[#219] morph: Refactor notary preparator

Resolve funlen linter for Preparator.Prepare method.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/morph/event/notary_preparator.go

@@ -103,55 +103,8 @@ func notaryPreparator(prm PreparatorPrm) NotaryPreparator {
 // transaction is expected to be received one more time
 // from the Notary service but already signed. This happens
 // since every notary call is a new notary request in fact.
-//
-// nolint: funlen
 func (p Preparator) Prepare(nr *payload.P2PNotaryRequest) (NotaryEvent, error) {
-	// notary request's main tx is expected to have
+	err := p.validateNotaryRequest(nr)
-	// three or four witnesses: one for proxy contract,
-	// one for alphabet multisignature, one optional for
-	// notary's invoker and one is for notary  contract
-	ln := len(nr.MainTransaction.Scripts)
-	switch ln {
-	case 3, 4:
-	default:
-		return nil, errUnexpectedWitnessAmount
-	}
-	invokerWitness := ln == 4
-
-	// alphabet node should handle only notary requests
-	// that have been sent unsigned(by storage nodes) =>
-	// such main TXs should have dummy scripts as an
-	// invocation script
-	//
-	// this check prevents notary flow recursion
-	if !bytes.Equal(nr.MainTransaction.Scripts[1].InvocationScript, p.dummyInvocationScript) {
-		return nil, ErrTXAlreadyHandled
-	}
-
-	currentAlphabet, err := p.alphaKeys()
-	if err != nil {
-		return nil, fmt.Errorf("could not fetch Alphabet public keys: %w", err)
-	}
-
-	err = p.validateCosigners(ln, nr.MainTransaction.Signers, currentAlphabet)
-	if err != nil {
-		return nil, err
-	}
-
-	// validate main TX's notary attribute
-	err = p.validateAttributes(nr.MainTransaction.Attributes, currentAlphabet, invokerWitness)
-	if err != nil {
-		return nil, err
-	}
-
-	// validate main TX's witnesses
-	err = p.validateWitnesses(nr.MainTransaction.Scripts, currentAlphabet, invokerWitness)
-	if err != nil {
-		return nil, err
-	}
-
-	// validate main TX expiration
-	err = p.validateExpiration(nr.FallbackTransaction)
 	if err != nil {
 		return nil, err
 	}
@@ -219,6 +172,60 @@ func (p Preparator) Prepare(nr *payload.P2PNotaryRequest) (NotaryEvent, error) {
 	}, nil
 }
 
+func (p Preparator) validateNotaryRequest(nr *payload.P2PNotaryRequest) error {
+	// notary request's main tx is expected to have
+	// three or four witnesses: one for proxy contract,
+	// one for alphabet multisignature, one optional for
+	// notary's invoker and one is for notary  contract
+	ln := len(nr.MainTransaction.Scripts)
+	switch ln {
+	case 3, 4:
+	default:
+		return errUnexpectedWitnessAmount
+	}
+	invokerWitness := ln == 4
+
+	// alphabet node should handle only notary requests
+	// that have been sent unsigned(by storage nodes) =>
+	// such main TXs should have dummy scripts as an
+	// invocation script
+	//
+	// this check prevents notary flow recursion
+	if !bytes.Equal(nr.MainTransaction.Scripts[1].InvocationScript, p.dummyInvocationScript) {
+		return ErrTXAlreadyHandled
+	}
+
+	currentAlphabet, err := p.alphaKeys()
+	if err != nil {
+		return fmt.Errorf("could not fetch Alphabet public keys: %w", err)
+	}
+
+	err = p.validateCosigners(ln, nr.MainTransaction.Signers, currentAlphabet)
+	if err != nil {
+		return err
+	}
+
+	// validate main TX's notary attribute
+	err = p.validateAttributes(nr.MainTransaction.Attributes, currentAlphabet, invokerWitness)
+	if err != nil {
+		return err
+	}
+
+	// validate main TX's witnesses
+	err = p.validateWitnesses(nr.MainTransaction.Scripts, currentAlphabet, invokerWitness)
+	if err != nil {
+		return err
+	}
+
+	// validate main TX expiration
+	err = p.validateExpiration(nr.FallbackTransaction)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func (p Preparator) validateParameterOpcodes(ops []Op) error {
 	l := len(ops)