improve ux when trying to open sqlite without protocol

optuna_dashboard/_storage_url.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import os.path
+from pathlib import Path
 import re
 from typing import TYPE_CHECKING
 
@@ -58,12 +59,25 @@
     return guess_storage_from_url(storage)
 
 
+def _has_sqlite_header(storage_url: str) -> bool:
+    storage_path = Path(storage_url)
+    SQLITE_HEADER = (
+        b"SQLite format 3\x00"  # see https://github.com/optuna/optuna-dashboard/pull/800
+    )
+    with storage_path.open(mode="rb") as f:
+        header = f.read(len(SQLITE_HEADER))
+    return header == SQLITE_HEADER
+
+
 def guess_storage_from_url(storage_url: str) -> BaseStorage:
     if storage_url.startswith("redis"):
         return get_journal_redis_storage(storage_url)
 
     if os.path.isfile(storage_url):
-        return get_journal_file_storage(storage_url)
+        if _has_sqlite_header(storage_url):
+            return get_rdb_storage("sqlite:///" + storage_url)
+        else:
+            return get_journal_file_storage(storage_url)
 
     if rfc1738_pattern.match(storage_url) is not None:
         return get_rdb_storage(storage_url)