[bug] Migration test fix (#10899)

* Pop instead of get

* Better error handling for unit registry

* Prevent caching during database migrations

* Remove debug msg

* Revert changes

src/backend/InvenTree/InvenTree/conversion.py

@@ -24,6 +24,22 @@ logger = structlog.get_logger('inventree')
 logging.getLogger('pint').setLevel(logging.ERROR)
 
 
+def can_cache_registry() -> bool:
+    """Return True if it is appropriate to cache the unit registry.
+
+    Prevent caching under certain conditions (such as database migration)
+    to prevent database access.
+    """
+    import InvenTree.ready
+
+    return not any([
+        InvenTree.ready.isImportingData(),
+        InvenTree.ready.isRunningBackup(),
+        InvenTree.ready.isRunningMigrations(),
+        InvenTree.ready.isInTestMode(),
+    ])
+
+
 def get_unit_registry_hash():
     """Return a hash representing the current state of the unit registry.
 
@@ -53,6 +69,9 @@ def set_unit_registry_hash(registry_hash: str):
     global _unit_registry_hash
     _unit_registry_hash = registry_hash
 
+    if not can_cache_registry():
+        return
+
     # Save to both the global settings and the session cache
     set_global_setting('_UNIT_REGISTRY_HASH', registry_hash)
     set_session_cache(_UNIT_REG_CACHE_KEY, registry_hash)
@@ -68,7 +87,7 @@ def get_unit_registry():
         return reload_unit_registry()
 
     # Check if the unit registry has changed
-    if _unit_registry_hash != get_unit_registry_hash():
+    if can_cache_registry() and _unit_registry_hash != get_unit_registry_hash():
         logger.info('Unit registry hash has changed, reloading unit registry')
         return reload_unit_registry()
 
@@ -103,33 +122,32 @@ def reload_unit_registry():
     reg.define('thousand = 1000')
 
     # Allow for custom units to be defined in the database
+    # Calculate a hash of all custom units
+    hash_md5 = md5()
+
     try:
         from common.models import CustomUnit
 
-        # Calculate a hash of all custom units
-        hash_md5 = md5()
-
-        for cu in CustomUnit.objects.all():
-            try:
-                fmt = cu.fmt_string()
-                reg.define(fmt)
-
-                hash_md5.update(fmt.encode('utf-8'))
-
-            except Exception as e:
-                logger.exception(
-                    'Failed to load custom unit: %s - %s', cu.fmt_string(), e
-                )
-
-        # Once custom units are loaded, save registry
-        _unit_registry = reg
-
-        # Update the unit registry hash
-        set_unit_registry_hash(hash_md5.hexdigest())
-
+        custom_units = list(CustomUnit.objects.all())
     except Exception:
-        # Database is not ready, or CustomUnit model is not available
-        pass
+        # Database is likely not ready
+        custom_units = []
+
+    for cu in custom_units:
+        try:
+            fmt = cu.fmt_string()
+            reg.define(fmt)
+
+            hash_md5.update(fmt.encode('utf-8'))
+
+        except Exception as e:
+            logger.exception('Failed to load custom unit: %s - %s', cu.fmt_string(), e)
+
+    # Once custom units are loaded, save registry
+    _unit_registry = reg
+
+    # Update the unit registry hash
+    set_unit_registry_hash(hash_md5.hexdigest())
 
     dt = time.time() - t_start
     logger.debug('Loaded unit registry in %.3f s', dt)

src/backend/InvenTree/common/models.py

@@ -704,7 +704,7 @@ class BaseInvenTreeSetting(models.Model):
         ):  # pragma: no cover
             return
 
-        attempts = int(kwargs.get('attempts', 3))
+        attempts = int(kwargs.pop('attempts', 3))
 
         filters = {
             'key__iexact': key,