fix mainnet attestation test

test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py

@@ -1,6 +1,7 @@
 from eth2spec.test.context import spec_state_test, expect_assertion_error, always_bls, with_all_phases, with_phases
 from eth2spec.test.helpers.attestations import (
     get_valid_attestation,
+    sign_aggregate_attestation,
     sign_attestation,
 )
 from eth2spec.test.helpers.state import (
@@ -207,20 +208,21 @@ def test_old_target_epoch(spec, state):
 @with_all_phases
 @spec_state_test
 def test_future_target_epoch(spec, state):
-    # TODO: fails in mainnet,
-    # something with committee indices resulting in out of range validator indices (we only have 512 test validators).
-    # Disabled in mainnet for now.
-    if spec.SHARD_COUNT > 8:
-        return
-
     assert spec.MIN_ATTESTATION_INCLUSION_DELAY < spec.SLOTS_PER_EPOCH * 2
 
     attestation = get_valid_attestation(spec, state)
 
-    state.slot += spec.MIN_ATTESTATION_INCLUSION_DELAY
-
+    participants = spec.get_attesting_indices(
+        state,
+        attestation.data,
+        attestation.aggregation_bits
+    )
     attestation.data.target.epoch = spec.get_current_epoch(state) + 1  # target epoch will be too new to handle
-    sign_attestation(spec, state, attestation)
+
+    # manually add signature for correct participants
+    attestation.signature = sign_aggregate_attestation(spec, state, attestation.data, participants)
+
+    state.slot += spec.MIN_ATTESTATION_INCLUSION_DELAY
 
     yield from run_attestation_processing(spec, state, attestation, False)