Designate NULL_REGION_ID; add check when constructing Region

Author: RyanSkonnord

src/sentry/types/region.py

@@ -43,9 +43,11 @@ class Region:
     """The region's category."""
 
     def __post_init__(self) -> None:
-        from sentry.utils.snowflake import REGION_ID
+        from sentry.utils.snowflake import NULL_REGION_ID, REGION_ID
 
         REGION_ID.validate(self.id)
+        if self.id == NULL_REGION_ID:
+            raise ValueError(f"Region ID {NULL_REGION_ID} is reserved for non-multi-region systems")
 
     def to_url(self, path: str) -> str:
         return self.address + path

src/sentry/utils/snowflake.py

@@ -61,6 +61,8 @@ def validate(self, value):
 MAX_AVAILABLE_REGION_SEQUENCES = 1 << REGION_SEQUENCE.length
 assert MAX_AVAILABLE_REGION_SEQUENCES > 0
 
+NULL_REGION_ID = 0
+
 
 def msb_0_ordering(value, width):
     """
@@ -80,7 +82,7 @@ def generate_snowflake_id(redis_key: str) -> int:
     try:
         segment_values[REGION_ID] = get_local_region().id
     except RegionContextError:  # expected if running in monolith mode
-        segment_values[REGION_ID] = 0
+        segment_values[REGION_ID] = NULL_REGION_ID
 
     current_time = datetime.now().timestamp()
     # supports up to 130 years