fix: update builder loop to wake up at target block time

src/config.rs

@@ -20,8 +20,9 @@ const QUINCEY_URL: &str = "QUINCEY_URL";
 const BUILDER_PORT: &str = "BUILDER_PORT";
 const SEQUENCER_KEY: &str = "SEQUENCER_KEY"; // empty (to use Quincey) OR AWS key ID (to use AWS signer) OR raw private key (to use local signer)
 const BUILDER_KEY: &str = "BUILDER_KEY"; // AWS key ID (to use AWS signer) OR raw private key (to use local signer)
-const INCOMING_TRANSACTIONS_BUFFER: &str = "INCOMING_TRANSACTIONS_BUFFER";
 const BLOCK_CONFIRMATION_BUFFER: &str = "BLOCK_CONFIRMATION_BUFFER";
+const CHAIN_OFFSET: &str = "CHAIN_OFFSET";
+const TARGET_SLOT_TIME: &str = "TARGET_SLOT_TIME";
 const BUILDER_REWARDS_ADDRESS: &str = "BUILDER_REWARDS_ADDRESS";
 const ROLLUP_BLOCK_GAS_LIMIT: &str = "ROLLUP_BLOCK_GAS_LIMIT";
 const TX_POOL_URL: &str = "TX_POOL_URL";
@@ -57,10 +58,12 @@ pub struct BuilderConfig {
     pub sequencer_key: Option<String>,
     /// Key to access Builder transaction submission wallet - AWS Key ID _OR_ local private key.
     pub builder_key: String,
-    /// Buffer in seconds that Builder will wait & accept incoming transactions before bundling them and submitting as a block.
-    pub incoming_transactions_buffer: u64,
     /// Buffer in seconds in which the `submitBlock` transaction must confirm on the Host chain.
     pub block_confirmation_buffer: u64,
+    /// The offset between Unix time and the chain's block times. For Holesky, this is 0; for Ethereum, 11.
+    pub chain_offset: u64,
+    /// The slot time at which the Builder should begin building a block. 0 to begin at the very start of the slot; 6 to begin in the middle; etc.
+    pub target_slot_time: u64,
     /// Address on Rollup to which Builder will receive user transaction fees.
     pub builder_rewards_address: Address,
     /// Gas limit for RU block.
@@ -147,8 +150,9 @@ impl BuilderConfig {
             builder_port: load_u16(BUILDER_PORT)?,
             sequencer_key: load_string_option(SEQUENCER_KEY),
             builder_key: load_string(BUILDER_KEY)?,
-            incoming_transactions_buffer: load_u64(INCOMING_TRANSACTIONS_BUFFER)?,
             block_confirmation_buffer: load_u64(BLOCK_CONFIRMATION_BUFFER)?,
+            chain_offset: load_u64(CHAIN_OFFSET)?,
+            target_slot_time: load_u64(TARGET_SLOT_TIME)?,
             builder_rewards_address: load_address(BUILDER_REWARDS_ADDRESS)?,
             rollup_block_gas_limit: load_u64(ROLLUP_BLOCK_GAS_LIMIT)?,
             tx_pool_url: load_url(TX_POOL_URL)?,

src/tasks/block.rs

@@ -4,6 +4,7 @@ use alloy::{
 };
 use alloy_primitives::{keccak256, Bytes, B256};
 use alloy_rlp::Buf;
+use std::time::{SystemTime, UNIX_EPOCH};
 use std::{sync::OnceLock, time::Duration};
 use tokio::{sync::mpsc, task::JoinHandle};
 use tracing::Instrument;
@@ -14,6 +15,9 @@ use super::oauth::Authenticator;
 use super::tx_poller::TxPoller;
 use crate::config::BuilderConfig;
 
+/// Ethereum's slot time in seconds.
+pub const ETHEREUM_SLOT_TIME: u64 = 12;
+
 #[derive(Debug, Default, Clone)]
 /// A block in progress.
 pub struct InProgressBlock {
@@ -108,7 +112,6 @@ impl InProgressBlock {
 
 /// BlockBuilder is a task that periodically builds a block then sends it for signing and submission.
 pub struct BlockBuilder {
-    pub incoming_transactions_buffer: u64,
     pub config: BuilderConfig,
     pub tx_poller: TxPoller,
     pub bundle_poller: BundlePoller,
@@ -119,7 +122,6 @@ impl BlockBuilder {
     pub fn new(config: &BuilderConfig, authenticator: Authenticator) -> Self {
         Self {
             config: config.clone(),
-            incoming_transactions_buffer: config.incoming_transactions_buffer,
             tx_poller: TxPoller::new(config),
             bundle_poller: BundlePoller::new(config, authenticator),
         }
@@ -155,15 +157,27 @@ impl BlockBuilder {
         self.bundle_poller.evict();
     }
 
+    // calculate the duration in seconds until the beginning of the next block slot.
+    fn secs_to_next_slot(&mut self) -> u64 {
+        let curr_timestamp: u64 = SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_secs();
+        let current_slot_time = (curr_timestamp - self.config.chain_offset) % ETHEREUM_SLOT_TIME;
+        (ETHEREUM_SLOT_TIME - current_slot_time) % ETHEREUM_SLOT_TIME
+    }
+
+    // add a buffer to the beginning of the block slot.
+    fn secs_to_next_target(&mut self) -> u64 {
+        self.secs_to_next_slot() + self.config.target_slot_time
+    }
+
     /// Spawn the block builder task, returning the inbound channel to it, and
     /// a handle to the running task.
     pub fn spawn(mut self, outbound: mpsc::UnboundedSender<InProgressBlock>) -> JoinHandle<()> {
         tokio::spawn(
             async move {
                 loop {
                     // sleep the buffer time
-                    tokio::time::sleep(Duration::from_secs(self.incoming_transactions_buffer))
-                        .await;
+                    tokio::time::sleep(Duration::from_secs(self.secs_to_next_target())).await;
+                    tracing::trace!("beginning block build cycle");
 
                     // Build a block
                     let mut in_progress = InProgressBlock::default();
@@ -178,6 +192,8 @@ impl BlockBuilder {
                             tracing::debug!("downstream task gone");
                             break;
                         }
+                    } else {
+                        tracing::debug!("no transactions, skipping block submission");
                     }
                 }
             }

src/tasks/oauth.rs

@@ -145,8 +145,9 @@ mod tests {
             builder_port: 8080,
             sequencer_key: None,
             builder_key: "0000000000000000000000000000000000000000000000000000000000000000".into(),
-            incoming_transactions_buffer: 1,
             block_confirmation_buffer: 1,
+            chain_offset: 0,
+            target_slot_time: 1,
             builder_rewards_address: Address::default(),
             rollup_block_gas_limit: 100_000,
             tx_pool_url: "http://localhost:9000/".into(),

tests/bundle_poller_test.rs

@@ -26,8 +26,9 @@ mod tests {
             builder_port: 8080,
             sequencer_key: None,
             builder_key: "0000000000000000000000000000000000000000000000000000000000000000".into(),
-            incoming_transactions_buffer: 1,
             block_confirmation_buffer: 1,
+            chain_offset: 0,
+            target_slot_time: 1,
             builder_rewards_address: Address::default(),
             rollup_block_gas_limit: 100_000,
             tx_pool_url: "http://localhost:9000/".into(),

tests/tx_poller_test.rs

@@ -74,8 +74,9 @@ mod tests {
             builder_port: 8080,
             sequencer_key: None,
             builder_key: "0000000000000000000000000000000000000000000000000000000000000000".into(),
-            incoming_transactions_buffer: 1,
             block_confirmation_buffer: 1,
+            chain_offset: 0,
+            target_slot_time: 1,
             builder_rewards_address: Address::default(),
             rollup_block_gas_limit: 100_000,
             tx_pool_url: "http://localhost:9000/".into(),