route_check: fix 'string indices must be integers' crash in FRR route mitigation

[cshivashgit]

What I did

ixed a route_check.py crash — TypeError: string indices must be integers, not 'str' — that occurs during FRR route mitigation when SUPPRESS_FIB_PENDING is enabled.

The ijson streaming refactor of fetch_routes() changed the data shape it produces: process_route_entry() started appending bare prefix strings to the missed/failed route lists. However, the downstream consumer mitigate_installed_not_offloaded_frr_routes() (and its unit test test_mitigate_routes) still expects dicts with prefix and protocol keys. When there are installed-but-not-offloaded FRR routes and FIB-suppress is enabled, mitigate indexes entry['prefix'] on a str, raising TypeError: string indices must be integers. This makes check_routes() return -1, so route_check.py reports a failure even though routing is healthy.

This is not caught by CI because the unit tests mock check_frr_pending_routes to return strings and patch mitigate_installed_not_offloaded_frr_routes to a no-op, so the real string-vs-dict mismatch never executes.

Crash:

Traceback (most recent call last):
  File "/usr/local/bin/route_check.py", line ..., in check_routes_for_namespace
    mitigate_installed_not_offloaded_frr_routes(namespace, rt_frr_miss, rt_appl)
  File "/usr/local/bin/route_check.py", line ..., in mitigate_installed_not_offloaded_frr_routes
    for entry in [entry for entry in missed_frr_rt if entry['prefix'] in rt_appl]:
TypeError: string indices must be integers, not 'str'

How I did it

• scripts/route_check.py
  • fetch_routes() / process_route_entry(): collect {'prefix': prefix, 'protocol': route_entry.get('protocol')} dicts instead of bare prefix strings, so mitigate has the protocol it needs for the FieldValuePairs notification it sends to fpmsyncd.
  • check_routes_for_namespace(): extract the prefix when populating results["missed_FRR_routes"] / results["failed_FRR_routes"] and the failed-routes log message, so the reported/result format remains a list of prefix strings (no change to external output or existing test data).
  • mitigate_installed_not_offloaded_frr_routes() is unchanged — it now receives the dict shape it always expected.
• tests/route_check_test.py
  • Updated the mock_fetch_routes helper to return the same {'prefix', 'protocol'} dict shape as the real fetch_routes, keeping the mock consistent with production behavior.

How to verify it

1. Run the unit tests:

   python3 -m pytest tests/route_check_test.py
   

   All cases pass, including test_mitigate_routes (which exercises the dict contract directly).

2. On a device with SUPPRESS_FIB_PENDING enabled, create an installed-but-not-offloaded FRR route condition (e.g. a selected BGP route present in APPL_DB whose offload flag has not been set in zebra) and run:

   route_check.py
   

   Before the fix this aborts with TypeError: string indices must be integers and a non-zero return code; after the fix it mitigates the route (sends the SWSS_RC_SUCCESS response) and returns success.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

Backport: this fix also needs to be ported to the 202605

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[cshivashgit]

Closing this PR as changes are already merged as part of #4649