add note about how slashings and exits can interact. add test

specs/phase0/validator.md

@@ -358,6 +358,10 @@ The `proof` for each deposit must be constructed against the deposit root contai
 
 Up to `MAX_VOLUNTARY_EXITS`, [`VoluntaryExit`](./beacon-chain.md#voluntaryexit) objects can be included in the `block`. The exits must satisfy the verification conditions found in [exits processing](./beacon-chain.md#voluntary-exits).
 
+*Note*: If a slashing for a validator is included in the same block as a
+voluntary exit, the voluntary exit will fail and cause the block to be invalid
+due to the slashing being processed first. Implementers must take heed of this
+operation interaction when packing blocks.
 
 #### Packaging into a `SignedBeaconBlock`
 

tests/core/pyspec/eth2spec/test/helpers/voluntary_exits.py

@@ -1,4 +1,19 @@
 from eth2spec.utils import bls
+from eth2spec.test.helpers.keys import privkeys
+
+
+def prepare_signed_exits(spec, state, indices):
+    domain = spec.get_domain(state, spec.DOMAIN_VOLUNTARY_EXIT)
+
+    def create_signed_exit(index):
+        exit = spec.VoluntaryExit(
+            epoch=spec.get_current_epoch(state),
+            validator_index=index,
+        )
+        signing_root = spec.compute_signing_root(exit, domain)
+        return spec.SignedVoluntaryExit(message=exit, signature=bls.Sign(privkeys[index], signing_root))
+
+    return [create_signed_exit(index) for index in indices]
 
 
 def sign_voluntary_exit(spec, state, voluntary_exit, privkey):

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

@@ -9,7 +9,7 @@ from eth2spec.test.helpers.block import (
     sign_block,
     transition_unsigned_block,
 )
-from eth2spec.test.helpers.keys import privkeys, pubkeys
+from eth2spec.test.helpers.keys import pubkeys
 from eth2spec.test.helpers.attester_slashings import (
     get_valid_attester_slashing_by_indices,
     get_valid_attester_slashing,
@@ -18,6 +18,7 @@ from eth2spec.test.helpers.attester_slashings import (
 from eth2spec.test.helpers.proposer_slashings import get_valid_proposer_slashing, check_proposer_slashing_effect
 from eth2spec.test.helpers.attestations import get_valid_attestation
 from eth2spec.test.helpers.deposits import prepare_state_and_deposit
+from eth2spec.test.helpers.voluntary_exits import prepare_signed_exits
 from eth2spec.test.helpers.shard_transitions import get_shard_transition_of_committee
 
 from eth2spec.test.context import (
@@ -794,20 +795,6 @@ def test_attestation(spec, state):
     assert spec.hash_tree_root(state.previous_epoch_attestations) == pre_current_attestations_root
 
 
-def prepare_signed_exits(spec, state, indices):
-    domain = spec.get_domain(state, spec.DOMAIN_VOLUNTARY_EXIT)
-
-    def create_signed_exit(index):
-        exit = spec.VoluntaryExit(
-            epoch=spec.get_current_epoch(state),
-            validator_index=index,
-        )
-        signing_root = spec.compute_signing_root(exit, domain)
-        return spec.SignedVoluntaryExit(message=exit, signature=bls.Sign(privkeys[index], signing_root))
-
-    return [create_signed_exit(index) for index in indices]
-
-
 # In phase1 a committee is computed for SHARD_COMMITTEE_PERIOD slots ago,
 # exceeding the minimal-config randao mixes memory size.
 # Applies to all voluntary-exit sanity block tests.

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

@@ -0,0 +1,53 @@
+from eth2spec.test.helpers.state import (
+    state_transition_and_sign_block,
+)
+from eth2spec.test.helpers.block import (
+    build_empty_block_for_next_slot,
+)
+from eth2spec.test.helpers.proposer_slashings import get_valid_proposer_slashing
+from eth2spec.test.helpers.voluntary_exits import prepare_signed_exits
+
+from eth2spec.test.context import (
+    spec_state_test,
+    with_all_phases,
+)
+
+
+def run_slash_and_exit(spec, state, slash_index, exit_index, valid=True):
+    # move state forward SHARD_COMMITTEE_PERIOD epochs to allow for exit
+    state.slot += spec.SHARD_COMMITTEE_PERIOD * spec.SLOTS_PER_EPOCH
+
+    yield 'pre', state
+
+    block = build_empty_block_for_next_slot(spec, state)
+
+    proposer_slashing = get_valid_proposer_slashing(
+        spec, state, slashed_index=slash_index, signed_1=True, signed_2=True)
+    signed_exit = prepare_signed_exits(spec, state, [exit_index])[0]
+
+    block.body.proposer_slashings.append(proposer_slashing)
+    block.body.voluntary_exits.append(signed_exit)
+
+    signed_block = state_transition_and_sign_block(spec, state, block, expect_fail=(not valid))
+
+    yield 'blocks', [signed_block]
+
+    if valid:
+        yield 'post', state
+    else:
+        yield 'post', None
+
+
+@with_all_phases
+@spec_state_test
+def test_slash_and_exit_same_index(spec, state):
+    validator_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
+    run_slash_and_exit(spec, state, validator_index, validator_index, valid=False)
+
+
+@with_all_phases
+@spec_state_test
+def test_slash_and_exit_diff_index(spec, state):
+    slash_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
+    exit_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-2]
+    run_slash_and_exit(spec, state, slash_index, exit_index)