Support for multiple workers in the same process

[lazka]

Description

When using granian with multiple event loops in one process then mcp errors out. It looks as if mcp can't handle being run multiple times in the same process?

# example.py
import contextlib
from starlette.applications import Starlette
from starlette.routing import Mount
from mcp.server.fastmcp import FastMCP

mcp = FastMCP("Demo", json_response=True)

@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b

@contextlib.asynccontextmanager
async def lifespan(app: Starlette):
    async with mcp.session_manager.run():
        yield

app = Starlette(
    routes=[
        Mount("/", app=mcp.streamable_http_app()),
    ],
    lifespan=lifespan,
)

uv add granian
uv run --python 3.14t granian --interface asgi --workers 2 example:app --port 8000

error:

Exception ignored while calling deallocator <function BaseEventLoop.__del__ at 0x58eff299fc0>:
Traceback (most recent call last):
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/asyncio/base_events.py", line 760, in __del__
    self.close()
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/asyncio/unix_events.py", line 70, in close
    super().close()
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/asyncio/selector_events.py", line 104, in close
    self._close_self_pipe()
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/asyncio/selector_events.py", line 111, in _close_self_pipe
    self._remove_reader(self._ssock.fileno())
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/asyncio/selector_events.py", line 296, in _remove_reader
    key = self._selector.get_map().get(fd)
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/selectors.py", line 70, in get
    fd = self._selector._fileobj_lookup(fileobj)
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/selectors.py", line 229, in _fileobj_lookup
    return _fileobj_to_fd(fileobj)
  File "/home/lazka/.local/share/uv/python/cpython-3.14.0+freethreaded-linux-x86_64-gnu/lib/python3.14t/selectors.py", line 42, in _fileobj_to_fd
    raise ValueError("Invalid file descriptor: {}".format(fd))
ValueError: Invalid file descriptor: -1

The error goes away if only one worker is used.

Any ideas how to work around this welcome.

References

https://github.com/emmett-framework/granian