Create test cases for testing L2 configuration.

Reference: https://github.com/sonic-net/SONiC/wiki/L2-Switch-mode

tests/l2/conftest.py

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+# conftest.py for L2 configuration.
+
+
+def pytest_addoption(parser):
+    # Target image to upgrade to.
+    parser.addoption(
+        "--target_image",
+        action="store",
+        default="",
+        help="Specify a target image for upgrade",
+        )

tests/l2/test_l2_configure.py

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+"""
+Tests related to L2 configuration
+"""
+import logging
+import pytest
+
+from tests.common import reboot
+from tests.common.config_reload import config_reload
+from tests.common.helpers.assertions import pytest_assert
+
+CONFIG_DB = '/etc/sonic/config_db.json'
+CONFIG_DB_BAK = '/etc/sonic/config_db.json.bak'
+TARGET_IMG_LOCALHOST = '/var/tmp/target_sonic_localhost.bin'
+TARGET_IMG_DUTHOST = '/var/tmp/target_sonic_duthost.bin'
+
+logger = logging.getLogger(__name__)
+
+pytestmark = [
+    pytest.mark.topology('any')
+]
+
+
+def test_l2_config_and_upgrade(request, duthosts, rand_one_dut_hostname, localhost):
+    """
+    @summary: A testcase that verifies DB migrator does not add bad data.
+        1. Cold reboot.
+        2. Configure switch into L2 mode.
+        3. Install a new image.
+        4. Reboot into the new image. DB migrator does its work.
+        5. Verify.
+
+    Args:
+        request: From pytest.
+        duthosts: set of DUTs.
+        rand_one_duthostname: sample a random DUT.
+        localhost: localhost object.
+        tbinfo: testbed info
+    """
+    # Setup.
+    duthost = duthosts[rand_one_dut_hostname]
+    hwsku = duthost.facts["hwsku"]
+
+    # Get target image path:
+    target_image = request.config.getoption('target_image', default=None)
+    if target_image is None:
+        pytest.skip("Skipping test due to missing --target_image.")
+    init_img = duthost.shell('sudo sonic-installer list | grep Current | cut -f2 -d " "')['stdout']
+
+    # Step 1: Reboot.
+    reboot(duthost, localhost, reboot_type="cold")
+
+    # Step 2: Configure DUT into L2 mode.
+    # Save original config
+    duthost.shell("sudo cp {} {}".format(CONFIG_DB, CONFIG_DB_BAK))
+    # Perform L2 configuration
+    l2_cfg = "sudo sonic-cfggen --preset l2 -p -H -k {}" \
+        " | sudo config load /dev/stdin -y".format(hwsku)
+    duthost.shell(l2_cfg)
+    duthost.shell("sudo config qos reload --no-dynamic-buffer")
+    duthost.shell("sudo config save -y")
+
+    # Step 3: Install new image and reboot.
+    init_img = duthost.shell('sudo sonic-installer list | grep Current | cut -f2 -d " "')['stdout']
+    logger.info("Init image: {}".format(init_img))
+    localhost.get_url(url=target_image, dest=TARGET_IMG_LOCALHOST)
+    duthost.copy(src=TARGET_IMG_LOCALHOST, dest=TARGET_IMG_DUTHOST)
+    duthost.shell("sudo sonic-installer install -y {}".format(TARGET_IMG_DUTHOST))
+    reboot(duthost, localhost, reboot_type="cold")
+
+    # Step 4: Verifies no config fro minigraph is written into ConfigDB.
+    new_img = duthost.shell('sudo sonic-installer list | grep Current | cut -f2 -d " "')['stdout']
+    logger.info("New image: {}".format(new_img))
+
+    try:
+        for table in ["TELEMETRY", "RESTAPI", "DEVICE_METADATA"]:
+            count = int(duthost.shell('redis-cli --scan --pattern "{}*" | wc -l'.format(table))['stdout'])
+            pytest_assert(count == 0, "{} table is not empty!".format(table))
+    except Exception:
+        raise
+    finally:
+        # Restore from L2 and new images, restore image first.
+        duthost.shell("sudo sonic-installer set-next-boot {}".format(init_img))
+        reboot(duthost, localhost, reboot_type="cold")
+        cur_img = duthost.shell('sudo sonic-installer list | grep Current | cut -f2 -d " "')['stdout']
+        logger.info("Current image: {}".format(cur_img))
+        duthost.shell("sudo cp {} {}".format(CONFIG_DB_BAK, CONFIG_DB))
+        config_reload(duthost)
+        duthost.shell("sudo rm {}".format(CONFIG_DB_BAK))