Skip unnecessary writes in default impl for modifyGet

datastore/datastore.nim

@@ -71,6 +71,9 @@ method modifyGet*(self: Datastore, key: Key, fn: ModifyGet): Future[?!seq[byte]]
   ## | some(u) | none     | delete u                     |
   ## | some(u) | some(v)  | replace u with v (if u != v) |
   ##
+  ## If `fn` raises an error, the value associated with `key` remains unchanged and raised error is wrapped
+  ## into a failure and returned as a result.
+  ##
   ## Note that `fn` can be called multiple times (when concurrent modification was detected). In such case
   ## only the last auxillary value is returned.
   ##

datastore/defaultimpl.nim

@@ -37,6 +37,10 @@ proc defaultModifyGetImpl*(
     except CatchableError as err:
       return failure(err)
 
+    if maybeCurrentData == maybeNewData:
+      # no need to change currently stored value (if any)
+      return aux.success
+
     if newData =? maybeNewData:
       if err =? (await self.put(key, newData)).errorOption:
         return failure(err)

datastore/sql/sqliteds.nim

@@ -66,7 +66,7 @@ method modifyGet*(self: SQLiteDatastore, key: Key, fn: ModifyGet): Future[?!seq[
       return failure(err)
 
     if maybeCurrentData == maybeNewData:
-      # no need to change currently stored value
+      # no need to change currently stored value (if any)
       break
 
     if err =? self.db.beginStmt.exec().errorOption: