[QoS] Disable pfc and pfcwd on service ports (#24145)

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#### Why I did it
Disable pfc and pfcwd on service ports since they carry lossy traffic and do not require any pfc config

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it
Finetune buffer and qos configs
#### How to verify it
Copy templates onto switch, reload the config and see the new config db
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#### Which release branch to backport (provide reason below if selected)

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- [x] 202006
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- [ ] 202205
- [ ] 202211
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [ ] 202411
- [X] 202505

#### Tested branch (Please provide the tested image version)

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- Please provide tested image version
- e.g.
- [x] 20201231.100
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- [ ] <!-- image version 1 -->
- [ ] <!-- image version 2 -->

#### Description for the changelog
<!--
Write a short (one line) summary that describes the changes in this
pull request for inclusion in the changelog:
-->
Disable PFC and PFCWD on service ports
<!--
 Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.
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#### Link to config_db schema for YANG module changes
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Provide a link to config_db schema for the table for which YANG model
is defined
Link should point to correct section on https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-yang-models/doc/Configuration.md
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All unit tests pass

```
collected 312 items

tests/test_cfggen.py .................................................................................... [ 26%]
tests/test_cfggen_from_yang.py .................. [ 32%]
tests/test_cfggen_pfx_filter.py . [ 33%]
tests/test_cfggen_platformJson.py ...... [ 34%]
tests/test_cfggen_t2_chassis_fe.py ...... [ 36%]
tests/test_chassis_cfggen.py ......................................................... [ 55%]
tests/test_frr.py ...... [ 57%]
tests/test_j2files.py ................................................ [ 72%]
tests/test_j2files_t2_chassis_fe.py ... [ 73%]
tests/test_minigraph_case.py ........................................... [ 87%]
tests/test_multinpu_cfggen.py ........................................ [100%]

===================================================================== 312 passed in 321.99s (0:05:21) ==================================
```

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

Author: mssonicbld

device/arista/x86_64-arista_7060x6_16pe_384c_b/Arista-7060X6-16PE-384C-B-O128S2/buffers_defaults_t0.j2

@@ -34,3 +34,8 @@
         }
     },
 {%- endmacro %}
+
+{%- macro generate_service_port_list(PORT_SERVICE) -%}
+    {%- if PORT_SERVICE.append("Ethernet512") %}{% endif %}
+    {%- if PORT_SERVICE.append("Ethernet513") %}{% endif %}
+{%- endmacro %}

device/arista/x86_64-arista_7060x6_16pe_384c_b/Arista-7060X6-16PE-384C-B-O128S2/buffers_defaults_t1.j2

@@ -34,3 +34,8 @@
         }
     },
 {%- endmacro %}
+
+{%- macro generate_service_port_list(PORT_SERVICE) -%}
+    {%- if PORT_SERVICE.append("Ethernet512") %}{% endif %}
+    {%- if PORT_SERVICE.append("Ethernet513") %}{% endif %}
+{%- endmacro %}

files/build_templates/qos_config.j2

@@ -5,6 +5,22 @@
 {%- set PORT_QOS_BYPASS = [] %}
 {%- set PORT_UPLINK = [] %}
 {%- set PORT_DOWNLINK = [] %}
+{%- set PORT_SERVICE = [] %}
+
+{% if template_exists("buffers.json.j2") %}
+    {% import "buffers.json.j2" as buffer_defs %}
+    {%- set filename_postfix = buffer_defs.default_topo | default("def", true) %}
+{% else %}
+    {%- set filename_postfix = "def" %}
+{% endif %}
+
+{# Allow includer to pre-package defs without needing an import #}
+{%- if defs is not defined %}
+    {# Import default values from device HWSKU folder #}
+    {%- if template_exists('buffers_defaults_%s.j2' % filename_postfix) %}
+        {%- import 'buffers_defaults_%s.j2' % filename_postfix as defs with context %}
+    {%- endif -%}
+{%- endif -%}
 
 {%- set voq_chassis = false %}
 {%- if DEVICE_METADATA is defined and DEVICE_METADATA['localhost']['switch_type'] is defined and  DEVICE_METADATA['localhost']['switch_type']  == 'voq' %}
@@ -37,6 +53,9 @@
 {%- if generate_qos_bypass_port_list is defined %}
     {%- if generate_qos_bypass_port_list(PORT_QOS_BYPASS) %} {% endif %}
 {%- endif %}
+{%- if defs is defined and defs.generate_service_port_list is defined %}
+    {%- if defs.generate_service_port_list(PORT_SERVICE) %}{% endif %}
+{%- endif %}
 
 {%- if PORT_ALL | sort_by_port_index %}{% endif %}
 
@@ -426,19 +445,28 @@
 {% endif %}
 {% endif %}
 {% if port not in PORT_DPC %}
+{% if port in PORT_SERVICE %}
+            "pfc_enable"      : "",
+            "pfcwd_sw_enable" : "",
+{% else %}
 {% if port in port_names_list_extra_queues %}
             "pfc_enable"      : "2,3,4,6",
 {% else %}
             "pfc_enable"      : "{{ LOSSLESS_TC|join(',') }}",
 {% endif %}
             "pfcwd_sw_enable" : "{{ LOSSLESS_TC|join(',') }}",
 {% endif %}
+{% endif %}
 {% if port not in PORT_DPC %}
             "tc_to_pg_map"    : "AZURE",
 {% else %}
             "tc_to_pg_map"    : "AZURE_DPC",
 {% endif %}
+{% if port in PORT_SERVICE %}
+            "pfc_to_queue_map": ""
+{% else %}
             "pfc_to_queue_map": "AZURE"
+{% endif %}
         }{% if not loop.last %},{% endif %}
 
 {% endfor %}

src/sonic-config-engine/sonic-cfggen

@@ -301,6 +301,17 @@ def _get_jinja2_env(paths):
     env.filters['b64encode'] = b64encode
     env.filters['b64decode'] = b64decode
 
+    # Introduce a 'template_exists' function to check if a template exists
+    # This is used to conditionally import a template since jinja2 import
+    # statement fails if the template does not exist and does not have native support
+    def template_exists(name):
+        try:
+            loader.get_source(env, name)
+            return True
+        except jinja2.exceptions.TemplateNotFound:
+            return False
+    env.globals['template_exists'] = template_exists
+
     return env
 
 def main():