block/dao: simplify trimming, avoid allocations

The only user of (*Block).Trim() is in DAO and it already has a nice buffer
usually, so creating another one makes no sense. It also simplifies error
handling a lot.

pkg/core/block/block.go

@@ -100,26 +100,16 @@ func New(stateRootEnabled bool) *Block {
 	}
 }
 
-// Trim returns a subset of the block data to save up space
-// in storage.
-// Notice that only the hashes of the transactions are stored.
-func (b *Block) Trim() ([]byte, error) {
-	buf := io.NewBufBinWriter()
-	numTx := len(b.Transactions)
-	buf.Grow(b.GetExpectedBlockSizeWithoutTransactions(numTx) + util.Uint256Size*numTx)
-	b.Header.EncodeBinary(buf.BinWriter)
+// EncodeTrimmed writes trimmed representation of the block data into w. Trimmed blocks
+// do not store complete transactions, instead they only store their hashes.
+func (b *Block) EncodeTrimmed(w *io.BinWriter) {
+	b.Header.EncodeBinary(w)
 
-	buf.WriteVarUint(uint64(numTx))
+	w.WriteVarUint(uint64(len(b.Transactions)))
 	for _, tx := range b.Transactions {
 		h := tx.Hash()
-		h.EncodeBinary(buf.BinWriter)
+		h.EncodeBinary(w)
 	}
-
-	if buf.Err != nil {
-		return nil, buf.Err
-	}
-
-	return buf.Bytes(), nil
 }
 
 // DecodeBinary decodes the block from the given BinReader, implementing

pkg/core/block/block_test.go

@@ -55,10 +55,11 @@ func TestDecodeBlock1(t *testing.T) {
 func TestTrimmedBlock(t *testing.T) {
 	block := getDecodedBlock(t, 1)
 
-	b, err := block.Trim()
-	require.NoError(t, err)
+	buf := io.NewBufBinWriter()
+	block.EncodeTrimmed(buf.BinWriter)
+	require.NoError(t, buf.Err)
 
-	r := io.NewBinReaderFromBuf(b)
+	r := io.NewBinReaderFromBuf(buf.Bytes())
 	trimmedBlock, err := NewTrimmedFromReader(false, r)
 	require.NoError(t, err)
 

pkg/core/dao/dao.go

@@ -648,11 +648,7 @@ func (dao *Simple) StoreAsBlock(block *block.Block, aer1 *state.AppExecResult, a
 		buf = dao.getDataBuf()
 	)
 	buf.WriteB(storage.ExecBlock)
-	b, err := block.Trim()
-	if err != nil {
-		return err
-	}
-	buf.WriteBytes(b)
+	block.EncodeTrimmed(buf.BinWriter)
 	if aer1 != nil {
 		aer1.EncodeBinary(buf.BinWriter)
 	}