Hi,
I'm trying to determine who is at fault, our YANG validator
implementation or the module ietf-netconf-nmda in RFC8526 .
There is a "when" expression at line 193 within this module
when 'derived-from-or-self(datastore, "ds:operational") ';
which is under this pseudo-branch within the module: / get-data ("rpc")
/ input ("input") / origin-filters ("choice").
Our implementation claims that the initial context node of the
expression does not contain a "datastore" child, resulting in a warning
for this expression, which seemed like an obvious bug at first, but does
not anymore.
The accessible tree for this expression is built using the following
bullet in Section 6.4.1:
o If the XPath expression is defined in a substatement to an "input"
statement in an "rpc" or "action" statement, the accessible tree
is the RPC or action operation instance, all state data in the
server, and the running configuration datastore. The root node
has top-level data nodes in all modules as children.
Additionally, for an RPC, the root node also has the node
representing the RPC operation being defined as a child. The node
representing the operation being defined has the operation's input
parameters as children.
"origin-filters" for which the "when" expression is defined is clearly a
substatement to an "input" statement in an "rpc" statement. Pseudo-
accessible tree below (matches what our implementation does).
<root>
+- get-data
+- datastore
+- // other data node siblings of datastore
+- // top-level data nodes of all modules
The initial context node is determined by using the following bullet in
Section 7.21.5:
o If the "when" statement is a child of a "uses", "choice", or
"case" statement, then the context node is the closest ancestor
node to the node with the "when" statement that is also a data
node. If no such node exists, the context node is the root node.
The accessible tree is tentatively altered during the processing
of the XPath expression by removing all instances (if any) of the
nodes added by the "uses", "choice", or "case" statement.
The "when" statement in question is clearly a child of a "choice"
statement without a data node ancestor. Neither "input" nor "rpc"
represent data nodes, as per Terminology section:
o data node: A node in the schema tree that can be instantiated in a
data tree. One of container, leaf, leaf-list, list, anydata, and
anyxml.
To our implementation this means that the initial context node is the
root node of the accessible tree, not the node that represents the
"rpc". Therefore the expression is attempting to refer to a node that is
undefined. It is expecting
when 'derived-from-or-self(get-data/datastore, "ds:operational") ';
or something along those lines instead.
Either way, something doesn't add up between RFC7950 and RFC8526.
There is a suspicious verified RFC7950 errata
(https://www.rfc-editor.org/errata/eid4794) dealing with initial context
nodes under augments, but there's nothing for the UCC ("uses", "choice",
"case") corner case. Perhaps there should be? Should Section 7.21.5 say
NEW:
o If the "when" statement is a child of a "uses", "choice", or
"case" statement, then the context node is the closest ancestor
node to the node with the "when" statement that is also a data
node, rpc, action or notification. If no such node exists, the
context node is the root node. The accessible tree is tentatively
altered during the processing of the XPath expression by removing
all instances (if any) of the nodes added by the "uses",
"choice", or "case" statement.
OLD:
o If the "when" statement is a child of a "uses", "choice", or
"case" statement, then the context node is the closest ancestor
node to the node with the "when" statement that is also a data
node. If no such node exists, the context node is the root node.
The accessible tree is tentatively altered during the processing
of the XPath expression by removing all instances (if any) of the
nodes added by the "uses", "choice", or "case" statement.
?
Jernej
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