issue_comments

That fixed the race condition in the datasette-graphql tests, which is the only place that I've been able to successfully replicate this. I'm going to land this change.

I can't replicate the race condition locally with or without this patch. I'm going to push the commit and then test the CI run from datasette-graphql that was failing against it.

The test suite passes with that change.

Second attempt at this: ```diff diff --git a/datasette/app.py b/datasette/app.py index 5976d8b..5f348cb 100644 --- a/datasette/app.py +++ b/datasette/app.py @@ -224,6 +224,7 @@ class Datasette: self.inspect_data = inspect_data self.immutables = set(immutables or []) self.databases = collections.OrderedDict() + self._refresh_schemas_lock = asyncio.Lock() self.crossdb = crossdb if memory or crossdb or not self.files: self.add_database(Database(self, is_memory=True), name="_memory") @@ -332,6 +333,12 @@ class Datasette: self.client = DatasetteClient(self)

 async def refresh_schemas(self):

• if self._refresh_schemas_lock.locked():
• return
• async with self._refresh_schemas_lock:
• await self._refresh_schemas() +
• async def _refresh_schemas(self): internal_db = self.databases["_internal"] if not self.internal_db_created: await init_internal_db(internal_db) ```

https://stackoverflow.com/a/25799871/6083 has a good example of using asyncio.Lock():

```python stuff_lock = asyncio.Lock()

async def get_stuff(url): async with stuff_lock: if url in cache: return cache[url] stuff = await aiohttp.request('GET', url) cache[url] = stuff return stuff ```

The only place that calls refresh_schemas() is here: https://github.com/simonw/datasette/blob/dd5ee8e66882c94343cd3f71920878c6cfd0da41/datasette/views/base.py#L120-L124

Ideally only one call to refresh_schemas() would be running at any one time.

The race condition happens inside this method - initially with the call to await init_internal_db(): https://github.com/simonw/datasette/blob/dd5ee8e66882c94343cd3f71920878c6cfd0da41/datasette/app.py#L334-L359

Here's the traceback I got from datasette-graphql (annoyingly only running the tests in GitHub Actions CI - I've not been able to replicate on my laptop yet):

``` tests/test_utils.py . [100%]

=================================== FAILURES =================================== ____ testgraphql_examples[path0] _____

ds = <datasette.app.Datasette object at 0x7f6b8b6f8fd0> path = PosixPath('/home/runner/work/datasette-graphql/datasette-graphql/examples/filters.md')

@pytest.mark.asyncio
@pytest.mark.parametrize(
    "path", (pathlib.Path(__file__).parent.parent / "examples").glob("*.md")
)
async def test_graphql_examples(ds, path):
    content = path.read_text()
    query = graphql_re.search(content)[1]
    try:
        variables = variables_re.search(content)[1]
    except TypeError:
        variables = "{}"
    expected = json.loads(json_re.search(content)[1])
    response = await ds.client.post(
        "/graphql",
        json={
            "query": query,
            "variables": json.loads(variables),
        },
    )

  assert response.status_code == 200, response.json()

E AssertionError: {'data': {'repos_arraycontains': None, 'users_contains': None, 'users_date': None, 'users_endswith': None, ...}, 'erro...", 'path': ['users_gt']}, {'locations': [{'column': 5, 'line': 34}], 'message': "'rows'", 'path': ['users_gte']}, ...]} E assert 500 == 200 E + where 500 = <Response [500 Internal Server Error]>.status_code

tests/test_graphql.py:142: AssertionError ----------------------------- Captured stderr call ----------------------------- table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists table databases already exists Traceback (most recent call last): File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/app.py", line 1171, in route_path response = await view(request, send) File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/views/base.py", line 151, in view request, **request.scope["url_route"]["kwargs"] File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/views/base.py", line 123, in dispatch_request await self.ds.refresh_schemas() File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/app.py", line 338, in refresh_schemas await init_internal_db(internal_db) File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/utils/internal_db.py", line 16, in init_internal_db block=True, File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/database.py", line 102, in execute_write return await self.execute_write_fn(_inner, block=block) File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/database.py", line 118, in execute_write_fn raise result File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/database.py", line 139, in _execute_writes result = task.fn(conn) File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/datasette/database.py", line 100, in _inner return conn.execute(sql, params or []) sqlite3.OperationalError: table databases already exists ```

This just broke the datasette-graphql test suite: https://github.com/simonw/datasette-graphql/issues/77 - I need to figure out a solution here.

Ideally I'd figure out a way to replicate this error in a concurrent unit test.

I started trying to use locks to resolve this but I've not figured out the right way to do that yet - here's my first experiment: ```diff diff --git a/datasette/app.py b/datasette/app.py index 9e15a16..1681c9d 100644 --- a/datasette/app.py +++ b/datasette/app.py @@ -217,6 +217,7 @@ class Datasette: self.inspect_data = inspect_data self.immutables = set(immutables or []) self.databases = collections.OrderedDict() + self._refresh_schemas_lock = threading.Lock() if memory or not self.files: self.add_database(Database(self, is_memory=True), name="_memory") # memory_name is a random string so that each Datasette instance gets its own @@ -324,6 +325,13 @@ class Datasette: self.client = DatasetteClient(self)

 async def refresh_schemas(self):

• return
• if self._refresh_schemas_lock.locked():
• return
• with self._refresh_schemas_lock:
• await self._refresh_schemas() +
• async def _refresh_schemas(self): internal_db = self.databases["_internal"] if not self.internal_db_created: await init_internal_db(internal_db) ```