Merge pull request #746 from ethereum/min-time-to-exit

Cannot exit until 2048 epochs (#675 item 21) [redo]

specs/core/0_beacon-chain.md

@@ -229,8 +229,9 @@ Code snippets appearing in `this style` are to be interpreted as Python code.
 | `MIN_SEED_LOOKAHEAD` | `2**0` (= 1) | epochs | 6.4 minutes |
 | `ACTIVATION_EXIT_DELAY` | `2**2` (= 4) | epochs | 25.6 minutes |
 | `EPOCHS_PER_ETH1_VOTING_PERIOD` | `2**4` (= 16) | epochs | ~1.7 hours |
-| `SLOTS_PER_HISTORICAL_ROOT` | `2**13` (= 8,192) | slots | ~13 hours | 
+| `SLOTS_PER_HISTORICAL_ROOT` | `2**13` (= 8,192) | slots | ~13 hours |
 | `MIN_VALIDATOR_WITHDRAWABILITY_DELAY` | `2**8` (= 256) | epochs | ~27 hours |
+| `PERSISTENT_COMMITTEE_PERIOD` | `2**11` (= 2,048)  | epochs | 9 days  |
 
 ### State list lengths
 
@@ -2465,6 +2466,8 @@ def process_voluntary_exit(state: BeaconState, exit: VoluntaryExit) -> None:
     assert validator.initiated_exit is False
     # Exits must specify an epoch when they become valid; they are not valid before then
     assert get_current_epoch(state) >= exit.epoch
+    # Must have been in the validator set long enough
+    assert get_current_epoch(state) - validator.activation_epoch >= PERSISTENT_COMMITTEE_PERIOD
     # Verify signature
     assert bls_verify(
         pubkey=validator.pubkey,