Add error logging to the timestamp migration and improved migration

This also changes the migration of items with 2 timestamps, so it still
migrates the valid one, if the other is invalid.

See wagtail#9590

wagtail/migrations/0088_fix_log_entry_json_timestamps.py

@@ -1,12 +1,15 @@
 # Generated by Django 3.2.6 on 2022-11-09 07:50
 
 import datetime
+import logging
 
 import django.core.serializers.json
 from django.conf import settings
 from django.db import migrations, models
 from django.utils import timezone, dateparse
 
+logger = logging.getLogger("wagtail.migrations")
+
 
 def legacy_to_iso_format(date_string, tz=None):
     dt = datetime.datetime.strptime(date_string, "%d %b %Y %H:%M")
@@ -43,7 +46,13 @@ def migrate_logs_with_created_only(model, converter):
             # to UTC by django.
             item.data["revision"]["created"] = converter(created)
         except ValueError:
-            # TODO TBD: log error?
+            logger.warning(
+                "Failed to migrate 'created' timestamp '%s' of %s %s (%s)",
+                item.data["revision"]["created"],
+                model.__name__,
+                item.pk,
+                converter.__name__,
+            )
             continue
         except KeyError:
             continue
@@ -62,24 +71,42 @@ def migrate_schedule_logs(model, converter):
         created = item.data["revision"]["created"]
         # May be unset for "wagtail.schedule.cancel"-logs.
         go_live_at = item.data["revision"].get("go_live_at")
+        changed = False
         try:
             # "created" is set to timezone.now() for new revisions ("wagtail.publish.schedule")
             # and to self.created_at for "wagtail.schedule.cancel", which is set to UTC
             # by django.
             item.data["revision"]["created"] = converter(created)
-
-            if go_live_at:
-                # The go_live_at date is set to the revision object's "go_live_at".
-                # The revision's object is created by deserializing the json data (see wagtail.models.Revision.as_object()),
-                # and this process converts all datetime objects to the local timestamp (see https://github.com/wagtail/django-modelcluster/blob/8666f16eaf23ca98afc160b0a4729864411c0563/modelcluster/models.py#L109-L115).
-                # That's the reason, why this date is the only date, which is not stored in the log's JSON as UTC, but in the default timezone.
+            changed = True
+        except ValueError:
+            logger.warning(
+                "Failed to migrate 'created' timestamp '%s' of %s %s (%s)",
+                created,
+                model.__name__,
+                item.pk,
+                converter.__name__,
+            )
+
+        if go_live_at:
+            # The go_live_at date is set to the revision object's "go_live_at".
+            # The revision's object is created by deserializing the json data (see wagtail.models.Revision.as_object()),
+            # and this process converts all datetime objects to the local timestamp (see https://github.com/wagtail/django-modelcluster/blob/8666f16eaf23ca98afc160b0a4729864411c0563/modelcluster/models.py#L109-L115).
+            # That's the reason, why this date is the only date, which is not stored in the log's JSON as UTC, but in the default timezone.
+            try:
                 item.data["revision"]["go_live_at"] = converter(
                     go_live_at, tz=timezone.get_default_timezone()
                 )
-        except ValueError:
-            # TODO TBD: log error?
-            continue
-        else:
+                changed = True
+            except ValueError:
+                logger.warning(
+                    "Failed to migrate 'go_live_at' timestamp '%s' of %s %s (%s)",
+                    go_live_at,
+                    model.__name__,
+                    item.pk,
+                    converter.__name__,
+                )
+
+        if changed:
             item.save(update_fields=["data"])