fix: return correct flashblock index in check_for_prestate

Author: alessandromazza98

crates/flashblocks/payload/src/generator.rs

@@ -274,6 +274,9 @@ impl<Builder, Client, Tasks> FlashblocksPayloadJobGenerator<Client, Tasks, Build
 where
     Builder: PayloadBuilder<BuiltPayload = OpBuiltPayload>,
 {
+    /// Check for any existing flashblock for the same payload id exists, and if so
+    /// reduces all existing flashblocks into a payload, returning the payload and the
+    /// next flashblock index to build on top of.
     fn check_for_pre_state(
         &self,
         attributes: &<Builder as PayloadBuilder>::Attributes,
@@ -303,7 +306,7 @@ where
                     None,
                 );
 
-                return Ok(Some((payload, flashblock.flashblock().index)));
+                return Ok(Some((payload, flashblock.flashblock().index + 1)));
             }
         }
 

crates/flashblocks/primitives/src/flashblocks.rs

@@ -155,6 +155,8 @@ impl Flashblock {
                 acc.base = next.flashblock.base;
             }
 
+            acc.index = next.flashblock.index;
+
             acc.metadata.fees = next.flashblock.metadata.fees;
             acc.metadata.flashblock_timestamp = next.flashblock.metadata.flashblock_timestamp;