document two uint64 -> int64 conversions (#2375)

* document two uint64 -> int64 conversions

* fix minimal preset slot time & calculation

beacon_chain/nimbus_validator_client.nim

@@ -13,7 +13,7 @@ import
   stew/shims/[tables, macros],
   chronos, confutils, metrics, json_rpc/[rpcclient, jsonmarshal],
   chronicles,
-  json_serialization/std/[options, sets, net],
+  json_serialization/std/[options, net],
 
   # Local modules
   spec/[datatypes, digest, crypto, helpers, network, signatures],

beacon_chain/sync/sync_manager.nim

@@ -1135,4 +1135,10 @@ proc getInfo*[A, B](man: SyncManager[A, B]): SyncInfo =
   ## Returns current synchronization information for RPC call.
   let wallSlot = man.getLocalWallSlot()
   let headSlot = man.getLocalHeadSlot()
-  SyncInfo(head_slot: headSlot, sync_distance: int64(wallSlot - headSlot))
+
+  # Even with 6 second slots (minimal presets), this overflows about 1.75
+  # trillion years into the chain's existence, if headSlot is 0.
+  let sync_distance = wallSlot - headSlot
+  doAssert sync_distance <= high(int64).uint64
+
+  SyncInfo(head_slot: headSlot, sync_distance: int64(sync_distance))

beacon_chain/time.nim

@@ -25,6 +25,9 @@ type
   BeaconTime* = distinct Duration ## Nanoseconds from beacon genesis time
 
 proc init*(T: type BeaconClock, genesis_time: uint64): T =
+  # ~290 billion years into the future
+  doAssert genesis_time <= high(int64).uint64
+
   let
     unixGenesis = fromUnix(genesis_time.int64)
     # GENESIS_SLOT offsets slot time, but to simplify calculations, we apply that

tests/official/test_fixture_const_sanity_check.nim

@@ -12,7 +12,7 @@ import
   # Status libraries
   stew/[byteutils, endians2],
   # Internals
-  ../../beacon_chain/spec/[datatypes, digest, presets],
+  ../../beacon_chain/spec/[datatypes, presets],
   # Test utilities
   ../testutil, ./fixtures_utils