[armhf][sonic-installer] Fix the issue of sonic-installer list after set-default and cleanup (#2479)

What I did
sonic-installer list will throw a exception when install images as the following step

(onie-nos-install ) Install first image A
(sonic-installer) Install Image B and not reboot it
(sonic-installer) set default to back to Image A
sudo sonic-installer cleanup
At this time, executing sonic-installer list will throw exception

How I did it
Modify the get_next_image() in uboot.py to check and return index 1 when the images list contains two elements.
This PR should work with #12609

This PR is needed by branch 202012 and 2022o5

Signed-off-by: mlok <[email protected]>

Author: mlok-nokia

sonic_installer/bootloader/uboot.py

@@ -40,7 +40,7 @@ def get_next_image(self):
         proc = subprocess.Popen("/usr/bin/fw_printenv -n boot_next", shell=True, text=True, stdout=subprocess.PIPE)
         (out, _) = proc.communicate()
         image = out.rstrip()
-        if "sonic_image_2" in image:
+        if "sonic_image_2" in image and len(images) == 2:
             next_image_index = 1
         else:
             next_image_index = 0

tests/installer_bootloader_uboot_test.py

@@ -35,6 +35,37 @@ def test_remove_image(run_command_patch):
     args, _ = args_list[0]
     assert exp_image_path in args[0]
 
+@patch("sonic_installer.bootloader.uboot.subprocess.Popen")
+@patch("sonic_installer.bootloader.uboot.run_command")
+def test_get_next_image(run_command_patch, popen_patch):
+    class MockProc():
+        commandline = "boot_next"
+        def communicate(self):
+            return MockProc.commandline, None
+
+    def mock_run_command(cmd):
+        # Remove leading string "/usr/bin/fw_setenv boot_next " -- the 29 characters
+        MockProc.commandline = cmd[29:]
+
+    # Constants
+    intstalled_images = [
+        f'{uboot.IMAGE_PREFIX}expeliarmus-{uboot.IMAGE_PREFIX}abcde',
+        f'{uboot.IMAGE_PREFIX}expeliarmus-abcde',
+    ]
+    
+    run_command_patch.side_effect = mock_run_command
+    popen_patch.return_value = MockProc()
+
+    bootloader = uboot.UbootBootloader()
+    bootloader.get_installed_images = Mock(return_value=intstalled_images)
+
+    bootloader.set_default_image(intstalled_images[1])
+    
+    # Verify get_next_image was executed with image path
+    next_image=bootloader.get_next_image()
+
+    assert next_image == intstalled_images[1]
+
 @patch("sonic_installer.bootloader.uboot.subprocess.Popen")
 @patch("sonic_installer.bootloader.uboot.run_command")
 def test_set_fips_uboot(run_command_patch, popen_patch):