transaction: avoid ReadArray()

Reflection adds some real cost to it:

name               old time/op    new time/op    delta
DecodeBinary-8       3.14µs ± 5%    2.89µs ± 3%   -8.19%  (p=0.000 n=10+10)
DecodeJSON-8         12.6µs ± 3%    13.0µs ± 1%   +3.77%  (p=0.000 n=10+10)
DecodeFromBytes-8    2.73µs ± 2%    2.37µs ± 1%  -13.12%  (p=0.000 n=9+9)

name               old alloc/op   new alloc/op   delta
DecodeBinary-8       1.82kB ± 0%    1.75kB ± 0%   -3.95%  (p=0.000 n=10+10)
DecodeJSON-8         3.49kB ± 0%    3.49kB ± 0%     ~     (all equal)
DecodeFromBytes-8    1.44kB ± 0%    1.37kB ± 0%   -5.00%  (p=0.000 n=10+10)

name               old allocs/op  new allocs/op  delta
DecodeBinary-8         29.0 ± 0%      26.0 ± 0%  -10.34%  (p=0.000 n=10+10)
DecodeJSON-8           58.0 ± 0%      58.0 ± 0%     ~     (all equal)
DecodeFromBytes-8      21.0 ± 0%      18.0 ± 0%  -14.29%  (p=0.000 n=10+10)

pkg/core/transaction/transaction.go

@@ -135,8 +135,30 @@ func (t *Transaction) decodeHashableFields(br *io.BinReader) {
 	t.SystemFee = int64(br.ReadU64LE())
 	t.NetworkFee = int64(br.ReadU64LE())
 	t.ValidUntilBlock = br.ReadU32LE()
-	br.ReadArray(&t.Signers, MaxAttributes)
+	nsigners := br.ReadVarUint()
-	br.ReadArray(&t.Attributes, MaxAttributes-len(t.Signers))
+	if br.Err != nil {
+		return
+	}
+	if nsigners > MaxAttributes {
+		br.Err = errors.New("too many signers")
+		return
+	} else if nsigners == 0 {
+		br.Err = errors.New("missing signers")
+		return
+	}
+	t.Signers = make([]Signer, nsigners)
+	for i := 0; i < int(nsigners); i++ {
+		t.Signers[i].DecodeBinary(br)
+	}
+	nattrs := br.ReadVarUint()
+	if nattrs > MaxAttributes-nsigners {
+		br.Err = errors.New("too many attributes")
+		return
+	}
+	t.Attributes = make([]Attribute, nattrs)
+	for i := 0; i < int(nattrs); i++ {
+		t.Attributes[i].DecodeBinary(br)
+	}
 	t.Script = br.ReadVarBytes(MaxScriptLength)
 	if br.Err == nil {
 		br.Err = t.isValid()
@@ -148,11 +170,18 @@ func (t *Transaction) decodeBinaryNoSize(br *io.BinReader) {
 	if br.Err != nil {
 		return
 	}
-	br.ReadArray(&t.Scripts, MaxAttributes)
+	nscripts := br.ReadVarUint()
-	if br.Err == nil && len(t.Signers) != len(t.Scripts) {
+	if nscripts > MaxAttributes {
+		br.Err = errors.New("too many witnesses")
+		return
+	} else if int(nscripts) != len(t.Signers) {
 		br.Err = fmt.Errorf("%w: %d vs %d", ErrInvalidWitnessNum, len(t.Signers), len(t.Scripts))
 		return
 	}
+	t.Scripts = make([]Witness, nscripts)
+	for i := 0; i < int(nscripts); i++ {
+		t.Scripts[i].DecodeBinary(br)
+	}
 
 	// Create the hash of the transaction at decode, so we dont need
 	// to do it anymore.