feat: implement builder registry and withdrawal flows

src/beacon_chain/mod.rs

@@ -2,5 +2,6 @@ pub mod constants;
 pub mod containers;
 pub mod process_payload_attestation;
 pub mod process_payload_bid;
+pub mod registry;
 pub mod types;
 pub mod withdrawals;

src/beacon_chain/registry.rs

@@ -0,0 +1,122 @@
+//! Builder registry and withdrawal flows (consensus-specs inspired).
+//!
+//! This module keeps the state surface minimal while enforcing:
+//! - bounded registry size (`BUILDER_REGISTRY_LIMIT`)
+//! - bounded pending withdrawals (`BUILDER_PENDING_WITHDRAWALS_LIMIT`)
+//! - withdrawability gating by epoch (`withdrawable_epoch`)
+//!
+//! It does not implement deposits; tests inject builders directly via the
+//! state trait to keep the scope small.
+
+use crate::beacon_chain::{
+    constants::{
+        BUILDER_PENDING_WITHDRAWALS_LIMIT, BUILDER_REGISTRY_LIMIT,
+        MAX_BUILDERS_PER_WITHDRAWALS_SWEEP,
+    },
+    containers::{Builder, BuilderPendingWithdrawal},
+    types::{BuilderIndex, Epoch, Gwei},
+};
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum RegistryError {
+    #[error("builder registry full (limit {0})")]
+    RegistryFull(u64),
+
+    #[error("builder {0} already exists")]
+    BuilderExists(BuilderIndex),
+
+    #[error("unknown builder {0}")]
+    UnknownBuilder(BuilderIndex),
+
+    #[error("builder {index} not withdrawable until epoch {required}, current {current}")]
+    NotWithdrawable {
+        index: BuilderIndex,
+        required: Epoch,
+        current: Epoch,
+    },
+
+    #[error("builder {index} balance {balance} insufficient for withdrawal {amount}")]
+    InsufficientBalance {
+        index: BuilderIndex,
+        balance: Gwei,
+        amount: Gwei,
+    },
+
+    #[error("pending withdrawals queue full (limit {0})")]
+    PendingQueueFull(u64),
+}
+
+pub trait RegistryState {
+    fn builder_count(&self) -> u64;
+    fn get_builder(&self, index: BuilderIndex) -> Option<Builder>;
+    fn insert_builder(&mut self, index: BuilderIndex, builder: Builder);
+    fn debit_builder_balance(&mut self, index: BuilderIndex, amount: Gwei);
+
+    fn push_pending_withdrawal(&mut self, w: BuilderPendingWithdrawal);
+    fn pending_withdrawals_len(&self) -> u64;
+    fn pop_pending_withdrawals(&mut self, max: u64) -> Vec<BuilderPendingWithdrawal>;
+}
+
+/// Register a builder if space permits.
+pub fn register_builder<S: RegistryState>(
+    state: &mut S,
+    index: BuilderIndex,
+    builder: Builder,
+) -> Result<(), RegistryError> {
+    if state.builder_count() >= BUILDER_REGISTRY_LIMIT {
+        return Err(RegistryError::RegistryFull(BUILDER_REGISTRY_LIMIT));
+    }
+    if state.get_builder(index).is_some() {
+        return Err(RegistryError::BuilderExists(index));
+    }
+    state.insert_builder(index, builder);
+    Ok(())
+}
+
+/// Request a withdrawal of builder balance into the execution layer.
+pub fn request_builder_withdrawal<S: RegistryState>(
+    state: &mut S,
+    index: BuilderIndex,
+    amount: Gwei,
+    current_epoch: Epoch,
+) -> Result<BuilderPendingWithdrawal, RegistryError> {
+    let builder = state
+        .get_builder(index)
+        .ok_or(RegistryError::UnknownBuilder(index))?;
+
+    if builder.withdrawable_epoch > current_epoch {
+        return Err(RegistryError::NotWithdrawable {
+            index,
+            required: builder.withdrawable_epoch,
+            current: current_epoch,
+        });
+    }
+    if builder.balance < amount {
+        return Err(RegistryError::InsufficientBalance {
+            index,
+            balance: builder.balance,
+            amount,
+        });
+    }
+    if state.pending_withdrawals_len() >= BUILDER_PENDING_WITHDRAWALS_LIMIT {
+        return Err(RegistryError::PendingQueueFull(
+            BUILDER_PENDING_WITHDRAWALS_LIMIT,
+        ));
+    }
+
+    let withdrawal = BuilderPendingWithdrawal {
+        fee_recipient: builder.execution_address,
+        amount,
+        builder_index: index,
+    };
+
+    state.debit_builder_balance(index, amount);
+    state.push_pending_withdrawal(withdrawal.clone());
+    Ok(withdrawal)
+}
+
+/// Pop up to `MAX_BUILDERS_PER_WITHDRAWALS_SWEEP` pending withdrawals for inclusion.
+pub fn sweep_pending_withdrawals<S: RegistryState>(state: &mut S) -> Vec<BuilderPendingWithdrawal> {
+    state.pop_pending_withdrawals(MAX_BUILDERS_PER_WITHDRAWALS_SWEEP)
+}

tests/unit/registry_test.rs

@@ -0,0 +1,117 @@
+use eip_7732::beacon_chain::{
+    constants::{
+        BUILDER_PENDING_WITHDRAWALS_LIMIT, BUILDER_REGISTRY_LIMIT,
+        MAX_BUILDERS_PER_WITHDRAWALS_SWEEP,
+    },
+    containers::{Builder, BuilderPendingWithdrawal},
+    registry::{register_builder, request_builder_withdrawal, sweep_pending_withdrawals, RegistryError, RegistryState},
+    types::*,
+};
+use std::collections::{HashMap, VecDeque};
+
+struct MockRegistry {
+    builders: HashMap<BuilderIndex, Builder>,
+    pending: VecDeque<BuilderPendingWithdrawal>,
+    pending_len_override: Option<u64>,
+}
+
+impl MockRegistry {
+    fn new() -> Self {
+        Self { builders: HashMap::new(), pending: VecDeque::new(), pending_len_override: None }
+    }
+    fn builder(index: BuilderIndex, balance: Gwei, withdrawable_epoch: Epoch) -> Builder {
+        Builder {
+            pubkey: [0u8; 48],
+            version: 0,
+            execution_address: [0u8; 20],
+            balance,
+            deposit_epoch: 0,
+            withdrawable_epoch,
+        }
+    }
+}
+
+impl RegistryState for MockRegistry {
+    fn builder_count(&self) -> u64 { self.builders.len() as u64 }
+    fn get_builder(&self, index: BuilderIndex) -> Option<Builder> {
+        self.builders.get(&index).cloned()
+    }
+    fn insert_builder(&mut self, index: BuilderIndex, builder: Builder) {
+        self.builders.insert(index, builder);
+    }
+    fn debit_builder_balance(&mut self, index: BuilderIndex, amount: Gwei) {
+        if let Some(b) = self.builders.get_mut(&index) {
+            b.balance -= amount;
+        }
+    }
+    fn push_pending_withdrawal(&mut self, w: BuilderPendingWithdrawal) {
+        self.pending.push_back(w);
+    }
+    fn pending_withdrawals_len(&self) -> u64 {
+        self.pending_len_override.unwrap_or(self.pending.len() as u64)
+    }
+    fn pop_pending_withdrawals(&mut self, max: u64) -> Vec<BuilderPendingWithdrawal> {
+        let mut out = Vec::new();
+        for _ in 0..max {
+            if let Some(w) = self.pending.pop_front() {
+                out.push(w);
+            } else {
+                break;
+            }
+        }
+        out
+    }
+}
+
+#[test]
+fn register_builder_succeeds() {
+    let mut reg = MockRegistry::new();
+    let b = MockRegistry::builder(1, 10_000, 5);
+    assert!(register_builder(&mut reg, 1, b).is_ok());
+    assert_eq!(reg.builder_count(), 1);
+}
+
+#[test]
+fn register_builder_duplicate_rejected() {
+    let mut reg = MockRegistry::new();
+    let b = MockRegistry::builder(1, 10_000, 5);
+    register_builder(&mut reg, 1, b.clone()).unwrap();
+    let err = register_builder(&mut reg, 1, b).unwrap_err();
+    assert!(matches!(err, RegistryError::BuilderExists(1)));
+}
+
+#[test]
+fn withdrawal_not_withdrawable_yet() {
+    let mut reg = MockRegistry::new();
+    register_builder(&mut reg, 1, MockRegistry::builder(1, 10_000, 10)).unwrap();
+    let err = request_builder_withdrawal(&mut reg, 1, 1_000, 5).unwrap_err();
+    assert!(matches!(err, RegistryError::NotWithdrawable { .. }));
+}
+
+#[test]
+fn withdrawal_insufficient_balance() {
+    let mut reg = MockRegistry::new();
+    register_builder(&mut reg, 1, MockRegistry::builder(1, 500, 0)).unwrap();
+    let err = request_builder_withdrawal(&mut reg, 1, 1_000, 0).unwrap_err();
+    assert!(matches!(err, RegistryError::InsufficientBalance { .. }));
+}
+
+#[test]
+fn pending_queue_limit_enforced() {
+    let mut reg = MockRegistry::new();
+    register_builder(&mut reg, 1, MockRegistry::builder(1, 10_000, 0)).unwrap();
+    reg.pending_len_override = Some(BUILDER_PENDING_WITHDRAWALS_LIMIT);
+    let err = request_builder_withdrawal(&mut reg, 1, 1, 0).unwrap_err();
+    assert!(matches!(err, RegistryError::PendingQueueFull(_)));
+}
+
+#[test]
+fn sweep_respects_max_builders_per_sweep() {
+    let mut reg = MockRegistry::new();
+    register_builder(&mut reg, 1, MockRegistry::builder(1, 10_000, 0)).unwrap();
+    for _ in 0..(MAX_BUILDERS_PER_WITHDRAWALS_SWEEP + 2) {
+        request_builder_withdrawal(&mut reg, 1, 1, 0).unwrap();
+    }
+    let swept = sweep_pending_withdrawals(&mut reg);
+    assert_eq!(swept.len() as u64, MAX_BUILDERS_PER_WITHDRAWALS_SWEEP);
+}