///
+	/// default: 50
+	#[serde(default = "default_remote_join_pdu_queue_capacity")]
+	pub remote_join_pdu_queue_capacity: usize,
+
 	/// Maximum age (in seconds) for cached federation transaction responses.
 	/// Entries older than this will be removed during cleanup.
 	///
@@ -3006,6 +3012,8 @@ fn default_max_fetch_prev_events() -> u16 { 1024 }
 
 fn default_max_concurrent_inbound_transactions() -> usize { 150 }
 
+fn default_remote_join_pdu_queue_capacity() -> usize { 50 }
+
 fn default_transaction_id_cache_max_age_secs() -> u64 { 60 * 60 * 2 }
 
 fn default_transaction_id_cache_max_entries() -> usize { 8192 }