Resolve IRC client hostnames off the accept loop (closes #156)

acceptLoop called newConn, which did the reverse DNS lookup, with its
three-second timeout, before the loop could accept the next
connection, so one client whose address had no answering reverse DNS
held up every other IRC connect. serve now does the lookup at its
start, on the connection's own goroutine, and the accept loop only
accepts.

The server keeps the lookup function in a field, as it does the ping
times, so that a test can give it one that blocks. The new test
checks that a second client registers while the first one's lookup
has not returned.

Model: opus-5-5

internal/ircserver/export_test.go

@@ -15,7 +15,8 @@ import (
 
 // NewTestServer creates a Server suitable for testing, whose
 // connections use interval and deadline in place of
-// pingInterval and pongDeadline.
+// pingInterval and pongDeadline, and resolve in place of
+// resolveHost.
 // The caller must call Stop() when finished.
 func NewTestServer(
 	log *slog.Logger,
@@ -23,6 +24,7 @@ func NewTestServer(
 	database *db.Database,
 	brk *broker.Broker,
 	interval, deadline time.Duration,
+	resolve func(ctx context.Context, addr string) string,
 ) *Server {
 	globs := &globals.Globals{
 		Appname:   "neoirc",
@@ -44,9 +46,16 @@ func NewTestServer(
 
 		pingInterval: interval,
 		pongDeadline: deadline,
+		resolveHost:  resolve,
 	}
 }
 
+// ResolveHost is the reverse DNS lookup that a server made by
+// New does for each client.
+func ResolveHost(ctx context.Context, addr string) string {
+	return resolveHost(ctx, addr)
+}
+
 // Start exposes the unexported start method for tests.
 func (s *Server) Start(addr string) error {
 	return s.start(context.Background(), addr)
@@ -97,10 +106,7 @@ func NewTestConn(
 	tcpConn net.Conn,
 	nick string,
 ) *Conn {
-	conn := newConn(
-		context.Background(), tcpConn, log,
-		nil, nil, cfg, nil,
-	)
+	conn := newConn(tcpConn, log, nil, nil, cfg, nil)
 	conn.nick = nick
 
 	return conn