Correctly decide if session involves a keyshare server

Previously this was decided by checking whether any of the scheme managers of any of the involved credetials and attributes that occur in the session uses a keyshare server. However, in case of disclosure and signing sessions, this could erroneously include attributes from disjunctions that have not been selected by the user.

irmaclient/session.go

@@ -389,7 +389,7 @@ func (session *session) do(proceed bool) {
 	}
 	session.Handler.StatusUpdate(session.Action, irma.StatusCommunicating)
 
-	if !session.irmaSession.Identifiers().Distributed(session.client.Configuration) {
+	if !session.Distributed() {
 		message, err := session.getProof()
 		if err != nil {
 			session.fail(&irma.SessionError{ErrorType: irma.ErrorCrypto, Err: err})
@@ -413,6 +413,31 @@ func (session *session) do(proceed bool) {
 	}
 }
 
+func (session *session) Distributed() bool {
+	var smi irma.SchemeManagerIdentifier
+	if session.Action == irma.ActionIssuing {
+		for _, credreq := range session.irmaSession.(*irma.IssuanceRequest).Credentials {
+			smi = credreq.CredentialTypeID.IssuerIdentifier().SchemeManagerIdentifier()
+			if session.client.Configuration.SchemeManagers[smi].Distributed() {
+				return true
+			}
+		}
+	}
+
+	if session.choice == nil || session.choice.Attributes == nil {
+		return false
+	}
+
+	for _, ai := range session.choice.Attributes {
+		smi = ai.Type.CredentialTypeIdentifier().IssuerIdentifier().SchemeManagerIdentifier()
+		if session.client.Configuration.SchemeManagers[smi].Distributed() {
+			return true
+		}
+	}
+
+	return false
+}
+
 func (session *session) KeyshareDone(message interface{}) {
 	session.sendResponse(message)
 }