refactor: make markRequested idempotent

Returns false on duplicate marking attempts instead of logging errors,
eliminating duplicate marking loop in blockPresenceHandler and
preventing duplicate block requests across concurrent flows.

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

codex/blockexchange/engine/engine.nim

@@ -78,10 +78,9 @@ declareCounter(
 )
 declareCounter(
   codex_block_exchange_requests_failed_total,
-  "Total number of block requests that failed after exhausting retries"
+  "Total number of block requests that failed after exhausting retries",
 )
 
-
 const
   DefaultMaxPeersPerRequest* = 10
   # The default max message length of nim-libp2p is 100 megabytes, meaning we can
@@ -296,7 +295,7 @@ proc downloadInternal(
       let scheduledPeer =
         if not self.pendingBlocks.isRequested(address):
           let peer = self.selectPeer(peers.with)
-          self.pendingBlocks.markRequested(address, peer.id)
+          discard self.pendingBlocks.markRequested(address, peer.id)
           peer.blockRequestScheduled(address)
           trace "Request block from block retry loop"
           await self.sendWantBlock(@[address], peer)
@@ -435,12 +434,11 @@ proc blockPresenceHandler*(
 
   let ourWantCids = ourWantList.filterIt(
     it in peerHave and not self.pendingBlocks.retriesExhausted(it) and
-      not self.pendingBlocks.isRequested(it)
+      self.pendingBlocks.markRequested(it, peer)
   ).toSeq
 
   for address in ourWantCids:
     self.pendingBlocks.decRetries(address)
-    self.pendingBlocks.markRequested(address, peer)
     peerCtx.blockRequestScheduled(address)
 
   if ourWantCids.len > 0:

codex/blockexchange/engine/pendingblocks.nim

@@ -155,15 +155,18 @@ func getRequestPeer*(self: PendingBlocksManager, address: BlockAddress): ?PeerId
   self.blocks.withValue(address, pending):
     result = pending[].requested
 
-proc markRequested*(self: PendingBlocksManager, address: BlockAddress, peer: PeerId) =
+proc markRequested*(
+    self: PendingBlocksManager, address: BlockAddress, peer: PeerId
+): bool =
   ## Marks this block as having been requested to a peer
   ##
 
   if self.isRequested(address):
-    error "Attempt to request block twice", address = address, peer = peer
+    return false
 
   self.blocks.withValue(address, pending):
     pending[].requested = peer.some
+  return true
 
 proc clearRequest*(
     self: PendingBlocksManager, address: BlockAddress, peer: ?PeerId = PeerId.none