[Nexthop][fboss2-dev] Add a "fboss2 config session diff" command.

[benoit-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

This adds just the command fboss2-dev config session diff.

Note: this change is part of a series, the previous one is #756, the next one is #758.

Test Plan

Unit tests.

Sample usage

[admin@fboss101 benoit]$ ./fboss2-dev show interface | head
+-----------+--------+-------+------+------+------------------------------+----------------------------+
| Interface | Status | Speed | VLAN | MTU  | Addresses                    | Description                |
--------------------------------------------------------------------------------------------------------
| eth1/1/1  | down   | 800G  | 2001 | 1500 | 10.0.0.0/24                  | Test port for diff command |
|           |        |       |      |      | 2400::/64                    |                            |
|           |        |       |      |      | fe80::b4db:91ff:fe95:ff07/64 |                            |
+-----------+--------+-------+------+------+------------------------------+----------------------------+
| eth1/2/1  | down   | 200G  | 2003 | 9216 | 11.0.0.0/24                  | Another test description   |
|           |        |       |      |      | 2401::/64                    |                            |
|           |        |       |      |      | fe80::b4db:91ff:fe95:ff07/64 |                            |
[admin@fboss101 benoit]$ ./fboss2-dev config interface eth1/1/1 mtu 9000
Successfully set MTU for interface(s) eth1/1/1 to 9000
[admin@fboss101 benoit]$ ./fboss2-dev config interface eth1/2/1 description 'This is a test.'
Successfully set description for interface(s) eth1/2/1
[admin@fboss101 benoit]$ ./fboss2-dev config session diff
--- /etc/coop/agent.conf	2025-11-05 12:49:14.497415902 -0800
+++ /home/admin/.fboss2/agent.conf	2025-11-05 12:57:27.123758309 -0800
@@ -369,7 +369,7 @@
         ],
         "isStateSyncDisabled": true,
         "isVirtual": false,
-        "mtu": 1500,
+        "mtu": 9000,
         "routerID": 0,
         "scope": 0,
         "type": 1,
@@ -2230,7 +2230,7 @@
       },
       {
         "conditionalEntropyRehash": false,
-        "description": "Another test description",
+        "description": "This is a test.",
         "drainState": 0,
         "expectedLLDPValues": {
           "2": "eth1/6/1"

[admin@fboss101 benoit]$ ./fboss2-dev config history
 Revision  Owner  Commit Time         
------------------------------------------
 r1        root   2025-11-05 13:26:58 
 r2        root   2025-11-05 13:27:35 
 r3        root   2025-11-05 13:28:13 
 r4        admin  2025-11-05 14:37:51 
 r5        admin  2025-11-05 14:38:31

You can also do fboss2-dev config diff r2 r4 or fboss2-dev config diff r1 current to compare committed revisions.

[meta-codesync]

@joseph5wu has imported this pull request. If you are a Meta employee, you can view this in D90277033.

[meta-codesync]

@joseph5wu merged this pull request in adf3fa4.