From f2c85957f2d8cc7705175d999e87ec4500ab3adb Mon Sep 17 00:00:00 2001
From: Thomas Letan <lthms@nomadic-labs.com>
Date: Tue, 2 Apr 2024 23:14:51 +0200
Subject: [PATCH] EVM Node: Fix off-by-one in the stream RPC for blueprints

When an observer starts, it connects to another EVM node to fetch blueprints,
starting from `next_blueprint_number'.

If (1) it was in sync with its EVM node endpoint, and (2) it is just stooped
and restarted, then from the current implementation of the stream RPC, it is
requesting a blueprint from the future, and the RPC fails with

    [
      {
        "kind": "temporary",
        "id": "failure",
        "msg": "Cannot start watching from a level in the future"
      }
    ]

This patch changes the behavior to allow an observer node to wait for the next
level.

It is not clear why the observer node was not failing without this patch...
---
 etherlink/bin_node/lib_dev/evm_services.ml |  7 +++---
 etherlink/tezt/tests/evm_sequencer.ml      | 27 ++++++++++++++++++++++
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/etherlink/bin_node/lib_dev/evm_services.ml b/etherlink/bin_node/lib_dev/evm_services.ml
index 5e2213a48182..6dcdd95835d7 100644
--- a/etherlink/bin_node/lib_dev/evm_services.ml
+++ b/etherlink/bin_node/lib_dev/evm_services.ml
@@ -41,10 +41,9 @@ let create_blueprint_watcher_service from_level =
   (* input source block creating a stream to observe the events *)
   let* head_info = Evm_context.head_info () in
   let (Qty next) = head_info.next_blueprint_number in
-  let head_level = Z.pred next in
   let* () =
-    if Z.(Compare.(head_level < of_int64 from_level)) then
-      Stdlib.failwith "Cannot start watching from a level in the future"
+    if Z.(Compare.(next < of_int64 from_level)) then
+      Stdlib.failwith "Cannot start watching from a level too far in the future"
     else return_unit
   in
 
@@ -52,7 +51,7 @@ let create_blueprint_watcher_service from_level =
   let next =
     let next_level_requested = ref Z.(of_int64 from_level) in
     fun () ->
-      if Z.Compare.(!next_level_requested <= head_level) then (
+      if Z.Compare.(!next_level_requested < next) then (
         let current_request = !next_level_requested in
         (next_level_requested := Z.(succ current_request)) ;
         let* blueprint = Evm_context.blueprint (Qty current_request) in
diff --git a/etherlink/tezt/tests/evm_sequencer.ml b/etherlink/tezt/tests/evm_sequencer.ml
index 250dd8a829a8..50dda3a10f12 100644
--- a/etherlink/tezt/tests/evm_sequencer.ml
+++ b/etherlink/tezt/tests/evm_sequencer.ml
@@ -1164,6 +1164,32 @@ let test_observer_applies_blueprint =
 
   unit
 
+let test_observer_applies_blueprint_when_restarted =
+  Protocol.register_test
+    ~__FILE__
+    ~tags:["evm"; "observer"]
+    ~title:"Can restart an Observer node"
+    ~uses
+  @@ fun protocol ->
+  (* Start the evm node *)
+  let* {sequencer; observer; _} =
+    setup_sequencer ~time_between_blocks:Nothing protocol
+  in
+
+  (* We produce a block and check the observer applies it. *)
+  let* _ = Evm_node.wait_for_blueprint_applied observer 1
+  and* _ = Rpc.produce_block sequencer in
+
+  (* We restart the observer *)
+  let* () = Evm_node.terminate observer in
+  let* () = Evm_node.run observer in
+
+  (* We produce a block and check the observer applies it. *)
+  let* _ = Evm_node.wait_for_blueprint_applied observer 2
+  and* _ = Rpc.produce_block sequencer in
+
+  unit
+
 let test_observer_forwards_transaction =
   Protocol.register_test
     ~__FILE__
@@ -2726,6 +2752,7 @@ let () =
   test_init_from_rollup_node_data_dir protocols ;
   test_init_from_rollup_node_with_delayed_inbox protocols ;
   test_observer_applies_blueprint protocols ;
+  test_observer_applies_blueprint_when_restarted protocols ;
   test_observer_forwards_transaction protocols ;
   test_sequencer_is_reimbursed protocols ;
   test_upgrade_kernel_auto_sync protocols ;
-- 
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