Add test_justify_previous_epoch_with_late_block to improve coverage (…

…#10)

tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py

@@ -790,3 +790,58 @@ def test_proposer_boost_root_same_slot_untimely_block(spec, state):
     })
 
     yield 'steps', test_steps
+
+
+@with_all_phases
+@spec_state_test
+@with_presets([MINIMAL], reason="too slow")
+def test_justify_previous_epoch_with_late_block(spec, state):
+    test_steps = []
+    # Initialization
+    store, anchor_block = get_genesis_forkchoice_store_and_block(spec, state)
+    yield 'anchor_state', state
+    yield 'anchor_block', anchor_block
+    current_time = state.slot * spec.config.SECONDS_PER_SLOT + store.genesis_time
+    on_tick_and_append_step(spec, store, current_time, test_steps)
+    assert store.time == current_time
+
+    for _ in range(2):
+        next_epoch(spec, state)
+
+    for _ in range(2):
+        state, store, _ = yield from apply_next_epoch_with_attestations(
+            spec, state, store, True, True, test_steps=test_steps)
+
+    assert state.current_justified_checkpoint.epoch == store.justified_checkpoint.epoch == 3
+    assert spec.get_current_epoch(state) == 4
+
+    # Move forward time to later epoch, and such that
+    # `compute_slots_since_epoch_start(get_current_slot(store)) == SAFE_SLOTS_TO_UPDATE_JUSTIFIED`
+    slot = spec.SLOTS_PER_EPOCH * 6 + spec.SAFE_SLOTS_TO_UPDATE_JUSTIFIED
+    on_tick_and_append_step(spec, store, store.genesis_time + slot * spec.config.SECONDS_PER_SLOT, test_steps)
+
+    next_slots(spec, state, spec.SLOTS_PER_EPOCH // 2)
+
+    # Now try to justify the checkpoint at epoch 4
+    # At the second half of the epoch, attest the current justification checkpoint
+    state, store, _ = yield from apply_next_slots_with_attestations(
+        spec, state, store, spec.SLOTS_PER_EPOCH // 2 - 1, True, False, test_steps)
+
+    assert spec.get_current_epoch(state) == 4
+    # skip last slot
+    next_slots(spec, state, 1)
+    # ensure that it passed epoch boundary
+    assert spec.get_current_epoch(state) == 5
+    assert state.current_justified_checkpoint.epoch == 3
+    assert store.justified_checkpoint.epoch == 3
+
+    # At the first half of the epoch, attest the *previous* justification checkpoint
+    state, store, _ = yield from apply_next_slots_with_attestations(
+        spec, state, store, spec.SLOTS_PER_EPOCH // 2, False, True, test_steps)
+
+    # During the above execution, `should_update_justified_checkpoint` function returns `True`
+    # via the last line of `should_update_justified_checkpoint`.
+    assert state.current_justified_checkpoint.epoch == 3
+    assert store.justified_checkpoint.epoch == 4
+
+    yield 'steps', test_steps