consensus: handle EBBs in rewindHeaderState

ouroboros-consensus/src/Ouroboros/Consensus/HeaderValidation.hs

@@ -201,12 +201,22 @@ rewindHeaderState cfg p HeaderState{..} = do
     chainState' <- rewindChainState (configConsensus cfg) headerStateChain p
     return $ HeaderState {
         headerStateChain  = chainState'
-      , headerStateTips   = Seq.dropWhileR rolledBack headerStateTips
+      , headerStateTips   = checkEBB $ Seq.dropWhileR tooNew headerStateTips
       , headerStateAnchor = headerStateAnchor
       }
   where
-    rolledBack :: AnnTip blk -> Bool
-    rolledBack t = annTipPoint t > p
+    tooNew :: AnnTip blk -> Bool
+    tooNew t = At (annTipSlotNo t) > pointSlot p
+
+    -- drops one more block if the target's successor shares its slot (this can
+    -- happen when the target is an EBB)
+    checkEBB :: StrictSeq (AnnTip blk) -> StrictSeq (AnnTip blk)
+    checkEBB tips = case tips of
+      -- t' and t have the same slot, t' is the target
+      tips'@(_ Seq.:|> t') Seq.:|> t | At (annTipSlotNo t) == pointSlot p
+                                     , annTipPoint t' == p -> tips'
+      -- either the target is the tip, or the target has been trimmed off
+      _                                                    -> tips
 
 {-------------------------------------------------------------------------------
   Validate header envelope