fix: reset builder withdrawable epoch only if balance swept

packages/state-transition/src/block/processBuilderDepositRequest.ts

@@ -33,11 +33,13 @@ export function processBuilderDepositRequest(
 
   const builder = state.builders.get(builderIndex);
 
-  // Increase balance by deposit amount
-  builder.balance += amount;
-
-  // If exited, reset the withdrawable epoch
-  if (builder.withdrawableEpoch !== FAR_FUTURE_EPOCH) {
+  // If the builder has exited and been fully swept (balance drained to 0), reset the
+  // withdrawable epoch so this top-up becomes withdrawable again. Must run before the
+  // balance increase, since the reset is gated on the current balance being 0.
+  if (builder.withdrawableEpoch !== FAR_FUTURE_EPOCH && builder.balance === 0) {
     builder.withdrawableEpoch = computeEpochAtSlot(state.slot) + state.config.MIN_BUILDER_WITHDRAWABILITY_DELAY;
   }
+
+  // Increase balance by deposit amount
+  builder.balance += amount;
 }

packages/state-transition/test/unit/block/processBuilderDepositRequest.test.ts

@@ -146,19 +146,19 @@ describe("processBuilderDepositRequest", () => {
     expect(builder.version).toBe(BUILDER_WITHDRAWAL_PREFIX);
   });
 
-  it("resets the withdrawable epoch when topping up an exited builder", () => {
+  it("resets the withdrawable epoch when topping up an exited, fully-swept builder", () => {
     const slot = SLOTS_PER_EPOCH * 2;
     const state = buildGloasState(slot);
     const pubkey = Uint8Array.from({length: 48}, (_, i) => i + 1);
     const executionAddress = Uint8Array.from({length: 20}, (_, i) => i + 1);
 
-    // Exited builder: finite withdrawableEpoch
+    // Exited and fully swept builder: finite withdrawableEpoch, zero balance
     state.builders.push(
       ssz.gloas.Builder.toViewDU({
         pubkey,
         version: BUILDER_WITHDRAWAL_PREFIX,
         executionAddress,
-        balance: 1_000_000_000,
+        balance: 0,
         depositEpoch: 0,
         withdrawableEpoch: 1,
       })
@@ -169,8 +169,36 @@ describe("processBuilderDepositRequest", () => {
     processBuilderDepositRequest(state, request);
 
     const builder = state.builders.get(0);
-    expect(builder.balance).toBe(2_000_000_000);
+    expect(builder.balance).toBe(1_000_000_000);
     const currentEpoch = Math.floor(slot / SLOTS_PER_EPOCH);
     expect(builder.withdrawableEpoch).toBe(currentEpoch + state.config.MIN_BUILDER_WITHDRAWABILITY_DELAY);
   });
+
+  it("does not reset the withdrawable epoch when topping up an exited builder with nonzero balance", () => {
+    const slot = SLOTS_PER_EPOCH * 2;
+    const state = buildGloasState(slot);
+    const pubkey = Uint8Array.from({length: 48}, (_, i) => i + 1);
+    const executionAddress = Uint8Array.from({length: 20}, (_, i) => i + 1);
+
+    // Exited builder that has NOT been swept: finite withdrawableEpoch, nonzero balance.
+    // Per spec, the reset is gated on balance == 0, so the withdrawableEpoch must be preserved.
+    state.builders.push(
+      ssz.gloas.Builder.toViewDU({
+        pubkey,
+        version: BUILDER_WITHDRAWAL_PREFIX,
+        executionAddress,
+        balance: 1_000_000_000,
+        depositEpoch: 0,
+        withdrawableEpoch: 1,
+      })
+    );
+
+    const request = makeBuilderDepositRequest({pubkey, executionAddress, amount: 1_000_000_000});
+
+    processBuilderDepositRequest(state, request);
+
+    const builder = state.builders.get(0);
+    expect(builder.balance).toBe(2_000_000_000);
+    expect(builder.withdrawableEpoch).toBe(1);
+  });
 });