[admin] Fixed FileNotFoundError openwisp#140

The change_view associated with BuildAdmin class was throwing
an exception when trying to delete a FirmwareImage object,
when the file associated with that object had already been
deleted from the file system.

There were two tasks according to my understanding:
1. Prevent the website to break and catch the error:
        the return expression in change_view has been put
in a try/catch expression, with instructions/hints/warnings
for the user using message_user

2. If an image file has been deleted from the filesystem,
automatically remove the FirmwareImage assiciated with it:

        The 'get_queryset' method for FirmwareImageInline class
has been over-written to automatically remove FirmwareImage objects
associated with deleted files and the information has been
logged with logger.

Fixes openwisp#140

openwisp_firmware_upgrader/admin.py

@@ -1,4 +1,5 @@
 import logging
+import os
 from datetime import timedelta
 
 import reversion
@@ -7,7 +8,7 @@
 from django.conf import settings
 from django.contrib import admin, messages
 from django.contrib.admin.helpers import ACTION_CHECKBOX_NAME
-from django.shortcuts import redirect
+from django.shortcuts import HttpResponseRedirect, redirect
 from django.template.response import TemplateResponse
 from django.urls import resolve, reverse
 from django.utils.safestring import mark_safe
@@ -84,6 +85,37 @@ def has_change_permission(self, request, obj=None):
             return False
         return True
 
+    def get_parent_object(self, request):
+        """
+        method to get the instance from model' s parent class
+        in context with the ForeignKey relation
+        """
+        resolved = resolve(request.path_info)
+        if resolved.kwargs:
+            return self.parent_model.objects.get(pk=resolved.kwargs["object_id"])
+        return None
+
+    def get_queryset(self, request):
+        """
+        overriding queryset to remove any FirmwareImage instances,
+        where the image file has been manually deleted by the user
+        from the file system
+        """
+        qs = super(FirmwareImageInline, self).get_queryset(request)
+        build = self.get_parent_object(request)
+        qs = qs.filter(build=build)
+        for imageObject in qs:
+            if imageObject.file is not None:
+                path = imageObject.file.path
+                if not os.path.isfile(path):
+                    try:
+                        type = imageObject.type
+                        imageObject.delete()
+                        logger.warning(f"Image object {type} was removed")
+                    except Exception:
+                        pass
+        return qs
+
 
 class CategoryFilter(MultitenantOrgFilter):
     multitenant_lookup = 'organization_id__in'
@@ -192,7 +224,21 @@ def change_view(self, request, object_id, form_url='', extra_context=None):
         extra_context = extra_context or {}
         upgrade_url = f'{app_label}_build_changelist'
         extra_context.update({'upgrade_url': upgrade_url})
-        return super().change_view(request, object_id, form_url, extra_context)
+        # preventing change_view to throw an error/exception
+        try:
+            return super().change_view(request, object_id, form_url, extra_context)
+        except Exception as e:
+            if type(e).__name__ == "FileNotFoundError":
+                self.message_user(
+                    request, "Please reload the page", level=logging.ERROR
+                )
+            else:
+                self.message_user(
+                    request,
+                    "Image objects have been removed or form data didn't validate",
+                    level=logging.ERROR,
+                )
+                return HttpResponseRedirect(request.path)
 
 
 class UpgradeOperationForm(forms.ModelForm):