fix: make attachment content loading properly asyncio-friendly #1143

[deepankarm]

A proposal to fix #1143

Since all invocations in llm.Attachment are doing IO using httpx non-async primitives, we can asyncify build_messages by executing it in a different thread using asyncio.to_thread.

The same can be verified using the following code (event loop block detected with pyleak).

import os
import asyncio

from llm.default_plugins.openai_models import AsyncChat
from llm.models import Attachment
from pyleak import no_event_loop_blocking

async def main():
    model = AsyncChat(
        model_id="gpt-4.1-2025-04-14",
        api_base="https://api.openai.com/v1",
        key=os.getenv("OPENAI_API_KEY"),
    )
    conversation = model.conversation()
    async with no_event_loop_blocking(action="raise"):
        await conversation.prompt(
            prompt="What is this image?",
            attachments=[
                Attachment(
                    url="https://upload.wikimedia.org/wikipedia/commons/thumb/7/74/A-Cat.jpg/1200px-A-Cat.jpg?20101227100718",
                ),
            ],
        )
    print(conversation.responses)

if __name__ == "__main__":
    asyncio.run(main())

Before

with messages = self.build_messages(prompt, conversation)

pyleak.eventloop.EventLoopBlockError: Detected 1 event loop blocks

Event Loop Block: block-1
  Duration: 0.305s (threshold: 0.100s)
  Timestamp: 1751442718.988
  Blocking Stack:
    File "/.../deepankarm/dummy/llm-run-1.py", line 30, in <module>
        asyncio.run(main())
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/asyncio/runners.py", line 194, in run
        return runner.run(main)
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/asyncio/runners.py", line 118, in run
        return self._loop.run_until_complete(task)
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/asyncio/base_events.py", line 671, in run_until_complete
        self.run_forever()
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/asyncio/base_events.py", line 638, in run_forever
        self._run_once()
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/asyncio/base_events.py", line 1971, in _run_once
        handle._run()
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/asyncio/events.py", line 84, in _run
        self._context.run(self._callback, *self._args)
      File "/.../deepankarm/dummy/llm-1.py", line 17, in main
        await conversation.prompt(
      File "/.../deepankarm/llm/llm/models.py", line 1382, in _force
        async for chunk in self:
      File "/.../deepankarm/llm/llm/models.py", line 1365, in __anext__
        chunk = await self._generator.__anext__()
      File "/.../deepankarm/llm/llm/default_plugins/openai_models.py", line 758, in execute
        messages = self.build_messages(prompt, conversation)
      File "/.../deepankarm/llm/llm/default_plugins/openai_models.py", line 593, in build_messages
        attachment_message.append(_attachment(attachment))
      File "/.../deepankarm/llm/llm/default_plugins/openai_models.py", line 437, in _attachment
        if attachment.resolve_type() == "application/pdf":
      File "/.../deepankarm/llm/llm/models.py", line 79, in resolve_type
        response = httpx.head(self.url)
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/site-packages/httpx/_api.py", line 267, in head
        return request(
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/site-packages/httpx/_api.py", line 102, in request
        with Client(
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/site-packages/httpx/_client.py", line 688, in __init__
        self._transport = self._init_transport(
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/site-packages/httpx/_client.py", line 731, in _init_transport
        return HTTPTransport(
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/site-packages/httpx/_transports/default.py", line 153, in __init__
        ssl_context = create_ssl_context(verify=verify, cert=cert, trust_env=trust_env)
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/site-packages/httpx/_config.py", line 40, in create_ssl_context
        ctx = ssl.create_default_context(cafile=certifi.where())
      File "/opt/homebrew/anaconda3/envs/ffa/lib/python3.12/ssl.py", line 708, in create_default_context
        context.load_verify_locations(cafile, capath, cadata)

After

with messages = await asyncify(self.build_messages, prompt, conversation)

[<AsyncResponse prompt='What is this image?' text='This image shows a close-up of a tabby cat. The cat has greenish-yellow eyes, a pink nose, and a coat with distinctive stripes and markings typical of a tabby pattern. The background appears to be outdoors, possibly on a tiled surface with some dry grass or plants.'>]

I've also added a unit test that detects event loop blocking using pytest.mark.pyleak with mocked a httpx.head response.

Disclaimer: I'm the author of pyleak.

[deepankarm]

I'm not sure it is the right file to add this test.