fix: use real hash for derived from latest (paradigmxyz#6561)

crates/rpc/rpc/src/debug.rs

@@ -410,9 +410,12 @@ where
         let mut at = block.parent_hash;
         let mut replay_block_txs = true;
 
-        // but if all transactions are to be replayed, we can use the state at the block itself
+        // if a transaction index is provided, we need to replay the transactions until the index
         let num_txs = transaction_index.index().unwrap_or(block.body.len());
-        if num_txs == block.body.len() {
+        // but if all transactions are to be replayed, we can use the state at the block itself
+        // this works with the exception of the PENDING block, because its state might not exist if
+        // built locally
+        if !target_block.is_pending() && num_txs == block.body.len() {
             at = block.hash();
             replay_block_txs = false;
         }

crates/rpc/rpc/src/eth/api/mod.rs

@@ -281,18 +281,18 @@ where
             let latest =
                 self.provider().latest_header()?.ok_or_else(|| EthApiError::UnknownBlockNumber)?;
 
-            let (mut latest_header, _block_hash) = latest.split();
+            let (mut latest_header, block_hash) = latest.split();
             // child block
             latest_header.number += 1;
-            // assumed child block is in the next slot
+            // assumed child block is in the next slot: 12s
             latest_header.timestamp += 12;
             // base fee of the child block
             let chain_spec = self.provider().chain_spec();
 
             latest_header.base_fee_per_gas = latest_header
                 .next_block_base_fee(chain_spec.base_fee_params(latest_header.timestamp));
 
-            let block_hash = latest_header.hash_slow();
+            // we're reusing the same block hash because we need this to lookup the block's state
             let latest = SealedHeader::new(latest_header, block_hash);
 
             PendingBlockEnvOrigin::DerivedFromLatest(latest)

crates/rpc/rpc/src/eth/api/pending_block.rs

@@ -309,7 +309,12 @@ where
 pub(crate) enum PendingBlockEnvOrigin {
     /// The pending block as received from the CL.
     ActualPending(SealedBlockWithSenders),
-    /// The header of the latest block
+    /// The _modified_ header of the latest block.
+    ///
+    /// This derives the pending state based on the latest header by modifying:
+    ///  - the timestamp
+    ///  - the block number
+    ///  - fees
     DerivedFromLatest(SealedHeader),
 }
 
@@ -330,7 +335,7 @@ impl PendingBlockEnvOrigin {
     /// Returns the [BlockId] that represents the state of the block.
     ///
     /// If this is the actual pending block, the state is the "Pending" tag, otherwise we can safely
-    /// identify the block by its hash.
+    /// identify the block by its hash (latest block).
     pub(crate) fn state_block_id(&self) -> BlockId {
         match self {
             PendingBlockEnvOrigin::ActualPending(_) => BlockNumberOrTag::Pending.into(),
@@ -339,6 +344,9 @@ impl PendingBlockEnvOrigin {
     }
 
     /// Returns the hash of the block the pending block should be built on.
+    ///
+    /// For the [PendingBlockEnvOrigin::ActualPending] this is the parent hash of the block.
+    /// For the [PendingBlockEnvOrigin::DerivedFromLatest] this is the hash of the _latest_ header.
     fn build_target_hash(&self) -> B256 {
         match self {
             PendingBlockEnvOrigin::ActualPending(block) => block.parent_hash,