Support parameterized SQL and block potentially harmful queries

You can now call arbitrary SQL like this:

    /flights?sql=select%20*%20from%20airports%20where%20country%20like%20:c&c=iceland

Unescaped, those querystring params look like this:

    sql = select * from airports where country like :c
    c = iceland

So SQL can be constructed with named parameters embedded in it, which will
then be read from the querystring and correctly escaped.

This means we can aggressively filter the SQL parameter for potentially
dangerous syntax. For the moment we enforce that it starts with a SELECT
statement and we ban the sequence "pragma" from it entirely.

If you need to use pragma in a query, you can use the new named parameter
mechanism.

Fixes #39

datasite/app.py

@@ -95,7 +95,7 @@ class BaseView(HTTPMethodView):
         rows.sort(key=lambda row: row[-1])
         return [str(r[1]) for r in rows]
 
-    async def execute(self, db_name, sql):
+    async def execute(self, db_name, sql, params=None):
         """Executes sql against db_name in a thread"""
         def sql_operation_in_thread():
             conn = getattr(connections, db_name, None)
@@ -111,7 +111,8 @@ class BaseView(HTTPMethodView):
                 setattr(connections, db_name, conn)
 
             with sqlite_timelimit(conn, SQL_TIME_LIMIT_MS):
-                rows = conn.execute(sql)
+                print('execute: ', sql, 'params=', params)
+                rows = conn.execute(sql, params or {})
             return rows
 
         return await asyncio.get_event_loop().run_in_executor(
@@ -135,7 +136,7 @@ class BaseView(HTTPMethodView):
             data, extra_template_data = await self.data(
                 request, name, hash, **kwargs
             )
-        except sqlite3.OperationalError as e:
+        except (sqlite3.OperationalError, InvalidSql) as e:
             data = {
                 'ok': False,
                 'error': str(e),
@@ -216,8 +217,13 @@ class DatabaseView(BaseView):
     template = 'database.html'
 
     async def data(self, request, name, hash):
-        sql = request.args.get('sql') or 'select * from sqlite_master'
-        rows = await self.execute(name, sql)
+        sql = 'select * from sqlite_master'
+        params = {}
+        if request.args.get('sql'):
+            params = request.raw_args
+            sql = params.pop('sql')
+            validate_sql_select(sql)
+        rows = await self.execute(name, sql, params)
         columns = [r[0] for r in rows.description]
         return {
             'database': name,
@@ -448,3 +454,15 @@ def app_factory(files, num_threads=3):
         '/<db_name:[^/]+>/<table:[^/]+?>/<pk_path:[^/]+?><as_json:(.jsono?)?$>'
     )
     return app
+
+
+class InvalidSql(Exception):
+    pass
+
+
+def validate_sql_select(sql):
+    sql = sql.strip().lower()
+    if not sql.startswith('select '):
+        raise InvalidSql('Statement must begin with SELECT')
+    if 'pragma' in sql:
+        raise InvalidSql('Statement may not contain PRAGMA')