_prepare_connection not called on write connections

[simonw]

I was trying to initalize SpatiaLite in a write connection:

>>> from datasette.app import Datasette
>>> ds = Datasette(memory=True, files=[], sqlite_extensions=["spatialite"])
>>> db = ds.add_memory_database('geo')
>>> await db.execute_write("select InitSpatialMetadata(1)")
UUID('3f143baa-4e3d-5842-a36f-4fa2f683b72f')
no such function: InitSpatialMetadata

It looks like the code that loads additional modules only works on read-only connections, not on write connections:

datasette/datasette/database.py

Lines 146 to 153 in 92a5280

	async def execute_fn(self, fn):
	def in_thread():
	conn = getattr(connections, self.name, None)
	if not conn:
	conn = self.connect()
	self.ds._prepare_connection(conn, self.name)
	setattr(connections, self.name, conn)
	return fn(conn)

Compared to:

datasette/datasette/database.py

Lines 124 to 132 in 92a5280

	def _execute_writes(self):
	# Infinite looping thread that protects the single write connection
	# to this database
	conn_exception = None
	conn = None
	try:
	conn = self.connect(write=True)
	except Exception as e:
	conn_exception = e

[simonw]

Made this change while working on this issue:

• Remove undocumented sqlite_functions mechanism #1567

I'm going to write a test for this that uses that sleep() SQL function from c35b84a.