session: Add method to set token signature without issuer

`Sign` method sets both issuer and signature of the token. There could
be a need to set signature only, e.g. for testing.

Now signature could be set via new method `SetSignature` also used by
`Sign` itself.

Refs #546.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

session/common.go

@@ -172,10 +172,7 @@ func (x commonData) signedData(w contextWriter) []byte {
 	return x.fillBody(w).StableMarshal(nil)
 }
 
-func (x *commonData) sign(signer user.Signer, w contextWriter) error {
-	x.issuer = signer.UserID()
-	x.issuerSet = true
-
+func (x *commonData) sign(signer neofscrypto.Signer, w contextWriter) error {
 	var sig neofscrypto.Signature
 
 	err := sig.Calculate(signer, x.signedData(w))

session/container.go

@@ -147,6 +147,14 @@ func (x *Container) UnmarshalJSON(data []byte) error {
 //
 // See also [Container.VerifySignature], [Container.SignedData].
 func (x *Container) Sign(signer user.Signer) error {
+	x.issuer = signer.UserID()
+	x.issuerSet = true
+	return x.SetSignature(signer)
+}
+
+// SetSignature allows to sign Container like [Container.Sign] but without
+// issuer setting.
+func (x *Container) SetSignature(signer neofscrypto.Signer) error {
 	return x.sign(signer, x.writeContext)
 }
 

session/container_test.go

@@ -560,6 +560,10 @@ func TestContainer_Issuer(t *testing.T) {
 func TestContainer_Sign(t *testing.T) {
 	val := sessiontest.Container()
 
+	require.NoError(t, val.SetSignature(test.RandomSignerRFC6979(t)))
+	require.Zero(t, val.Issuer())
+	require.True(t, val.VerifySignature())
+
 	require.NoError(t, val.Sign(test.RandomSignerRFC6979(t)))
 
 	require.True(t, val.VerifySignature())

session/object.go

@@ -7,6 +7,7 @@ import (
 	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/nspcc-dev/neofs-api-go/v2/session"
 	cid "github.com/nspcc-dev/neofs-sdk-go/container/id"
+	neofscrypto "github.com/nspcc-dev/neofs-sdk-go/crypto"
 	oid "github.com/nspcc-dev/neofs-sdk-go/object/id"
 	"github.com/nspcc-dev/neofs-sdk-go/user"
 )
@@ -181,6 +182,14 @@ func (x *Object) UnmarshalJSON(data []byte) error {
 //
 // See also [Object.VerifySignature], [Object.SignedData].
 func (x *Object) Sign(signer user.Signer) error {
+	x.issuer = signer.UserID()
+	x.issuerSet = true
+	return x.SetSignature(signer)
+}
+
+// SetSignature allows to sign Object like [Object.Sign] but without issuer
+// setting.
+func (x *Object) SetSignature(signer neofscrypto.Signer) error {
 	return x.sign(signer, x.writeContext)
 }
 

session/object_test.go

@@ -623,6 +623,10 @@ func TestObject_Issuer(t *testing.T) {
 func TestObject_Sign(t *testing.T) {
 	val := sessiontest.Object()
 
+	require.NoError(t, val.SetSignature(test.RandomSignerRFC6979(t)))
+	require.Zero(t, val.Issuer())
+	require.True(t, val.VerifySignature())
+
 	require.NoError(t, val.Sign(test.RandomSignerRFC6979(t)))
 
 	require.True(t, val.VerifySignature())