Handle the case when a shard may not have a committee at slot.

Block is invalid if contains ShardBlobHeader lacking committee
Reject Gossip ShardBlobHeader and ShardBlob messages which lacks committee

specs/sharding/beacon-chain.md

@@ -455,8 +455,15 @@ def compute_shard_from_committee_index(state: BeaconState, slot: Slot, index: Co
 
 ```python
 def compute_committee_index_from_shard(state: BeaconState, slot: Slot, shard: Shard) -> CommitteeIndex:
+    """
+    Returns either committee index for ``shard`` at ``slot`` or ``None`` if no committee 
+    """
     active_shards = get_active_shard_count(state, compute_epoch_at_slot(slot))
-    return CommitteeIndex((active_shards + shard - get_start_shard(state, slot)) % active_shards)    
+    index = (active_shards + shard - get_start_shard(state, slot)) % active_shards
+    if index >= get_committee_count_per_slot(state, compute_epoch_at_slot(slot)):
+        return None
+    else:
+        return CommitteeIndex(index)     
 ```
 
 
@@ -559,6 +566,7 @@ def update_pending_votes(state: BeaconState, attestation: Attestation) -> None:
 def process_shard_header(state: BeaconState,
                          signed_header: SignedShardBlobHeader) -> None:
     header = signed_header.message
+    committee_index = compute_committee_index_from_shard(state, header.slot, header.shard)
     # Verify the header is not 0, and not from the future.
     assert Slot(0) < header.slot <= state.slot
     header_epoch = compute_epoch_at_slot(header.slot)
@@ -566,6 +574,8 @@ def process_shard_header(state: BeaconState,
     assert header_epoch in [get_previous_epoch(state), get_current_epoch(state)]
     # Verify that the shard is active
     assert header.shard < get_active_shard_count(state, header_epoch)
+    # Verify that shard has a committee at slot
+    assert committee_index is not None
     # Verify that the block root matches,
     # to ensure the header will only be included in this specific Beacon Chain sub-tree.
     assert header.body_summary.beacon_block_root == get_block_root_at_slot(state, header.slot - 1)
@@ -595,8 +605,7 @@ def process_shard_header(state: BeaconState,
     assert header_root not in [pending_header.root for pending_header in pending_headers]
 
     # Include it in the pending list
-    index = compute_committee_index_from_shard(state, header.slot, header.shard)
-    committee_length = len(get_beacon_committee(state, header.slot, index))
+    committee_length = len(get_beacon_committee(state, header.slot, committee_index))
     pending_headers.append(PendingShardHeader(
         slot=header.slot,
         shard=header.shard,
@@ -693,6 +702,9 @@ def process_pending_headers(state: BeaconState) -> None:
 
             # The entire committee (and its balance)
             index = compute_committee_index_from_shard(state, slot, shard)
+            if index is None:
+                # the shard had no committee on this slot
+                continue
             full_committee = get_beacon_committee(state, slot, index)
             # The set of voters who voted for each header (and their total balances)
             voting_sets = [

specs/sharding/p2p-interface.md

@@ -117,6 +117,8 @@ The following validations MUST pass before forwarding the `signed_blob` (with in
   (a client MAY queue future blobs for processing at the appropriate slot).
 - _[IGNORE]_ The `blob` is new enough to be still be processed --
   i.e. validate that `compute_epoch_at_slot(blob.slot) >= get_previous_epoch(state)`
+- _[REJECT]_ The shard should have a committee at slot --
+  i.e. validate that `compute_committee_index_from_shard(state, blob.slot, blob.shard) is not None`
 - _[REJECT]_ The shard blob is for the correct subnet --
   i.e. `compute_subnet_for_shard_blob(state, blob.slot, blob.shard) == subnet_id`  
 - _[IGNORE]_ The blob is the first blob with valid signature received for the `(blob.proposer_index, blob.slot, blob.shard)` combination.
@@ -141,6 +143,8 @@ The following validations MUST pass before forwarding the `signed_shard_header`
 - _[IGNORE]_ The `header` is new enough to be still be processed --
   i.e. validate that `compute_epoch_at_slot(header.slot) >= get_previous_epoch(state)`
 - _[IGNORE]_ The header is the first header with valid signature received for the `(header.proposer_index, header.slot, header.shard)` combination.
+- _[REJECT]_ The shard should have a committee at slot --
+  i.e. validate that `compute_committee_index_from_shard(state, header.slot, header.shard) is not None`
 - _[REJECT]_ The proposer signature, `signed_shard_header.signature`, is valid with respect to the `proposer_index` pubkey.
 - _[REJECT]_ The header is proposed by the expected `proposer_index` for the block's slot
   in the context of the current shuffling (defined by `header.body_summary.beacon_block_root`/`slot`).