s3api: report the bound addresses from the listen hook

`WithOnListen` tells an embedder when the S3 server is serving, but not
where: the callback takes no arguments, and the server reports the
addresses it bound nowhere else. An embedder that wants an ephemeral port
therefore cannot ask for port 0; it has to pick a free port itself,
release it, and pass it in, which loses to any other process that binds
the same port in between.

Add `WithOnListenAddrs`, which passes the callback the address of every
listener the server bound, in port-specification order, so an embedder can
serve on `127.0.0.1:0` and learn the port the kernel chose. `WithOnListen`
is unchanged. `MultiListener` gains `Addrs`, the every-listener counterpart
of `Addr`, to supply them.

s3api/server.go

@@ -61,6 +61,7 @@ type S3ApiServer struct {
 	middlewares      []middlewareMount
 	socketPerm       os.FileMode
 	onListen         func()
+	onListenAddrs    func(addrs []net.Addr)
 }
 
 type routeMount struct {
@@ -355,6 +356,16 @@ func WithOnListen(fn func()) Option {
 	return func(s *S3ApiServer) { s.onListen = fn }
 }
 
+// WithOnListenAddrs is WithOnListen with the addresses the server bound
+// passed to the callback: one per listener, in the order of the port
+// specifications given to ServeMultiPort, with a specification that resolves
+// to several addresses contributing one each. This is how a caller that asked
+// for port 0 learns which port the kernel chose; the server does not report
+// it anywhere else.
+func WithOnListenAddrs(fn func(addrs []net.Addr)) Option {
+	return func(s *S3ApiServer) { s.onListenAddrs = fn }
+}
+
 // ServeMultiPort creates listeners for multiple port specifications and serves
 // on all of them simultaneously. This supports listening on multiple ports and/or
 // addresses (e.g., [":7070", "localhost:8080", "0.0.0.0:9090"]).
@@ -389,10 +400,15 @@ func (sa *S3ApiServer) ServeMultiPort(ports []string) error {
 	// Combine all listeners
 	finalListener := netutil.NewMultiListener(listeners...)
 
-	if sa.onListen != nil {
-		fn := sa.onListen
+	if sa.onListen != nil || sa.onListenAddrs != nil {
+		fn, fnAddrs := sa.onListen, sa.onListenAddrs
 		sa.app.Hooks().OnListen(func(fiber.ListenData) error {
-			fn()
+			if fn != nil {
+				fn()
+			}
+			if fnAddrs != nil {
+				fnAddrs(finalListener.Addrs())
+			}
 			return nil
 		})
 	}