Merge pull request #2452 from Nashatyrev/fix/no-committee-for-shard-case

[Sharding] Handle the case when committee count < active shards

specs/sharding/p2p-interface.md

@@ -124,6 +124,8 @@ 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)` doesn't raise an error
 - _[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.
@@ -152,6 +154,8 @@ The following validations MUST pass before forwarding the `signed_shard_blob_hea
 - _[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)` doesn't raise an error
 - _[REJECT]_ The proposer signature, `signed_shard_blob_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`).