Adds test that affects the proposer selection

tests/core/pyspec/eth2spec/test/phase0/sanity/test_slots.py

@@ -1,11 +1,16 @@
 from eth2spec.test.helpers.forks import (
     is_post_capella,
 )
-from eth2spec.test.helpers.state import get_state_root
 from eth2spec.test.context import (
     spec_state_test,
     with_all_phases,
 )
+from eth2spec.test.helpers.state import (
+    get_state_root,
+    next_epoch,
+    next_slot,
+    transition_to
+)
 
 
 @with_all_phases
@@ -87,3 +92,49 @@ def test_historical_accumulator(spec, state):
         assert len(state.historical_summaries) == len(pre_historical_summaries) + 1
     else:
         assert len(state.historical_roots) == len(pre_historical_roots) + 1
+
+
+@with_all_phases
+@spec_state_test
+def test_balance_change_affects_proposer(spec, state):
+    # Brute-force an instance where a validator's balance change prevents it from proposing.
+    # We must brute-force this because sometimes the balance change doesn't make a difference.
+    # Give this approach 100 attempts to find such a case.
+    for _ in range(100):
+        original_state = state.copy()
+
+        # Get the proposer of the first slot in the next epoch
+        next_epoch_state = state.copy()
+        next_epoch(spec, next_epoch_state)
+        proposer_next_epoch = spec.get_beacon_proposer_index(next_epoch_state)
+
+        # Reduce the validator's balance, making it less likely to propose
+        # The validator's effective balance will be updated during epoch processing
+        spec.decrease_balance(state, proposer_next_epoch, 10 * spec.EFFECTIVE_BALANCE_INCREMENT)
+
+        # Check if the proposer changed as a result of the balance change
+        tmp_state = state.copy()
+        next_epoch(spec, tmp_state)
+        if proposer_next_epoch != spec.get_beacon_proposer_index(tmp_state):
+            # Use this state
+            break
+        else:
+            # Try another state
+            state = original_state.copy()
+            next_epoch(spec, state)
+
+    # Transition to the last slot of the current epoch
+    slot = state.slot + spec.SLOTS_PER_EPOCH - (state.slot % spec.SLOTS_PER_EPOCH) - 1
+    transition_to(spec, state, slot)
+
+    yield 'pre', state
+    yield 'slots', 1
+
+    # Transition to the next epoch
+    next_slot(spec, state)
+
+    yield 'post', state
+
+    # Verify that the proposer changed because of the balance change
+    proposer_next_epoch_after_change = spec.get_beacon_proposer_index(state)
+    assert proposer_next_epoch != proposer_next_epoch_after_change