table_exists() now uses async SQL, refs #420

datasette/app.py

@@ -300,8 +300,13 @@ class Datasette:
             conn.execute('PRAGMA cache_size=-{}'.format(self.config("cache_size_kb")))
         pm.hook.prepare_connection(conn=conn)
 
-    def table_exists(self, database, table):
-        return table in self.inspect().get(database, {}).get("tables")
+    async def table_exists(self, database, table):
+        results = await self.execute(
+            database,
+            "select 1 from sqlite_master where type='table' and name=?",
+            params=(table,)
+        )
+        return bool(results.rows)
 
     def inspect(self):
         " Inspect the database and return a dictionary of table metadata "
@@ -410,19 +415,8 @@ class Datasette:
             for p in ps
         ]
 
-    async def execute(
-        self,
-        db_name,
-        sql,
-        params=None,
-        truncate=False,
-        custom_time_limit=None,
-        page_size=None,
-    ):
-        """Executes sql against db_name in a thread"""
-        page_size = page_size or self.page_size
-
-        def sql_operation_in_thread():
+    async def execute_against_connection_in_thread(self, db_name, fn):
+        def in_thread():
             conn = getattr(connections, db_name, None)
             if not conn:
                 info = self.inspect()[db_name]
@@ -441,7 +435,25 @@ class Datasette:
                     )
                 self.prepare_connection(conn)
                 setattr(connections, db_name, conn)
+            return fn(conn)
 
+        return await asyncio.get_event_loop().run_in_executor(
+            self.executor, in_thread
+        )
+
+    async def execute(
+        self,
+        db_name,
+        sql,
+        params=None,
+        truncate=False,
+        custom_time_limit=None,
+        page_size=None,
+    ):
+        """Executes sql against db_name in a thread"""
+        page_size = page_size or self.page_size
+
+        def sql_operation_in_thread(conn):
             time_limit_ms = self.sql_time_limit_ms
             if custom_time_limit and custom_time_limit < time_limit_ms:
                 time_limit_ms = custom_time_limit
@@ -476,8 +488,8 @@ class Datasette:
             else:
                 return Results(rows, False, cursor.description)
 
-        return await asyncio.get_event_loop().run_in_executor(
-            self.executor, sql_operation_in_thread
+        return await self.execute_against_connection_in_thread(
+            db_name, sql_operation_in_thread
         )
 
     def app(self):

datasette/utils.py

@@ -801,10 +801,11 @@ def get_plugins(pm):
 FORMATS = ('csv', 'json', 'jsono')
 
 
-def resolve_table_and_format(table_and_format, table_exists):
+async def resolve_table_and_format(table_and_format, table_exists):
     if '.' in table_and_format:
         # Check if a table exists with this exact name
-        if table_exists(table_and_format):
+        it_exists = await table_exists(table_and_format)
+        if it_exists:
             return table_and_format, None
     # Check if table ends with a known format
     for _format in FORMATS:

datasette/views/base.py

@@ -155,7 +155,7 @@ class BaseView(RenderMixin):
             r.headers["Access-Control-Allow-Origin"] = "*"
         return r
 
-    def resolve_db_name(self, request, db_name, **kwargs):
+    async def resolve_db_name(self, request, db_name, **kwargs):
         databases = self.ds.inspect()
         hash = None
         name = None
@@ -180,11 +180,13 @@ class BaseView(RenderMixin):
 
         if not correct_hash_provided:
             if "table_and_format" in kwargs:
-                table, _format = resolve_table_and_format(
+                async def async_table_exists(t):
+                    return await self.ds.table_exists(name, t)
+                table, _format = await resolve_table_and_format(
                     table_and_format=urllib.parse.unquote_plus(
                         kwargs["table_and_format"]
                     ),
-                    table_exists=lambda t: self.ds.table_exists(name, t)
+                    table_exists=async_table_exists
                 )
                 kwargs["table"] = table
                 if _format:
@@ -221,7 +223,7 @@ class BaseView(RenderMixin):
         assert NotImplemented
 
     async def get(self, request, db_name, **kwargs):
-        database, hash, correct_hash_provided, should_redirect = self.resolve_db_name(
+        database, hash, correct_hash_provided, should_redirect = await self.resolve_db_name(
             request, db_name, **kwargs
         )
         if should_redirect:
@@ -328,11 +330,13 @@ class BaseView(RenderMixin):
         if not _format:
             _format = (kwargs.pop("as_format", None) or "").lstrip(".")
         if "table_and_format" in kwargs:
-            table, _ext_format = resolve_table_and_format(
+            async def async_table_exists(t):
+                return await self.ds.table_exists(database, t)
+            table, _ext_format = await resolve_table_and_format(
                 table_and_format=urllib.parse.unquote_plus(
                     kwargs["table_and_format"]
                 ),
-                table_exists=lambda t: self.ds.table_exists(database, t)
+                table_exists=async_table_exists
             )
             _format = _format or _ext_format
             kwargs["table"] = table