sanity check fixes for chassis; and cache cleanup at the end of pytest session

tests/common/plugins/sanity_check/checks.py

@@ -106,7 +106,7 @@ def _find_down_ports(dut, phy_interfaces, ip_interfaces):
 def check_interfaces(duthosts):
     def _check():
         check_results = []
-        for dut in duthosts:
+        for dut in duthosts.frontend_nodes:
             logger.info("Checking interfaces status on %s..." % dut.hostname)
 
             networking_uptime = dut.get_networking_uptime().seconds
@@ -161,7 +161,7 @@ def _check():
 def check_bgp(duthosts):
     def _check():
         check_results = []
-        for dut in duthosts:
+        for dut in duthosts.frontend_nodes:
             def _check_bgp_status_helper():
                 asic_check_results = []
                 bgp_facts = dut.bgp_facts(asic_index='all')

tests/conftest.py

@@ -880,3 +880,19 @@ def duthost_console(localhost, creds, request):
                        console_password=creds['console_password'][vars['console_type']])
     yield host
     host.disconnect()
+
+@pytest.fixture(scope='session')
+def cleanup_cache_for_session(request):
+    """
+    This fixture allows developers to cleanup the cached data for all DUTs in the testbed before test.
+    Use cases:
+      - Running tests where some 'facts' about the DUT that get cached are changed.
+      - Running tests/regression without running test_pretest which has a test to clean up cache (PR#2978)
+      - Test case development phase to work out testbed information changes.
+
+    This fixture is not automatically applied, if you want to use it, you have to add a call to it in your tests.
+    """
+    tbname, tbinfo = get_tbinfo(request)
+    cache.cleanup(zone=tbname)
+    for a_dut in tbinfo['duts']:
+        cache.cleanup(zone=a_dut)