rocksdb: only create single read batch in bg call (#578)

* only create single batch per bg call

* iterate over blocks as they were read

* catch error in case uncaught

* skip read if we do not have block

lib/replicator.js

@@ -1830,26 +1830,19 @@ module.exports = class Replicator {
  async _resolveBlocksLocally () {
  // TODO: check if fork compat etc. Requires that we pass down truncation info
 
- let clear = null
+ const clear = []
+ const blocks = []
 
+ const reader = this.core.storage.createReadBatch()
  for (const b of this._blocks) {
  if (this.core.bitfield.get(b.index) === false) continue
-
- const reader = this.core.storage.createReadBatch()
-
- try {
- const block = this.core.blocks.get(reader, b.index)
- reader.tryFlush()
- b.resolve(await block)
- } catch (err) {
- b.reject(err)
- }
-
- if (clear === null) clear = []
- clear.push(b)
+ blocks.push(this._resolveLocalBlock(b, reader, clear))
  }
+ reader.tryFlush()
+
+ await Promise.all(blocks)
 
- if (clear === null) return
+ if (!clear.length) return
 
  // Currently the block tracker does not support deletes during iteration, so we make
  // sure to clear them afterwards.
@@ -1858,6 +1851,17 @@ module.exports = class Replicator {
  }
  }
 
+ async _resolveLocalBlock (b, reader, resolved) {
+ try {
+ b.resolve(await this.core.blocks.get(reader, b.index))
+ } catch (err) {
+ b.reject(err)
+ return
+ }
+
+ resolved.push(b)
+ }
+
  _resolveBlockRequest (tracker, index, value, req) {
  const b = tracker.remove(index)
  if (b === null) return false