Reset withdrawable epoch when depositing to an exited builder (#5373)

@brech1 [has pointed
out](https://notes.ethereum.org/@brech1/crowd-validator-sweep) an
easy-to-abuse way of slowing down the validator sweep. This PR
implements his suggested solution of resetting the builder's
withdrawable epoch on deposit top-ups so that it cannot re-use those
funds to sustain this attack. My napkin math tells me that it would
require 75 ETH (~$150k) per minute to sustain this attack. This solution
would prevent an attack from quickly getting that ETH back to
re-deposit.

Author: jtraglia

specs/gloas/beacon-chain.md

@@ -1587,9 +1587,16 @@ def apply_deposit_for_builder(
         if is_valid_deposit_signature(pubkey, withdrawal_credentials, amount, signature):
             add_builder_to_registry(state, pubkey, withdrawal_credentials, amount, slot)
     else:
-        # Increase balance by deposit amount
         builder_index = builder_pubkeys.index(pubkey)
-        state.builders[builder_index].balance += amount
+        builder = state.builders[builder_index]
+
+        # Increase balance by deposit amount
+        builder.balance += amount
+
+        # If exited, reset the withdrawable epoch
+        if builder.withdrawable_epoch != FAR_FUTURE_EPOCH:
+            epoch = get_current_epoch(state)
+            builder.withdrawable_epoch = epoch + MIN_BUILDER_WITHDRAWABILITY_DELAY
 ```
 
 ###### Modified `process_deposit_request`

tests/core/pyspec/eth_consensus_specs/test/gloas/block_processing/test_process_deposit_request.py

@@ -205,6 +205,56 @@ def test_process_deposit_request__builder_top_up_large(spec, state):
     )
 
 
+@with_gloas_and_later
+@spec_state_test
+def test_process_deposit_request__exited_builder_top_up_resets_withdrawable_epoch(spec, state):
+    """
+    Test top-up to an exited builder resets its withdrawable epoch.
+
+    Input State Configured:
+        - Existing builder at index 0 that has exited (withdrawable_epoch in the past)
+
+    Output State Verified:
+        - Builder balance increased (top-up)
+        - withdrawable_epoch reset to current_epoch + MIN_BUILDER_WITHDRAWABILITY_DELAY
+    """
+    builder_pubkey = state.builders[0].pubkey
+    amount = spec.MIN_DEPOSIT_AMOUNT
+    pre_builder_count = len(state.builders)
+
+    # Advance an epoch and mark builder 0 as exited (withdrawable_epoch in the past)
+    deposit_request = prepare_process_deposit_request(
+        spec,
+        state,
+        for_builder=True,
+        pubkey=builder_pubkey,
+        amount=amount,
+        signed=True,
+        advance_epochs=1,
+        builder_modifications={0: {"withdrawable_epoch": "current_epoch-1"}},
+    )
+    pre_state = state.copy()
+    expected_withdrawable_epoch = (
+        spec.get_current_epoch(state) + spec.config.MIN_BUILDER_WITHDRAWABILITY_DELAY
+    )
+    # Sanity check: the exited epoch differs from the expected reset value
+    assert state.builders[0].withdrawable_epoch != expected_withdrawable_epoch
+
+    yield from run_deposit_request_processing(spec, state, deposit_request)
+
+    assert_process_deposit_request(
+        spec,
+        state,
+        pre_state,
+        deposit_request=deposit_request,
+        is_builder_deposit=True,
+        expected_builder_count=pre_builder_count,
+        expected_builder_index=0,
+        expected_builder_balance_delta=amount,
+        expected_builder_withdrawable_epoch=expected_withdrawable_epoch,
+    )
+
+
 #
 # Invalid signature tests
 #