perf: optimize further fetchBatched

- Reduce DefaultFetchBatch to prevent blockData explosion
- Add chunked onBatch processing to prevent memory accumulation
- Implement memory buffer reuse to reduce GC allocation churn
- Fix short-circuit evaluation

Part of https://github.com/codex-storage/nim-codex/issues/974

codex/node.nim

@@ -52,7 +52,9 @@ export logutils
 logScope:
   topics = "codex node"
 
-const DefaultFetchBatch = 1_000_000
+const
+  DefaultFetchBatch = 8192
+  MaxOnBatchBlocks = 128
 
 type
   Contracts* =
@@ -186,18 +188,14 @@ proc fetchBatched*(
   #     (i: int) => self.networkStore.getBlock(BlockAddress.init(cid, i))
   #   )
 
+  var addresses = newSeqOfCap[BlockAddress](batchSize)
   while not iter.finished:
-    let addresses = collect:
+    addresses.setLen(0)
-      for i in 0 ..< batchSize:
+    for i in 0 ..< batchSize:
-        if not iter.finished:
+      if not iter.finished:
-          let address = BlockAddress.init(cid, iter.next())
+        let address = BlockAddress.init(cid, iter.next())
-          if not (await address in self.networkStore) or fetchLocal:
+        if fetchLocal or not (await address in self.networkStore):
-            address
+          addresses.add(address)
-
-    proc successful(
-        blk: ?!bt.Block
-    ): Future[bool] {.async: (raises: [CancelledError]).} =
-      return blk.isSuccess()
 
     let blockResults = await self.networkStore.getBlocks(addresses)
 
@@ -218,11 +216,17 @@ proc fetchBatched*(
       if not onBatch.isNil:
         blockData.add(blk)
 
+        if blockData.len >= MaxOnBatchBlocks:
+          if batchErr =? (await onBatch(blockData)).errorOption:
+            return failure(batchErr)
+          blockData = @[]
+
     if failedBlocks > 0:
       return failure("Some blocks failed (Result) to fetch (" & $failedBlocks & ")")
 
-    if not onBatch.isNil and batchErr =? (await onBatch(blockData)).errorOption:
+    if not onBatch.isNil and blockData.len > 0:
-      return failure(batchErr)
+      if batchErr =? (await onBatch(blockData)).errorOption:
+        return failure(batchErr)
 
     if not iter.finished:
       await idleAsync()