Cache the lookup pepper in the HashingMetadataStore. (#475)

changelog.d/475.misc

@@ -0,0 +1 @@
+Cache the lookup pepper in the `HashingMetadataStore`.

sydent/db/hashing_metadata.py

@@ -26,13 +26,18 @@
 class HashingMetadataStore:
     def __init__(self, sydent: "Sydent") -> None:
         self.sydent = sydent
+        self._cached_lookup_pepper: Optional[str] = None
 
     def get_lookup_pepper(self) -> Optional[str]:
         """Return the value of the current lookup pepper from the db
 
         :return: A pepper if it exists in the database, or None if one does
                  not exist
         """
+
+        if self._cached_lookup_pepper is not None:
+            return self._cached_lookup_pepper
+
         cur = self.sydent.db.cursor()
         res = cur.execute("select lookup_pepper from hashing_metadata")
         # Annotation safety: lookup_pepper is marked as varchar(256) in the
@@ -52,6 +57,8 @@ def get_lookup_pepper(self) -> Optional[str]:
         if isinstance(pepper, bytes):
             pepper = pepper.decode("UTF-8")
 
+        self._cached_lookup_pepper = pepper
+
         return pepper
 
     def store_lookup_pepper(
@@ -84,6 +91,9 @@ def store_lookup_pepper(
         # Commit the queued db transactions so that adding a new pepper and hashing is atomic
         self.sydent.db.commit()
 
+        # Update the cached pepper (only once the transaction has committed successfully!)
+        self._cached_lookup_pepper = pepper
+
     def _rehash_threepids(
         self,
         cur: Cursor,