Schema Mount: top-level rpc of a mounted module is rejected inside a NETCONF <action> ("Unexpected RPC element") #2622

Summary:
RFC 8528 Section 5 says that for an RPC defined in a mounted module, "clients can invoke this operation as if it were defined as an action for the corresponding mount point" (RFC 7950 Section 7.15). A client therefore sends the mounted RPC inside a NETCONF element, nested under the mount point in the parent data tree.

libyang rejects this with Unexpected RPC element "".

Root cause:

1. lydxml_env_netconf_rpc() (src/parser_xml.c:1341-1353) parses the envelope and sets int_opts = LYD_INTOPT_NO_SIBLINGS | LYD_INTOPT_ACTION. LYD_INTOPT_RPC is set only when there is no .
2. In the mounted context, the mounted module's top-level rpc is still a schema node with nodetype == LYS_RPC.
3. lyd_parser_check_schema() (src/parser_common.c:155-162) accepts a LYS_RPC node only when int_opts contains LYD_INTOPT_RPC or LYD_INTOPT_REPLY. Under LYD_INTOPT_ACTION it jumps to error_node_inval and logs Unexpected RPC element "". (src/parser_common.c:195).

The check assumes the node type alone selects the envelope: LYS_RPC for , LYS_ACTION for . RFC 8528 breaks this assumption, because a top-level RPC of a mounted module is nested in the parent data tree and is invoked as an action.

src/plugins_exts/schema_mount.c has no RPC/action handling, and before this report tests/utests/extensions/test_schema_mount.c had no RPC, action or notification case.

Affected version: libyang 6.4.13

Please see attached JIRA-schema-mount-rpc-as action-summary.md for potential fix and 0001-test-mounted-rpc-as-action-reproducer.patch to add unit test to reproduce the issue.

0001-path-fallback-to-ext-lookup.patch
JIRA-mount_data_require_parent_context-summary.md