Sysrepo subscription with filter not working as expected

[rbu9fe]

I'm registering a module change subscription with the flag enabled=True and the initial callback doesn't always provide the change information that is expected.

Assuming that the sysrepo operational datastore has this contents

{
  "ietf-interfaces:interfaces": {
    "interface": [
      {
        "name": "eno0",
        "type": "iana-if-type:ethernetCsmacd",
        "enabled": "true"
      }
    ]
  }
}

then this code reproduces the issue:

import sysrepo

xpaths = (
    "/ietf-interfaces:interfaces/interface[name='eno0']/name",  # OK
    "/ietf-interfaces:interfaces/interface[name='eno0']/type",  # OK
    "/ietf-interfaces:interfaces/interface[name='eno0']/enabled",  # OK
    "/ietf-interfaces:interfaces/interface[type='iana-if-type:ethernetCsmacd']/name",  # ERROR
    "/ietf-interfaces:interfaces/interface[type='iana-if-type:ethernetCsmacd']/type",  # OK
    "/ietf-interfaces:interfaces/interface[type='iana-if-type:ethernetCsmacd']/enabled",  # ERROR
    "/ietf-interfaces:interfaces/interface[enabled='true']/name",  # ERROR
    "/ietf-interfaces:interfaces/interface[enabled='true']/type",  # ERROR
    "/ietf-interfaces:interfaces/interface[enabled='true']/enabled",  # OK
)

connection = sysrepo.SysrepoConnection()
for xpath in xpaths:
    global n_changes
    session = connection .start_session("operational")

    session.subscribe_module_change(
        "ietf-interfaces",
        xpath,
        change_cb,
        passive=True,
        done_only=True,
        enabled=True,
    )

    print(f"{xpath}: {n_changes != 0}")

It expects that each subscription is executed during subscription already (due to the enabled=True flag) and prints the number of changes the callback found. Some work and some don't - see the # OK' and '# ERROR' comments behind the xpaths. Looks like it works only when filtering the list key (i.e. name`) or when the filtered element is part of the result anyway, but not when I filter the list items for property A but query property B.

This works fine when using the libyang API directly, so I assume it has something to do with how the changes are collected as the filter string itself is passed as-is to the C API.

[rbu9fe]

The root cause of this issue is that the change callback implementation filters all available changes by the subscribed xpath here. Even though initially the entire datastore is part of the changes, in my opinion the sysrepo C API doesn't work as expected when the filter component is only used for filtering but is not part of the final result. I created this issue in the sysrepo project.

[rbu9fe]

Ok the sysrepo maintainer claims that it can't be implemented in a different way. Is there a special reason why the Python wrapper behaves differently than the C API, where one needs to call sr_get_changes manually to actually get the change list? There the caller has the filter under his own control.

[rbu9fe]

Anyway, the current implemenation doesn't work when using such subscription xpaths that filter elements, where the filtered node is not part of the result. Since the C API or sr_get_changes is already providing only filtered data, we must not filter again. I provided a pull request to fix this here.