From 7051325ba4becde9390f8748c099ec9a21750c89 Mon Sep 17 00:00:00 2001
From: Benjamin Canou <benjamin@canou.fr>
Date: Mon, 17 Sep 2018 09:18:26 +0200
Subject: [PATCH] P2p, client: fix network protocol version negociation

---
 src/bin_client/main_client.ml |  2 +-
 src/lib_base/p2p_version.ml   | 10 +++++++---
 src/lib_base/p2p_version.mli  | 12 ++++++++++--
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/bin_client/main_client.ml b/src/bin_client/main_client.ml
index 55af0cb36..984c30f04 100644
--- a/src/bin_client/main_client.ml
+++ b/src/bin_client/main_client.ml
@@ -97,7 +97,7 @@ let check_network ctxt =
       default () ;
       Lwt.return_none
   | Ok versions ->
-      match String.split_on_char '_' (List.hd versions).name with
+      match String.split_on_char '_' (P2p_version.best versions).name with
       | "SANDBOXED" :: _ ->
           sandbox () ;
           Lwt.return_some `Sandbox
diff --git a/src/lib_base/p2p_version.ml b/src/lib_base/p2p_version.ml
index 3cb6a4e90..198778c7f 100644
--- a/src/lib_base/p2p_version.ml
+++ b/src/lib_base/p2p_version.ml
@@ -42,8 +42,6 @@ let encoding =
        (req "major" uint16)
        (req "minor" uint16))
 
-(* the common version for a pair of peers, if any, is the maximum one,
-   in lexicographic order *)
 let common la lb =
   let la = List.sort (fun l r -> compare r l) la in
   let lb = List.sort (fun l r -> compare r l) lb in
@@ -51,6 +49,12 @@ let common la lb =
     | [], _ | _, [] -> None
     | ((a :: ta) as la), ((b :: tb) as lb) ->
         if a = b then Some a
-        else if a < b then find (ta, lb)
+        else if a > b then find (ta, lb)
         else find (la, tb)
   in find (la, lb)
+
+let best lv =
+  if lv = [] then
+    invalid_arg "P2p_version.best"
+  else
+    List.hd (List.sort (fun l r -> compare r l) lv)
diff --git a/src/lib_base/p2p_version.mli b/src/lib_base/p2p_version.mli
index 9b52282f5..edc8c11bc 100644
--- a/src/lib_base/p2p_version.mli
+++ b/src/lib_base/p2p_version.mli
@@ -23,16 +23,24 @@
 (*                                                                           *)
 (*****************************************************************************)
 
-(** Protocol version *)
+(** Network protocol version. *)
 
+(** Type of a network protocol version. *)
 type t = {
   name : string ;
   major : int ;
   minor : int ;
 }
-(** Type of a protocol version. *)
 
 val pp : Format.formatter -> t -> unit
 val encoding : t Data_encoding.t
+
+(** Selects the prefered common version for a pair of version
+    lists. Used during network protocol negociation. If any, it is the
+    maximum one, in lexicographic order (name, then major, minor). *)
 val common : t list -> t list -> t option
 
+(** Gives the prefered version in a list: the one selected by
+    {!common} among the list of compatible ones. *)
+val best : t list -> t
+