Lwt_utils: connect: add a ?timeout argument

This is to replace the default UNIX timeout that can be very long (30s
or so). In the context of baking, it is not acceptable to wait for
such a long time to connect to e.g. a signer daemon whenever there is
multiple addresses available for load balancing.

src/lib_stdlib_unix/lwt_utils_unix.ml

@@ -288,7 +288,7 @@ module Socket = struct
     | Error (`Msg _) -> host
     | Ok ipaddr -> Ipaddr.to_string ipaddr
 
-  let connect = function
+  let connect ?(timeout=5.) = function
     | Unix path ->
         let addr = Lwt_unix.ADDR_UNIX path in
         let sock = Lwt_unix.socket PF_UNIX SOCK_STREAM 0 in
@@ -300,22 +300,24 @@ module Socket = struct
         | [] ->
             failwith "could not resolve host '%s'" host
         | addrs ->
-            let rec try_connect = function
+            let rec try_connect acc = function
               | [] ->
-                  failwith "could not connect to '%s'" host
+                  Lwt.return
+                    (Error (failure "could not connect to '%s'" host :: List.rev acc))
               | { Unix.ai_family; ai_socktype; ai_protocol; ai_addr } :: addrs ->
                   let sock = Lwt_unix.socket ai_family ai_socktype ai_protocol in
-                  Lwt.catch
+                  protect ~on_error:begin fun e ->
-                    (fun () ->
+                    Lwt_unix.close sock >>= fun () ->
-                       Lwt_unix.connect sock ai_addr >>= fun () ->
+                    Lwt.return (Error e)
-                       return sock)
+                  end begin fun () ->
-                    (fun exn ->
+                    with_timeout (Lwt_unix.sleep timeout) (fun _c ->
-                       Format.printf "@{<error>@{<title>Unable to connect to %s@}@}@.\
+                        Lwt_unix.connect sock ai_addr >>= fun () ->
-                                     \  @[<h 0>%a@]@."
+                        return sock)
-                         host Format.pp_print_text (Printexc.to_string exn) ;
+                  end >>= function
-                       Lwt_unix.close sock >>= fun () ->
+                  | Ok sock -> return sock
-                       try_connect addrs) in
+                  | Error e ->
-            try_connect addrs
+                      try_connect (e @ acc) addrs in
+            try_connect [] addrs
 
   let bind ?(backlog = 10) = function
     | Unix path ->

src/lib_stdlib_unix/lwt_utils_unix.mli

@@ -78,7 +78,18 @@ module Socket : sig
     | Unix of string
     | Tcp of string * string * Unix.getaddrinfo_option list
 
-  val connect: addr -> Lwt_unix.file_descr tzresult Lwt.t
+  val connect:
+    ?timeout:float -> addr -> Lwt_unix.file_descr tzresult Lwt.t
+  (** [connect ?timeout addr] tries connecting to [addr] and returns
+      the resulting socket file descriptor on success. When using TCP,
+      [Unix.getaddrinfo] is used to resolve the hostname and service
+      (port). The different socket addresses returned by
+      [Unix.getaddrinfo] are tried sequentially, and the [?timeout]
+      argument (default: 5s) governs how long it waits to get a
+      connection. If a connection is not obtained in less than
+      [?timeout], the connection is canceled and and the next socket
+      address (if it exists) is tried. *)
+
   val bind:
     ?backlog:int -> addr -> Lwt_unix.file_descr list tzresult Lwt.t