From c16129a02d72733bfa60dca517cbef44d0f457ec Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gr=C3=A9goire=20Henry?= <gregoire.henry@tezos.com>
Date: Thu, 10 May 2018 17:00:26 +0200
Subject: [PATCH] Shell: early detection of branches that forks before the
 checkpoint.

---
 src/lib_shell/bootstrap_pipeline.ml | 36 ++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/src/lib_shell/bootstrap_pipeline.ml b/src/lib_shell/bootstrap_pipeline.ml
index 5c3f702b8..323f8b3d5 100644
--- a/src/lib_shell/bootstrap_pipeline.ml
+++ b/src/lib_shell/bootstrap_pipeline.ml
@@ -31,14 +31,32 @@ type t = {
   mutable errors: Error_monad.error list ;
 }
 
-let fetch_step pipeline (step : Block_locator.step)  =
+let assert_acceptable_header pipeline
+    ?(first = false) hash (header : Block_header.t) =
+  let chain_state = Distributed_db.chain_state pipeline.chain_db in
+  fail_unless
+    (Time.(now () >= header.shell.timestamp))
+    (Future_block_header hash) >>=? fun () ->
+  State.Chain.checkpoint chain_state >>= fun (level, checkpoint) ->
+  fail_when
+    (Int32.equal header.shell.level level &&
+     not (Block_hash.equal checkpoint hash))
+    (Checkpoint_error (hash, Some pipeline.peer_id)) >>=? fun () ->
+  (* Early detection of fork point before the current checkpoint. *)
+  Chain.head chain_state >>= fun head ->
+  let bootstraping = (State.Block.header head).shell.level < level in
+  fail_when (first && not bootstraping && header.shell.level < level)
+    (Checkpoint_error (hash, Some pipeline.peer_id)) >>=? fun () ->
+  return ()
+
+let fetch_step pipeline ?first (step : Block_locator.step)  =
   lwt_log_info "fetching step %a -> %a (%d%s) from peer %a."
     Block_hash.pp_short step.block
     Block_hash.pp_short step.predecessor
     step.step
     (if step.strict_step then "" else " max")
     P2p_peer.Id.pp_short pipeline.peer_id >>= fun () ->
-  let rec fetch_loop acc hash cpt =
+  let rec fetch_loop ?first acc hash cpt =
     Lwt_unix.yield () >>= fun () ->
     if cpt < 0 then
       lwt_log_info "invalid step from peer %a (too long)."
@@ -61,15 +79,13 @@ let fetch_step pipeline (step : Block_locator.step)  =
           pipeline.chain_db ~peer:pipeline.peer_id
           hash ()
       end >>=? fun header ->
-      fail_unless
-        (Time.(now () >= header.shell.timestamp))
-        (Future_block_header hash) >>=? fun () ->
+      assert_acceptable_header ?first pipeline hash header >>=? fun () ->
       lwt_debug "fetched block header %a from peer %a."
         Block_hash.pp_short hash
         P2p_peer.Id.pp_short pipeline.peer_id >>= fun () ->
       fetch_loop ((hash, header) :: acc) header.shell.predecessor (cpt - 1)
   in
-  fetch_loop [] step.block step.step >>=? fun headers ->
+  fetch_loop ?first [] step.block step.step >>=? fun headers ->
   iter_s
     begin fun header ->
       protect ~canceler:pipeline.canceler begin fun () ->
@@ -86,8 +102,12 @@ let headers_fetch_worker_loop pipeline =
        the point of view of the node sending the locator *)
     let seed = {Block_locator.sender_id=pipeline.peer_id; receiver_id=sender_id } in
     let steps = Block_locator.to_steps seed pipeline.locator in
-    iter_s (fetch_step pipeline) steps >>=? fun () ->
-    return ()
+    match steps with
+    | [] -> return ()
+    | step :: steps ->
+        fetch_step pipeline ~first:true step >>=? fun () ->
+        iter_s (fetch_step pipeline) steps >>=? fun () ->
+        return ()
   end >>= function
   | Ok () ->
       lwt_log_info "fetched all step from peer %a."