simplify message syncing logic

1. Allow duplicate blocks from bitswap. This shouldn't happen, but there's no
reason to bail (just log loudly).
2. Simplify logic to very explicitly check to make sure we're fetching every
block.

chain/sub/incoming.go

@@ -120,12 +120,6 @@ func FetchMessagesByCids(
 			return err
 		}
 
-		// FIXME: We already sort in `fetchCids`, we are duplicating too much work,
-		//  we don't need to pass the index.
-		if out[i] != nil {
-			return fmt.Errorf("received duplicate message")
-		}
-
 		out[i] = msg
 		return nil
 	})
@@ -149,10 +143,6 @@ func FetchSignedMessagesByCids(
 			return err
 		}
 
-		if out[i] != nil {
-			return fmt.Errorf("received duplicate message")
-		}
-
 		out[i] = smsg
 		return nil
 	})
@@ -184,24 +174,29 @@ func fetchCids(
 		return fmt.Errorf("duplicate CIDs in fetchCids input")
 	}
 
-	fetchedBlocks := bserv.GetBlocks(ctx, cids)
-
-	for i := 0; i < len(cids); i++ {
-		select {
-		case block, ok := <-fetchedBlocks:
-			if !ok {
-				return fmt.Errorf("failed to fetch all messages")
-			}
-
-			ix, ok := cidIndex[block.Cid()]
-			if !ok {
-				return fmt.Errorf("received message we didnt ask for")
-			}
-
-			if err := cb(ix, block); err != nil {
-				return err
-			}
+	for block := range bserv.GetBlocks(ctx, cids) {
+		ix, ok := cidIndex[block.Cid()]
+		if !ok {
+			// Ignore duplicate/unexpected blocks. This shouldn't
+			// happen, but we can be safe.
+			log.Errorw("received duplicate/unexpected block when syncing", "cid", block.Cid())
+			continue
 		}
+
+		// Record that we've received the block.
+		delete(cidIndex, block.Cid())
+
+		if err := cb(ix, block); err != nil {
+			return err
+		}
+	}
+
+	if len(cidIndex) > 0 {
+		err := ctx.Err()
+		if err == nil {
+			err = fmt.Errorf("failed to fetch %d messages for unknown reasons", len(cidIndex))
+		}
+		return err
 	}
 
 	return nil