apuchyova although the description is really kind of self-explanatory it would be better to actually see the the error.

Also it seems like you played around with action support in netconf SB, so I believe you have some models and data connected to this issue. It would be really cool if you can attach them here so the reproduction will be easier.

Based on the proposed unit test:

[ERROR] Tests run: 27, Failures: 0, Errors: 2, Skipped: 0, Time elapsed: 1.387 s <<< FAILURE! - in org.opendaylight.netconf.sal.connect.netconf.schema.mapping.NetconfMessageTransformerTest
[ERROR] testRpcEmptyBodyWithOutputDefinedSchemaResult(org.opendaylight.netconf.sal.connect.netconf.schema.mapping.NetconfMessageTransformerTest)  Time elapsed: 0.011 s  <<< ERROR!
java.lang.IllegalArgumentException: Failed to parse RPC response [rpc-reply: null]
	at org.opendaylight.netconf.sal.connect.netconf.schema.mapping.NetconfMessageTransformer.parseResult(NetconfMessageTransformer.java:329)
	at org.opendaylight.netconf.sal.connect.netconf.schema.mapping.NetconfMessageTransformer.toRpcResult(NetconfMessageTransformer.java:294)
	at org.opendaylight.netconf.sal.connect.netconf.schema.mapping.NetconfMessageTransformerTest.testRpcEmptyBodyWithOutputDefinedSchemaResult(NetconfMessageTransformerTest.java:240)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[...]
Caused by: javax.xml.stream.XMLStreamException: Schema for node with name ok and namespace urn:ietf:params:xml:ns:netconf:base:1.0 does not exist at AbsoluteSchemaPath{path=[(urn:example:rpcs-actions-outputs)rpc-with-output, (urn:example:rpcs-actions-outputs)output]}
	at org.opendaylight.yangtools.yang.data.codec.xml.XmlParserStream.read(XmlParserStream.java:522)
	at org.opendaylight.yangtools.yang.data.codec.xml.XmlParserStream.parse(XmlParserStream.java:270)
	at org.opendaylight.yangtools.yang.data.codec.xml.XmlParserStream.traverse(XmlParserStream.java:297)
	at org.opendaylight.netconf.sal.connect.netconf.schema.mapping.NetconfMessageTransformer.parseResult(NetconfMessageTransformer.java:326)
	... 30 more

Which means XML parser was asked to parse </ok> as a child, which means we are parsing a NETCONF concept in Yangtools – which is wrong.

From the parser perspective, we an option of injecting proper parsing here, given that we have mechanics for dealing with RFC8258 mounts. From yang-data-codec-xml perspective whether it is a mount point node or NETCONF XML node are completely indiscernible: both are something that occurs under a DataNodeContainer and it has a QName.

Hence the correct fix is make pretend that

module netconf-internal {
   // Conflicts with ietf-netconf
   namespace "urn:ietf:params:xml:ns:netconf:base:1.0"

   container ok {
       presence "Indicates a no-response explicit acknowledgement";
   }
}

is implicitly schema-mounted into the <rpc-reply> SchemaNode.

So the mechanics goes:

• NETCONF passes down an appropriate MountPointContext
• yang-data-codec-xml executes lookup to a fictious mount point
• NETCONF turns that fiction into an empty ContainerNode matching the output statement
• yang-data-codec-xml results in that ContainerNode being wrapped in a MountPointNode pointing to the fictious mount point

... or this actually is a bug Sorry about the above.

<rpc-reply> contains either <data> or <ok>, hence that part should be handled by NETCONF, not by yang-data-codec-xml – unless I am missing something.

NetconfMessageTransformer.java throws error when it is trying to parse the <ok/> as a child in schema node. This is because when the rpc has output parameters i.e, child nodes, xmlparser tries to parse them. So, I'm planning the fix is if dont send the <ok/> of the <rpc-reply> to xmlparser, it wont parse the <ok/> as a child node. Xmlparser will try to find the output parameters else it treat the non mandatory nodes as transient nodes. I have validated this fix and it works fine.
Please provide your input on this to proceed further.