avoid unnecessary async copies in broadcast (#3830)

beacon_chain/networking/eth2_network.nim

@@ -2419,31 +2419,30 @@ proc addAsyncValidator*[MsgType](node: Eth2Node,
 proc unsubscribe*(node: Eth2Node, topic: string) =
   node.pubsub.unsubscribeAll(topic)
 
-proc broadcast(node: Eth2Node, topic: string, msg: auto):
+proc gossipEncode(msg: auto): seq[byte] =
+  let uncompressed = SSZ.encode(msg)
+  # This function only for messages we create. A message this large amounts to
+  # an internal logic error.
+  doAssert uncompressed.len <= maxGossipMaxSize()
+
+  snappy.encode(uncompressed)
+
+proc broadcast(node: Eth2Node, topic: string, msg: seq[byte]):
     Future[Result[void, cstring]] {.async.} =
-  try:
-    let uncompressed = SSZ.encode(msg)
+  let peers = await node.pubsub.publish(topic, msg)
 
-    # This is only for messages we create. A message this large amounts to an
-    # internal logic error.
-    doAssert uncompressed.len <= maxGossipMaxSize()
+  # TODO remove workaround for sync committee BN/VC log spam
+  if peers > 0 or find(topic, "sync_committee_") != -1:
+    inc nbc_gossip_messages_sent
+    return ok()
+  else:
+    # Increments libp2p_gossipsub_failed_publish metric
+    return err("No peers on libp2p topic")
 
-    let compressed =
-      try: snappy.encode(uncompressed)
-      except InputTooLarge:
-        raiseAssert "More than 4gb? not likely.."
-
-    let peers = await node.pubsub.publish(topic, compressed)
-
-    # TODO remove workaround for sync committee BN/VC log spam
-    if peers > 0 or find(topic, "sync_committee_") != -1:
-      inc nbc_gossip_messages_sent
-      return ok()
-    else:
-      # Increments libp2p_gossipsub_failed_publish metric
-      return err("No peers on libp2p topic")
-  except IOError as exc:
-    raiseAssert exc.msg # TODO in-memory compression shouldn't fail
+proc broadcast(node: Eth2Node, topic: string, msg: auto):
+    Future[Result[void, cstring]] =
+  # Avoid {.async.} copies of message while broadcasting
+  broadcast(node, topic, gossipEncode(msg))
 
 proc subscribeAttestationSubnets*(
     node: Eth2Node, subnets: AttnetBits, forkDigest: ForkDigest) =