Test for sql_time_limit_ms + sqlite_functions mechanism

Added a unit test for the sql_time_limit_ms option.

To test this, I needed to add a custom SQLite sleep() function. I've added a
simple mechanism to the Datasette class for registering custom functions.

I also had to modify the sqlite_timelimit() function. It makes use of a magic
value, N, which is the number of SQLite virtual machine instructions that
should execute in between calls to my termination decision function.

The value of N was not finely grained enough for my test to work - so I've
added logic that says that if the time limit is less than 50ms, N is set to 1.
This got the tests working.

Refs #95

datasette/app.py

@@ -108,6 +108,12 @@ class BaseView(HTTPMethodView):
             return name, expected, should_redirect
         return name, expected, None
 
+    def prepare_connection(self, conn):
+        conn.row_factory = sqlite3.Row
+        conn.text_factory = lambda x: str(x, 'utf-8', 'replace')
+        for name, num_args, func in self.ds.sqlite_functions:
+            conn.create_function(name, num_args, func)
+
     async def execute(self, db_name, sql, params=None, truncate=False):
         """Executes sql against db_name in a thread"""
         def sql_operation_in_thread():
@@ -119,8 +125,7 @@ class BaseView(HTTPMethodView):
                     uri=True,
                     check_same_thread=False,
                 )
-                conn.row_factory = sqlite3.Row
-                conn.text_factory = lambda x: str(x, 'utf-8', 'replace')
+                self.prepare_connection(conn)
                 setattr(connections, db_name, conn)
 
             with sqlite_timelimit(conn, self.ds.sql_time_limit_ms):
@@ -525,6 +530,7 @@ class Datasette:
         self.cors = cors
         self._inspect = inspect_data
         self.metadata = metadata or {}
+        self.sqlite_functions = []
 
     def inspect(self):
         if not self._inspect:

datasette/utils.py

@@ -85,13 +85,21 @@ class CustomJSONEncoder(json.JSONEncoder):
 @contextmanager
 def sqlite_timelimit(conn, ms):
     deadline = time.time() + (ms / 1000)
+    # n is the number of SQLite virtual machine instructions that will be
+    # executed between each check. It's hard to know what to pick here.
+    # After some experimentation, I've decided to go with 1000 by default and
+    # 1 for time limits that are less than 50ms
+    n = 1000
+    if ms < 50:
+        n = 1
 
     def handler():
         if time.time() >= deadline:
             return 1
-    conn.set_progress_handler(handler, 10000)
+
+    conn.set_progress_handler(handler, n)
     yield
-    conn.set_progress_handler(None, 10000)
+    conn.set_progress_handler(None, n)
 
 
 class InvalidSql(Exception):