chains Merkle shreds in broadcast fake shreds (#35061)

The commit migrates
    turbine/src/broadcast_stage/broadcast_fake_shreds_run.rs
to use chained Merkle shreds variant.

ledger/src/shred.rs

@@ -692,7 +692,7 @@ pub mod layout {
         Ok(flags & ShredFlags::SHRED_TICK_REFERENCE_MASK.bits())
     }
 
-    pub(crate) fn get_merkle_root(shred: &[u8]) -> Option<Hash> {
+    pub fn get_merkle_root(shred: &[u8]) -> Option<Hash> {
         match get_shred_variant(shred).ok()? {
             ShredVariant::LegacyCode | ShredVariant::LegacyData => None,
             ShredVariant::MerkleCode(proof_size, chained) => {

turbine/src/broadcast_stage.rs

@@ -66,6 +66,8 @@ pub enum Error {
     Blockstore(#[from] solana_ledger::blockstore::BlockstoreError),
     #[error(transparent)]
     ClusterInfo(#[from] solana_gossip::cluster_info::ClusterInfoError),
+    #[error("Invalid Merkle root, slot: {slot}, index: {index}")]
+    InvalidMerkleRoot { slot: Slot, index: u64 },
     #[error(transparent)]
     Io(#[from] std::io::Error),
     #[error(transparent)]
@@ -76,8 +78,14 @@ pub enum Error {
     Send,
     #[error(transparent)]
     Serialize(#[from] std::boxed::Box<bincode::ErrorKind>),
+    #[error("Shred not found, slot: {slot}, index: {index}")]
+    ShredNotFound { slot: Slot, index: u64 },
     #[error(transparent)]
     TransportError(#[from] solana_sdk::transport::TransportError),
+    #[error("Unknown last index, slot: {0}")]
+    UnknownLastIndex(Slot),
+    #[error("Unknown slot meta, slot: {0}")]
+    UnknownSlotMeta(Slot),
 }
 
 type Result<T> = std::result::Result<T, Error>;

turbine/src/broadcast_stage/broadcast_fake_shreds_run.rs

@@ -1,7 +1,7 @@
 use {
     super::*,
     solana_entry::entry::Entry,
-    solana_ledger::shred::{ProcessShredsStats, ReedSolomonCache, Shredder},
+    solana_ledger::shred::{self, ProcessShredsStats, ReedSolomonCache, Shredder},
     solana_sdk::{hash::Hash, signature::Keypair},
 };
 
@@ -45,6 +45,21 @@ impl BroadcastRun for BroadcastFakeShredsRun {
             .expect("Database error")
             .map(|meta| meta.consumed)
             .unwrap_or(0) as u32;
+        let chained_merkle_root = match next_shred_index.checked_sub(1) {
+            None => broadcast_utils::get_chained_merkle_root_from_parent(
+                bank.slot(),
+                bank.parent_slot(),
+                blockstore,
+            )
+            .unwrap(),
+            Some(index) => {
+                let shred = blockstore
+                    .get_data_shred(bank.slot(), u64::from(index))
+                    .unwrap()
+                    .unwrap();
+                shred::layout::get_merkle_root(&shred).unwrap()
+            }
+        };
 
         let num_entries = receive_results.entries.len();
 
@@ -60,7 +75,7 @@ impl BroadcastRun for BroadcastFakeShredsRun {
             keypair,
             &receive_results.entries,
             last_tick_height == bank.max_tick_height(),
-            None, // chained_merkle_root
+            Some(chained_merkle_root),
             next_shred_index,
             self.next_code_index,
             true, // merkle_variant
@@ -82,7 +97,7 @@ impl BroadcastRun for BroadcastFakeShredsRun {
             keypair,
             &fake_entries,
             last_tick_height == bank.max_tick_height(),
-            None, // chained_merkle_root
+            Some(chained_merkle_root),
             next_shred_index,
             self.next_code_index,
             true, // merkle_variant

turbine/src/broadcast_stage/broadcast_utils.rs

@@ -1,12 +1,15 @@
 use {
-    super::Result,
+    super::{Error, Result},
     bincode::serialized_size,
     crossbeam_channel::Receiver,
     solana_entry::entry::Entry,
-    solana_ledger::shred::ShredData,
+    solana_ledger::{
+        blockstore::Blockstore,
+        shred::{self, ShredData},
+    },
     solana_poh::poh_recorder::WorkingBankEntry,
     solana_runtime::bank::Bank,
-    solana_sdk::clock::Slot,
+    solana_sdk::{clock::Slot, hash::Hash},
     std::{
         sync::Arc,
         time::{Duration, Instant},
@@ -96,6 +99,34 @@ pub(super) fn recv_slot_entries(receiver: &Receiver<WorkingBankEntry>) -> Result
     })
 }
 
+// Returns the Merkle root of the last erasure batch of the parent slot.
+pub(super) fn get_chained_merkle_root_from_parent(
+    slot: Slot,
+    parent: Slot,
+    blockstore: &Blockstore,
+) -> Result<Hash> {
+    if slot == parent {
+        debug_assert_eq!(slot, 0u64);
+        return Ok(Hash::default());
+    }
+    debug_assert!(parent < slot, "parent: {parent} >= slot: {slot}");
+    let index = blockstore
+        .meta(parent)?
+        .ok_or_else(|| Error::UnknownSlotMeta(parent))?
+        .last_index
+        .ok_or_else(|| Error::UnknownLastIndex(parent))?;
+    let shred = blockstore
+        .get_data_shred(parent, index)?
+        .ok_or(Error::ShredNotFound {
+            slot: parent,
+            index,
+        })?;
+    shred::layout::get_merkle_root(&shred).ok_or(Error::InvalidMerkleRoot {
+        slot: parent,
+        index,
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use {