fix(dot/sync): fix block request and response logic

dot/state/block.go

@@ -494,6 +494,16 @@ func (bs *BlockState) AddBlockToBlockTree(header *types.Header) error {
 	return bs.bt.AddBlock(header, uint64(arrivalTime.UnixNano()))
 }
 
+// GetAllBlocksAtNumber returns all unfinalised blocks with the given number
+func (bs *BlockState) GetAllBlocksAtNumber(num *big.Int) ([]common.Hash, error) {
+	header, err := bs.GetHeaderByNumber(num)
+	if err != nil {
+		return nil, err
+	}
+
+	return bs.GetAllBlocksAtDepth(header.ParentHash), nil
+}
+
 // GetAllBlocksAtDepth returns all hashes with the depth of the given hash plus one
 func (bs *BlockState) GetAllBlocksAtDepth(hash common.Hash) []common.Hash {
 	return bs.bt.GetAllBlocksAtDepth(hash)

dot/sync/chain_sync.go

@@ -897,6 +897,12 @@ func workerToRequests(w *worker) ([]*network.BlockRequestMessage, error) {
 		} else {
 			// in tip-syncing mode, we know the hash of the block on the fork we wish to sync
 			start, _ = variadic.NewUint64OrHash(w.startHash)
+
+			// if we're doing descending requests and not at the last (highest starting) request,
+			// then use number as start block
+			if w.direction == network.Descending && i != numRequests-1 {
+				start, _ = variadic.NewUint64OrHash(startNumber)
+			}
 		}
 
 		var end *common.Hash
@@ -911,7 +917,21 @@ func workerToRequests(w *worker) ([]*network.BlockRequestMessage, error) {
 			Direction:     w.direction,
 			Max:           &max,
 		}
-		startNumber += maxResponseSize
+
+		switch w.direction {
+		case network.Ascending:
+			startNumber += maxResponseSize
+		case network.Descending:
+			startNumber -= maxResponseSize
+		}
+	}
+
+	// if our direction is descending, we want to send out the request with the lowest
+	// startNumber first
+	if w.direction == network.Descending {
+		for i, j := 0, len(reqs)-1; i < j; i, j = i+1, j-1 {
+			reqs[i], reqs[j] = reqs[j], reqs[i]
+		}
 	}
 
 	return reqs, nil

dot/sync/chain_sync_test.go

@@ -405,6 +405,30 @@ func TestWorkerToRequests(t *testing.T) {
 				},
 			},
 		},
+		{
+			w: &worker{
+				startNumber:  big.NewInt(1 + maxResponseSize + (maxResponseSize / 2)),
+				targetNumber: big.NewInt(1),
+				direction:    network.Descending,
+				requestData:  bootstrapRequestData,
+			},
+			expected: []*network.BlockRequestMessage{
+				{
+					RequestedData: network.RequestedDataHeader + network.RequestedDataBody + network.RequestedDataJustification,
+					StartingBlock: *variadic.MustNewUint64OrHash(1 + (maxResponseSize / 2)),
+					EndBlockHash:  nil,
+					Direction:     network.Descending,
+					Max:           &max64,
+				},
+				{
+					RequestedData: bootstrapRequestData,
+					StartingBlock: *variadic.MustNewUint64OrHash(1 + maxResponseSize + (maxResponseSize / 2)),
+					EndBlockHash:  nil,
+					Direction:     network.Descending,
+					Max:           &max128,
+				},
+			},
+		},
 	}
 
 	for i, tc := range testCases {

dot/sync/interface.go

@@ -55,6 +55,9 @@ type BlockState interface {
 	StoreRuntime(common.Hash, runtime.Instance)
 	GetHighestFinalisedHeader() (*types.Header, error)
 	GetFinalisedNotifierChannel() chan *types.FinalisationInfo
+	GetHeaderByNumber(num *big.Int) (*types.Header, error)
+	GetAllBlocksAtNumber(num *big.Int) ([]common.Hash, error)
+	IsDescendantOf(parent, child common.Hash) (bool, error)
 }
 
 // StorageState is the interface for the storage state