Fix /eth/v1/node/syncing (#3720)

* Fix REST `/eth/v1/node/syncing` call to return values even if SyncManager is not running.

* Use syncManager.inProgress as is_syncing indicator.

beacon_chain/rpc/rest_node_api.nim

@@ -253,7 +253,19 @@ proc installNodeApiHandlers*(router: var RestRouter, node: BeaconNode) =
 
   # https://ethereum.github.io/beacon-APIs/#/Node/getSyncingStatus
   router.api(MethodGet, "/eth/v1/node/syncing") do () -> RestApiResponse:
-    return RestApiResponse.jsonResponse(node.syncManager.getInfo())
+    let
+      wallSlot = node.beaconClock.now().slotOrZero()
+      headSlot = node.dag.head.slot
+      distance = wallSlot - headSlot
+      info = RestSyncInfo(
+        head_slot: headSlot, sync_distance: distance,
+        is_syncing:
+          if isNil(node.syncManager):
+            false
+          else:
+            node.syncManager.inProgress
+      )
+    return RestApiResponse.jsonResponse(info)
 
   # https://ethereum.github.io/beacon-APIs/#/Node/getHealth
   router.api(MethodGet, "/eth/v1/node/health") do () -> RestApiResponse:

beacon_chain/sync/sync_manager.nim

@@ -669,14 +669,3 @@ proc syncLoop[A, B](man: SyncManager[A, B]) {.async.} =
 proc start*[A, B](man: SyncManager[A, B]) =
   ## Starts SyncManager's main loop.
   man.syncFut = man.syncLoop()
-
-proc getInfo*[A, B](man: SyncManager[A, B]): RestSyncInfo =
-  ## Returns current synchronization information for RPC call.
-  let wallSlot = man.getLocalWallSlot()
-  let headSlot = man.getLocalHeadSlot()
-  let sync_distance = wallSlot - headSlot
-  RestSyncInfo(
-    head_slot: headSlot,
-    sync_distance: sync_distance,
-    is_syncing: man.inProgress
-  )