Shell: add error for blocks inconsistent with the checkpoint

src/lib_shell_services/validation_errors.ml

@@ -231,6 +231,7 @@ let () =
 (************************ Validator errors ********************************)
 
 type error +=  Inactive_chain of Chain_id.t
+type error += Checkpoint_error of Block_hash.t * P2p_peer.Id.t option
 
 let () =
   (* Inactive network *)
@@ -246,4 +247,28 @@ let () =
           Chain_id.pp chain)
     Data_encoding.(obj1 (req "inactive_chain" Chain_id.encoding))
     (function Inactive_chain chain -> Some chain | _ -> None)
-    (fun chain -> Inactive_chain chain)
+    (fun chain -> Inactive_chain chain) ;
+  register_error_kind
+    `Branch
+    ~id:"node.validator.checkpoint_error"
+    ~title: "Block incompatble with the current checkpoint."
+    ~description: "The block belongs to a branch that is not compatible \
+                   with the current checkpoint."
+    ~pp: (fun ppf (block, peer) ->
+        match peer with
+        | None ->
+            Format.fprintf ppf
+              "The block %a is incompatible with the current checkpoint."
+              Block_hash.pp_short block
+        | Some peer ->
+            Format.fprintf ppf
+              "The peer %a send us a block which is a sibling \
+               of the configured checkpoint (%a)."
+              P2p_peer.Id.pp peer
+              Block_hash.pp_short block)
+    Data_encoding.(obj2
+                     (req "block" Block_hash.encoding)
+                     (opt "peer" P2p_peer.Id.encoding))
+    (function Checkpoint_error (block, peer) -> Some (block, peer) | _ -> None)
+    (fun (block, peer) -> Checkpoint_error (block, peer))
+

src/lib_shell_services/validation_errors.mli

@@ -45,3 +45,4 @@ type error +=
 (************************ Validator errors ********************************)
 
 type error +=  Inactive_chain of Chain_id.t
+type error += Checkpoint_error of Block_hash.t * P2p_peer.Id.t option