Implement caching for unit registry: (#10870)

* Implement caching for unit registry:

- Registry could become out of sync across sessions
- Implement a simple caching system

* Simplify code

src/backend/InvenTree/InvenTree/conversion.py

@@ -2,30 +2,76 @@
 
 import logging
 import re
+from hashlib import md5
 from typing import Optional
 
 from django.core.exceptions import ValidationError
 from django.utils.translation import gettext_lazy as _
 
 import pint
-
-_unit_registry = None
-
 import structlog
 
+from common.settings import get_global_setting, set_global_setting
+from InvenTree.cache import get_session_cache, set_session_cache
+
+_UNIT_REG_CACHE_KEY = 'unit_registry_hash'
+_unit_registry = None
+_unit_registry_hash: str = ''
+
 logger = structlog.get_logger('inventree')
 
 # Disable log output for Pint library
 logging.getLogger('pint').setLevel(logging.ERROR)
 
 
+def get_unit_registry_hash():
+    """Return a hash representing the current state of the unit registry.
+
+    We use this to determine if we need to reload the unit registry,
+    due to changes in the database.
+    """
+    # Look in the session cache first (faster, and potentially newer)
+    registry_hash = get_session_cache(_UNIT_REG_CACHE_KEY)
+
+    if registry_hash is None:
+        registry_hash = get_global_setting(
+            '_UNIT_REGISTRY_HASH', create=False, backup_value=''
+        )
+
+        if registry_hash:
+            set_session_cache(_UNIT_REG_CACHE_KEY, registry_hash)
+
+    return registry_hash
+
+
+def set_unit_registry_hash(registry_hash: str):
+    """Save the hash representing the current state of the unit registry.
+
+    Because most of the registry is static, we only need to consider the
+    CustomUnit entries in the database.
+    """
+    global _unit_registry_hash
+    _unit_registry_hash = registry_hash
+
+    # 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)
+
+
 def get_unit_registry():
     """Return a custom instance of the Pint UnitRegistry."""
     global _unit_registry
+    global _unit_registry_hash
 
     # Cache the unit registry for speedier access
     if _unit_registry is None:
         return reload_unit_registry()
+
+    # Check if the unit registry has changed
+    if _unit_registry_hash != get_unit_registry_hash():
+        logger.info('Unit registry hash has changed, reloading unit registry')
+        return reload_unit_registry()
+
     return _unit_registry
 
 
@@ -60,9 +106,16 @@ def reload_unit_registry():
     try:
         from common.models import CustomUnit
 
+        # Calculate a hash of all custom units
+        hash_md5 = md5()
+
         for cu in CustomUnit.objects.all():
             try:
-                reg.define(cu.fmt_string())
+                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
@@ -71,6 +124,9 @@ def reload_unit_registry():
         # Once custom units are loaded, save registry
         _unit_registry = reg
 
+        # Update the unit registry hash
+        set_unit_registry_hash(hash_md5.hexdigest())
+
     except Exception:
         # Database is not ready, or CustomUnit model is not available
         pass