Implement GetBlockHeader function

The function queries electrum server for a block header for a block at
the given height and then returns it in the format expected by the
bitcoin.Chain interface.

Author: nkuba

pkg/bitcoin/electrum/block.go

@@ -0,0 +1,38 @@
+package electrum
+
+import (
+	"bytes"
+	"encoding/hex"
+
+	"github.com/btcsuite/btcd/wire"
+	"github.com/checksum0/go-electrum/electrum"
+
+	"github.com/keep-network/keep-core/pkg/bitcoin"
+)
+
+// convertBlockHeader transforms a BlockHeader returned from Electrum protocol to
+// the format expected by the bitcoin.Chain interface.
+func convertBlockHeader(electrumResult *electrum.GetBlockHeaderResult) (*bitcoin.BlockHeader, error) {
+	headerBytes, err := hex.DecodeString(electrumResult.Header)
+	if err != nil {
+		return nil, err
+	}
+
+	buf := bytes.NewBuffer(headerBytes)
+
+	var b wire.BlockHeader
+	if err := b.Deserialize(buf); err != nil {
+		return nil, err
+	}
+
+	result := &bitcoin.BlockHeader{
+		Version:                 b.Version,
+		PreviousBlockHeaderHash: bitcoin.Hash(b.PrevBlock),
+		MerkleRootHash:          bitcoin.Hash(b.MerkleRoot),
+		Time:                    uint32(b.Timestamp.Unix()),
+		Bits:                    b.Bits,
+		Nonce:                   b.Nonce,
+	}
+
+	return result, nil
+}

pkg/bitcoin/electrum/electrum.go

@@ -141,9 +141,32 @@ func (c *Connection) GetLatestBlockHeight() (uint, error) {
 	panic("not implemented")
 }
 
+// GetBlockHeader gets the block header for the given block height. If the
+// block with the given height was not found on the chain, this function
+// returns an error.
 func (c *Connection) GetBlockHeader(
 	blockHeight uint,
 ) (*bitcoin.BlockHeader, error) {
-	// TODO: Implementation.
-	panic("not implemented")
+	var getBlockHeaderResult *electrum.GetBlockHeaderResult
+	err := wrappers.DoWithDefaultRetry(c.requestRetryTimeout, func(ctx context.Context) error {
+		result, err := c.client.GetBlockHeader(c.ctx, uint32(blockHeight), 0)
+		if err != nil {
+			return fmt.Errorf(
+				"GetBlockHeader failed: [%w]",
+				err,
+			)
+		}
+		getBlockHeaderResult = result
+		return nil
+	})
+	if err != nil {
+		return nil, fmt.Errorf("failed to get block header: [%w]", err)
+	}
+
+	blockHeader, err := convertBlockHeader(getBlockHeaderResult)
+	if err != nil {
+		return nil, fmt.Errorf("failed to convert block header: %w", err)
+	}
+
+	return blockHeader, nil
 }