Middleware stacking and recursive initialization

[Kludex]

Discussed in #1161

^{Originally posted by euri10 April 1, 2021}
long story short I was writing a logging middleware whose handlers are initialized in the __init__ of the middleware and ended up with multiple logs.

it seems to me the __init__ of the middlewarea are recursively called because of the build_middleware_stack method and I'm not sure if this is intended or a bug.

here's a test case that shows what I think is an issue:

class SimpleInitializableMiddleware:
    def __init__(self, app, counter: int):
        self.app = app
        self.counter = counter
        print(self.counter)


def test_middleware_stack_init():
    app = Starlette()
    app.add_middleware(SimpleInitializableMiddleware, counter=1)
    app.add_middleware(SimpleInitializableMiddleware, counter=2)
    app.add_middleware(SimpleInitializableMiddleware, counter=3)
    app.add_middleware(SimpleInitializableMiddleware, counter=4)

I would expect to see 1 2 3 4 as the output.
however it prints 1 1 2 1 2 3 1 2 3 4 because for each add_middlware call it goes down the stack and reinitialize it.

what you think @tomchristie ?

[Kludex]

A possible solution would be to deprecate, and further remove add_middleware().

[Kludex]

• We are going with Lazily build middleware stack #2017. Check more about it over there.

This will be available on Starlette 0.24.0.