`WsgiToAsgiInstance.build_environ` crashes when `scope.server` is present, but set to None

[graysonchao]

Some ASGI servers, such as uvicorn, will sometimes pass a scope object to WsgiToAsgi where scope["server"] is set to None. This causes the app to crash when build_environ tries to access scope["server"] like a tuple (here).

However, extending the check to include that scope["server"] is not None would be an imperfect fix, because it would set SERVER_NAME and SERVER_PORT to "localhost:80" when in some cases that's not true - for example, when launching uvicorn bound to a Unix socket. This latter issue likely needs to be fixed in the server code, though.

[b0ses]

I've spent days googling around this issue with radio silence on how to fix it, but I'm super thankful someone else has come across it. Including the traceback here for visibility:

Traceback (most recent call last):
  File ".../uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../engineio/async_drivers/asgi.py", line 75, in __call__
    await self.other_asgi_app(scope, receive, send)
  File ".../asgiref/wsgi.py", line 21, in __call__
    await WsgiToAsgiInstance(self.wsgi_application)(scope, receive, send)
  File ".../asgiref/wsgi.py", line 51, in __call__
    await self.run_wsgi_app(body)
  File ".../asgiref/sync.py", line 502, in __call__
    ret = await asyncio.shield(exec_coro)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../asgiref/sync.py", line 556, in thread_handler
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File ".../asgiref/wsgi.py", line 141, in run_wsgi_app
    environ = self.build_environ(self.scope, body)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../asgiref/wsgi.py", line 77, in build_environ
    environ["SERVER_NAME"] = scope["server"][0]

And have gotten around it by updating the if 'server' in scope: check. Like @graysonchao said though, this is isn't the most ideal fix.

[carltongibson]

A PR adding a test and updating the check to ≈ if scope.get("server") is not None: would be great here.

The spec has:

server (Iterable[Unicode string, Optional[int]]) – Either a two-item iterable of [host, port], where host is the listening address for this server, and port is the integer listening port, or [path, None] where path is that of the unix socket. Optional; if missing defaults to None.

The type-hint there should probably allow |None, since the whole key is optional (and I suspect uvicorn is just filling the hole)

[patx]

@carltongibson PR created