era: fix verifier at empty slots

* avoid returning zero-byte block data to REST/p2p when loading era
files

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -252,7 +252,7 @@ proc getBlockSSZ*(dag: ChainDAGRef, bid: BlockId, bytes: var seq[byte]): bool =
       getBlockSSZ(
         dag.era, getStateField(dag.headState, historical_roots).asSeq,
         dag.headState.historical_summaries().asSeq,
-        bid.slot, bytes).isOk)
+        bid.slot, bytes).isOk() and bytes.len > 0)
 
 proc getBlockSZ*(dag: ChainDAGRef, bid: BlockId, bytes: var seq[byte]): bool =
   # Load the snappy-frame-compressed ("SZ") SSZ-encoded data of a block into
@@ -265,7 +265,7 @@ proc getBlockSZ*(dag: ChainDAGRef, bid: BlockId, bytes: var seq[byte]): bool =
       getBlockSZ(
         dag.era, getStateField(dag.headState, historical_roots).asSeq,
         dag.headState.historical_summaries().asSeq,
-        bid.slot, bytes).isOk)
+        bid.slot, bytes).isOk and bytes.len > 0)
 
 proc getForkedBlock*(
     dag: ChainDAGRef, bid: BlockId): Opt[ForkedTrustedSignedBeaconBlock] =

beacon_chain/era_db.nim

@@ -104,12 +104,23 @@ proc getBlockSZ*(
 
 proc getBlockSSZ*(
     f: EraFile, slot: Slot, bytes: var seq[byte]): Result[void, string] =
+  ## Get raw SSZ bytes of the block at the given slot - may overwrite
+  ## `bytes` on error.
+  ##
+  ## Sets `bytes` to an empty seq and returns success if there is no block at
+  ## the given slot, according to the index
   var tmp: seq[byte]
   ? f.getBlockSZ(slot, tmp)
+
   let
     len = uncompressedLenFramed(tmp).valueOr:
       return err("Cannot read uncompressed length, era file corrupt?")
 
+  if len == 0:
+    # Given slot is empty
+    reset(bytes)
+    return ok()
+
   if len > int.high.uint64:
     return err("Invalid uncompressed size")
 
@@ -298,6 +309,11 @@ proc getBlockSSZ*(
     db: EraDB, historical_roots: openArray[Eth2Digest],
     historical_summaries: openArray[HistoricalSummary], slot: Slot,
     bytes: var seq[byte]): Result[void, string] =
+  ## Get raw SSZ bytes of the block at the given slot - may overwrite
+  ## `bytes` on error.
+  ##
+  ## Sets `bytes` to an empty seq and returns success if there is no block at
+  ## the given slot according to the index
   let
     f = ? db.getEraFile(historical_roots, historical_summaries, slot.era + 1)
 
@@ -307,13 +323,15 @@ proc getBlock*(
     db: EraDB, historical_roots: openArray[Eth2Digest],
     historical_summaries: openArray[HistoricalSummary], slot: Slot,
     root: Opt[Eth2Digest], T: type ForkyTrustedSignedBeaconBlock): Opt[T] =
-  var tmp: seq[byte]
+  var bytes: seq[byte]
   ? db.getBlockSSZ(
-    historical_roots, historical_summaries, slot, tmp).mapErr(proc(x: auto) = discard)
+    historical_roots, historical_summaries, slot, bytes).mapConvertErr(void)
+  if bytes.len() == 0:
+    return Opt.none(T)
 
   result.ok(default(T))
   try:
-    readSszBytes(tmp, result.get(), updateRoot = root.isNone)
+    readSszBytes(bytes, result.get(), updateRoot = root.isNone)
     if root.isSome():
       result.get().root = root.get()
   except CatchableError:
@@ -350,6 +368,8 @@ proc getState*(
     state: var ForkedHashedBeaconState): Result[void, string] =
   var bytes: seq[byte]
   ? db.getStateSSZ(historical_roots, historical_summaries, slot, bytes)
+  if bytes.len() == 0:
+    return err("State not found")
 
   try:
     state = readSszForkedHashedBeaconState(db.cfg, slot, bytes)