Fix vs test failure in test_buffer_traditional (sonic-net#1881)

Ref: sonic-net#1875

The test expects buffer PGs created for all ports.
However, the vs image is deployed as cable length being 0m,
which provides the buffer PGs to be created and fails the test.

To fix the failure:
- Config the cable length of port under test to 300m if it was 0m
- Startup the port after cable length configured
  By doing that the buffer PG can be configured correctly
- Collect the orig_profiles after port is started up

Signed-off-by: Stephen Sun <stephens@nvidia.com>

tests/test_buffer_traditional.py

@@ -68,17 +68,20 @@ def setup_teardown_test(self, dvs):
         finally:
             self.teardown()
 
-    @pytest.mark.skip(reason="Failing. Under investigation")
     def test_zero_cable_len_profile_update(self, dvs, setup_teardown_test):
         self.pg_name_map = setup_teardown_test
         orig_cable_len = None
         orig_speed = None
         try:
-            dvs.runcmd("config interface startup {}".format(self.INTF))
-            self.orig_profiles = self.get_asic_buf_profile()
             # get orig cable length and speed
             fvs = self.config_db.get_entry("CABLE_LENGTH", "AZURE")
             orig_cable_len = fvs[self.INTF]
+            if orig_cable_len == "0m":
+                fvs[self.INTF] = "300m"
+                cable_len_before_test = "300m"
+                self.config_db.update_entry("CABLE_LENGTH", "AZURE", fvs)
+            else:
+                cable_len_before_test = orig_cable_len
             fvs = self.config_db.get_entry("PORT", self.INTF)
             orig_speed = fvs["speed"]
 
@@ -88,9 +91,15 @@ def test_zero_cable_len_profile_update(self, dvs, setup_teardown_test):
                 test_speed = "100000"
             test_cable_len = "0m"
 
+            dvs.runcmd("config interface startup {}".format(self.INTF))
+            # Make sure the buffer PG has been created
+            orig_lossless_profile = "pg_lossless_{}_{}_profile".format(orig_speed, cable_len_before_test)
+            self.app_db.wait_for_entry("BUFFER_PROFILE_TABLE", orig_lossless_profile)
+            self.orig_profiles = self.get_asic_buf_profile()
+
             # check if the lossless profile for the test speed is already present
             fvs = dict()
-            new_lossless_profile = "pg_lossless_{}_{}_profile".format(test_speed, orig_cable_len)
+            new_lossless_profile = "pg_lossless_{}_{}_profile".format(test_speed, cable_len_before_test)
             fvs = self.app_db.get_entry("BUFFER_PROFILE_TABLE", new_lossless_profile)
             if len(fvs):
                 profile_exp_cnt_diff = 0
@@ -110,15 +119,14 @@ def test_zero_cable_len_profile_update(self, dvs, setup_teardown_test):
             self.app_db.wait_for_deleted_entry("BUFFER_PROFILE_TABLE", test_lossless_profile)
 
             # buffer pgs should still point to the original buffer profile
-            orig_lossless_profile = "pg_lossless_{}_{}_profile".format(orig_speed, orig_cable_len)
             self.app_db.wait_for_field_match("BUFFER_PG_TABLE", self.INTF + ":3-4", {"profile": "[BUFFER_PROFILE_TABLE:{}]".format(orig_lossless_profile)})
             fvs = dict()
             for pg in self.pg_name_map:
                 fvs["SAI_INGRESS_PRIORITY_GROUP_ATTR_BUFFER_PROFILE"] = self.buf_pg_profile[pg]
                 self.asic_db.wait_for_field_match("ASIC_STATE:SAI_OBJECT_TYPE_INGRESS_PRIORITY_GROUP", self.pg_name_map[pg], fvs)
 
-            # change cable length to 'orig_cable_len'
-            self.change_cable_len(orig_cable_len)
+            # change cable length to 'cable_len_before_test'
+            self.change_cable_len(cable_len_before_test)
 
             # change intf speed to 'test_speed'
             dvs.runcmd("config interface speed {} {}".format(self.INTF, test_speed))