[yang-mgmt] Treat empty CONFIG DB leaf-list (['']) as empty so libyang3 accepts empty pattern-constrained leaf-lists

[deepak-singhal0408]

Why I did it

config apply-patch, config replace and config rollback fail on master for any configuration that contains an empty pattern-constrained leaf-list. BGP_ALLOWED_PREFIXES is the common trigger, because a deployment often populates only prefixes_v4 or only prefixes_v6, leaving the other leaf-list empty.

An empty leaf-list is stored in CONFIG DB as field@ = "". The Python ConfigDBConnector deserialises that empty string into a single empty-string element (['']), which is the CONFIG DB representation of "no elements", not a real element. Before the libyang1 to libyang3 port (#26584) the '' sentinel was tolerated by the validation library. libyang3 validates the '' element against the leaf type's pattern (for example the ipv4-prefix pattern of BGP_ALLOWED_PREFIXES.prefixes_v4) and rejects the whole config:

Unsatisfied pattern - "" does not conform to "(([0-9]| ... )/(([0-9])|([1-2][0-9])|(3[0-2]))( (le|ge) ...)?"
Data path: .../BGP_ALLOWED_PREFIXES_LIST[deployment='DEPLOYMENT_ID'][id='0']/prefixes_v4

This is a read-side (config-to-YANG validation) regression. #26584 did add a guard for the sibling case of an empty leaf-list that has a schema default "", but missed the pattern-typed, no-default leaf-list case (['']), which is exactly BGP_ALLOWED_PREFIXES.

This fixes the read-side regression behind sonic-mgmt test generic_config_updater/test_bgp_prefix.py (tracked by sonic-mgmt issue 25549). It is complementary to, and does not replace, the separate write-side effort that avoids writing prefixes_v4@ = "" into CONFIG DB in the first place (sonic-buildimage issue 27818, sonic-utilities PR 4478). Even with that write-side fix, existing configs and other tables can still present an empty leaf-list, so the validation layer should accept it.

Work item tracking

• Microsoft ADO (number only): 38183186

How I did it

In the CONFIG DB to YANG translation in src/sonic-yang-mgmt/sonic_yang_ext.py (the leaf-list branch of _findYangTypedValue), drop the empty-string sentinel so an empty leaf-list is represented as an empty list [], which libyang3 accepts. The drop is gated on the leaf-list having no schema default (not list(leaf.defaults())), so the existing empty-with-default handling introduced by #26584 is left byte-for-byte unchanged.

Added a regression unit test (test_empty_leaflist_collapses_to_empty) that loads BGP_ALLOWED_PREFIXES with an empty prefixes_v4 leaf-list and asserts the config validates and the leaf-list collapses to [].

How to verify it

Unit test (sonic-yang-mgmt suite): with the fix the new test passes and the full suite is green; reverting only the two changed lines makes just test_empty_leaflist_collapses_to_empty fail with the Unsatisfied pattern - "" error above.

End-to-end on a KVM t1 testbed (vms-kvm-t1-lag), baseline master image built after #26584 (so it carries the regression), matched before/after with only sonic_yang_ext.py swapped:

DUT state	generic_config_updater/test_bgp_prefix.py::test_bgp_prefix_tc1_suite
before (unpatched baseline)	2 failed, 2 errors — config apply-patch returns non-zero, teardown config rollback failed
after (fix applied)	2 passed

Also verified the mechanism directly: with BGP_ALLOWED_PREFIXES|DEPLOYMENT_ID|0 holding prefixes_v4@ = "", a subsequent valid config apply-patch fails with the Unsatisfied pattern - "" error before the fix and succeeds after it.

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• 202608

Tested branch (Please provide the tested image version)

• master — SONiC.master.1145536-931dbfb70 (build 2026-06-22)

Description for the changelog

[yang-mgmt] Treat an empty CONFIG DB leaf-list (['']) as empty so libyang3 does not reject empty pattern-constrained leaf-lists such as BGP_ALLOWED_PREFIXES.

Link to config_db schema for YANG module changes

No YANG model change. BGP_ALLOWED_PREFIXES schema: https://github.com/sonic-net/sonic-buildimage/blob/master/src/sonic-yang-models/doc/Configuration.md#bgp_allowed_prefixes

A picture of a cute animal (not mandatory but encouraged)

🦔

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

This PR fixes a regression in sonic-yang-mgmt’s CONFIG_DB→YANG translation after the libyang1→libyang3 port: it treats the CONFIG DB empty leaf-list sentinel ([''], originating from field@ = "") as an actual empty leaf-list ([]) so libyang3 does not reject pattern-constrained leaf-lists like BGP_ALLOWED_PREFIXES.prefixes_v4.

Changes:

• Collapse the [''] empty leaf-list sentinel into [] during _findYangTypedValue() leaf-list conversion (only for leaf-lists without schema defaults).
• Add a regression unit test covering BGP_ALLOWED_PREFIXES validation and ensuring the sentinel does not survive into the translated data.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/sonic-yang-mgmt/sonic_yang_ext.py	Drops the empty-string sentinel when translating leaf-list values to avoid libyang3 pattern validation failures.
src/sonic-yang-mgmt/tests/libyang-python-tests/test_sonic_yang.py	Adds a regression test asserting [''] is treated as an empty leaf-list and validation succeeds.

[deepak-singhal0408]

This is a read-side fix for the regression from the libyang1->libyang3 port. @bhouse-nexthop @qiluo-msft @vaibhavhd @rimunagala, could you help review the change? Thanks,

[bhouse-nexthop]

This look valid to me, thanks!

[deepak-singhal0408]

@lguohan @yxieca, could you help merge this PR. thanks,

[deepak-singhal0408]

202605 branch verification ✅

Verified this fix on the 202605 release branch with a matched A/B on a KVM t1-lag testbed (vlab-03), 202605 nightly 202605.1157773-9c84048a4 (Debian 13/trixie, Python 3.13 / libyang3). The failing test is xfail-marked (issue 25549), so it was run with --runxfail for a real pass/fail verdict. The patched sonic_yang_ext.py was hot-deployed onto the pristine 202605 image; the yang file is the only variable.

Test: generic_config_updater/test_bgp_prefix.py::test_bgp_prefix_tc1_suite ([None-empty], [None-1010:1010]), --runxfail.

Build	tests	failures	errors
UNFIXED (pristine 202605 sonic_yang_ext.py)	2	2	2
FIXED (patched leaf-list sentinel drop)	2	0	0

regression_confirmed = true, fix_confirmed = true on 202605 — the empty CONFIG DB leaf-list (['']) fails libyang3 validation on the unfixed build (GCU apply-patch errors), and both parametrizations pass with the fix.

[mssonicbld]

Cherry-pick PR to 202605: #28308

[mssonicbld]

@deepak-singhal0408 cherry pick PR didn't pass PR checker after retry. Please help check! Thanks.
#28308

_{---Powered by SONiC BuildBot}