Hydrate destination queues at startup (#1179)

* Hydrate destination queues at startup

* Review comments

federationsender/storage/postgres/queue_pdus_table.go

@@ -63,6 +63,9 @@ const selectQueuePDUsCountSQL = "" +
 	"SELECT COUNT(*) FROM federationsender_queue_pdus" +
 	" WHERE server_name = $1"
 
+const selectQueueServerNamesSQL = "" +
+	"SELECT DISTINCT server_name FROM federationsender_queue_pdus"
+
 type queuePDUsStatements struct {
 	insertQueuePDUStmt                *sql.Stmt
 	deleteQueueTransactionPDUsStmt    *sql.Stmt
@@ -70,6 +73,7 @@ type queuePDUsStatements struct {
 	selectQueuePDUsByTransactionStmt  *sql.Stmt
 	selectQueueReferenceJSONCountStmt *sql.Stmt
 	selectQueuePDUsCountStmt          *sql.Stmt
+	selectQueueServerNamesStmt        *sql.Stmt
 }
 
 func (s *queuePDUsStatements) prepare(db *sql.DB) (err error) {
@@ -95,6 +99,9 @@ func (s *queuePDUsStatements) prepare(db *sql.DB) (err error) {
 	if s.selectQueuePDUsCountStmt, err = db.Prepare(selectQueuePDUsCountSQL); err != nil {
 		return
 	}
+	if s.selectQueueServerNamesStmt, err = db.Prepare(selectQueueServerNamesSQL); err != nil {
+		return
+	}
 	return
 }
 
@@ -190,3 +197,24 @@ func (s *queuePDUsStatements) selectQueuePDUs(
 
 	return result, rows.Err()
 }
+
+func (s *queuePDUsStatements) selectQueueServerNames(
+	ctx context.Context, txn *sql.Tx,
+) ([]gomatrixserverlib.ServerName, error) {
+	stmt := sqlutil.TxStmt(txn, s.selectQueueServerNamesStmt)
+	rows, err := stmt.QueryContext(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "queueFromStmt: rows.close() failed")
+	var result []gomatrixserverlib.ServerName
+	for rows.Next() {
+		var serverName gomatrixserverlib.ServerName
+		if err = rows.Scan(&serverName); err != nil {
+			return nil, err
+		}
+		result = append(result, serverName)
+	}
+
+	return result, rows.Err()
+}

federationsender/storage/postgres/storage.go

@@ -264,3 +264,11 @@ func (d *Database) GetPendingPDUCount(
 ) (int64, error) {
 	return d.selectQueuePDUCount(ctx, nil, serverName)
 }
+
+// GetPendingServerNames returns the server names that have PDUs
+// waiting to be sent.
+func (d *Database) GetPendingServerNames(
+	ctx context.Context,
+) ([]gomatrixserverlib.ServerName, error) {
+	return d.selectQueueServerNames(ctx, nil)
+}