368 lines
13 KiB
Python
368 lines
13 KiB
Python
################################################################
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# The core state machine
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################################################################
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#
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# Rule 1: everything that affects the state machine and state transitions must
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# live here in this file. As much as possible goes into the table-based
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# representation, but for the bits that don't quite fit, the actual code and
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# state must nonetheless live here.
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#
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# Rule 2: this file does not know about what role we're playing; it only knows
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# about HTTP request/response cycles in the abstract. This ensures that we
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# don't cheat and apply different rules to local and remote parties.
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#
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#
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# Theory of operation
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# ===================
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#
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# Possibly the simplest way to think about this is that we actually have 5
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# different state machines here. Yes, 5. These are:
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#
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# 1) The client state, with its complicated automaton (see the docs)
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# 2) The server state, with its complicated automaton (see the docs)
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# 3) The keep-alive state, with possible states {True, False}
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# 4) The SWITCH_CONNECT state, with possible states {False, True}
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# 5) The SWITCH_UPGRADE state, with possible states {False, True}
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#
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# For (3)-(5), the first state listed is the initial state.
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#
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# (1)-(3) are stored explicitly in member variables. The last
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# two are stored implicitly in the pending_switch_proposals set as:
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# (state of 4) == (_SWITCH_CONNECT in pending_switch_proposals)
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# (state of 5) == (_SWITCH_UPGRADE in pending_switch_proposals)
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#
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# And each of these machines has two different kinds of transitions:
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#
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# a) Event-triggered
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# b) State-triggered
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#
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# Event triggered is the obvious thing that you'd think it is: some event
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# happens, and if it's the right event at the right time then a transition
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# happens. But there are somewhat complicated rules for which machines can
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# "see" which events. (As a rule of thumb, if a machine "sees" an event, this
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# means two things: the event can affect the machine, and if the machine is
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# not in a state where it expects that event then it's an error.) These rules
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# are:
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#
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# 1) The client machine sees all h11.events objects emitted by the client.
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#
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# 2) The server machine sees all h11.events objects emitted by the server.
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#
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# It also sees the client's Request event.
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#
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# And sometimes, server events are annotated with a _SWITCH_* event. For
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# example, we can have a (Response, _SWITCH_CONNECT) event, which is
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# different from a regular Response event.
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#
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# 3) The keep-alive machine sees the process_keep_alive_disabled() event
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# (which is derived from Request/Response events), and this event
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# transitions it from True -> False, or from False -> False. There's no way
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# to transition back.
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#
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# 4&5) The _SWITCH_* machines transition from False->True when we get a
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# Request that proposes the relevant type of switch (via
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# process_client_switch_proposals), and they go from True->False when we
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# get a Response that has no _SWITCH_* annotation.
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#
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# So that's event-triggered transitions.
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#
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# State-triggered transitions are less standard. What they do here is couple
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# the machines together. The way this works is, when certain *joint*
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# configurations of states are achieved, then we automatically transition to a
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# new *joint* state. So, for example, if we're ever in a joint state with
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#
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# client: DONE
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# keep-alive: False
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#
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# then the client state immediately transitions to:
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#
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# client: MUST_CLOSE
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#
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# This is fundamentally different from an event-based transition, because it
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# doesn't matter how we arrived at the {client: DONE, keep-alive: False} state
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# -- maybe the client transitioned SEND_BODY -> DONE, or keep-alive
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# transitioned True -> False. Either way, once this precondition is satisfied,
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# this transition is immediately triggered.
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#
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# What if two conflicting state-based transitions get enabled at the same
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# time? In practice there's only one case where this arises (client DONE ->
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# MIGHT_SWITCH_PROTOCOL versus DONE -> MUST_CLOSE), and we resolve it by
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# explicitly prioritizing the DONE -> MIGHT_SWITCH_PROTOCOL transition.
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#
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# Implementation
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# --------------
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#
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# The event-triggered transitions for the server and client machines are all
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# stored explicitly in a table. Ditto for the state-triggered transitions that
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# involve just the server and client state.
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#
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# The transitions for the other machines, and the state-triggered transitions
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# that involve the other machines, are written out as explicit Python code.
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#
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# It'd be nice if there were some cleaner way to do all this. This isn't
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# *too* terrible, but I feel like it could probably be better.
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#
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# WARNING
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# -------
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#
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# The script that generates the state machine diagrams for the docs knows how
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# to read out the EVENT_TRIGGERED_TRANSITIONS and STATE_TRIGGERED_TRANSITIONS
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# tables. But it can't automatically read the transitions that are written
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# directly in Python code. So if you touch those, you need to also update the
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# script to keep it in sync!
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from typing import cast, Dict, Optional, Set, Tuple, Type, Union
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from ._events import *
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from ._util import LocalProtocolError, Sentinel
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# Everything in __all__ gets re-exported as part of the h11 public API.
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__all__ = [
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"CLIENT",
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"SERVER",
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"IDLE",
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"SEND_RESPONSE",
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"SEND_BODY",
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"DONE",
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"MUST_CLOSE",
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"CLOSED",
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"MIGHT_SWITCH_PROTOCOL",
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"SWITCHED_PROTOCOL",
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"ERROR",
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]
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class CLIENT(Sentinel, metaclass=Sentinel):
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pass
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class SERVER(Sentinel, metaclass=Sentinel):
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pass
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# States
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class IDLE(Sentinel, metaclass=Sentinel):
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pass
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class SEND_RESPONSE(Sentinel, metaclass=Sentinel):
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pass
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class SEND_BODY(Sentinel, metaclass=Sentinel):
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pass
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class DONE(Sentinel, metaclass=Sentinel):
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pass
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class MUST_CLOSE(Sentinel, metaclass=Sentinel):
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pass
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class CLOSED(Sentinel, metaclass=Sentinel):
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pass
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class ERROR(Sentinel, metaclass=Sentinel):
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pass
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# Switch types
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class MIGHT_SWITCH_PROTOCOL(Sentinel, metaclass=Sentinel):
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pass
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class SWITCHED_PROTOCOL(Sentinel, metaclass=Sentinel):
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pass
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class _SWITCH_UPGRADE(Sentinel, metaclass=Sentinel):
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pass
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class _SWITCH_CONNECT(Sentinel, metaclass=Sentinel):
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pass
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EventTransitionType = Dict[
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Type[Sentinel],
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Dict[
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Type[Sentinel],
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Dict[Union[Type[Event], Tuple[Type[Event], Type[Sentinel]]], Type[Sentinel]],
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],
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]
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EVENT_TRIGGERED_TRANSITIONS: EventTransitionType = {
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CLIENT: {
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IDLE: {Request: SEND_BODY, ConnectionClosed: CLOSED},
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SEND_BODY: {Data: SEND_BODY, EndOfMessage: DONE},
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DONE: {ConnectionClosed: CLOSED},
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MUST_CLOSE: {ConnectionClosed: CLOSED},
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CLOSED: {ConnectionClosed: CLOSED},
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MIGHT_SWITCH_PROTOCOL: {},
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SWITCHED_PROTOCOL: {},
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ERROR: {},
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},
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SERVER: {
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IDLE: {
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ConnectionClosed: CLOSED,
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Response: SEND_BODY,
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# Special case: server sees client Request events, in this form
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(Request, CLIENT): SEND_RESPONSE,
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},
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SEND_RESPONSE: {
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InformationalResponse: SEND_RESPONSE,
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Response: SEND_BODY,
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(InformationalResponse, _SWITCH_UPGRADE): SWITCHED_PROTOCOL,
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(Response, _SWITCH_CONNECT): SWITCHED_PROTOCOL,
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},
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SEND_BODY: {Data: SEND_BODY, EndOfMessage: DONE},
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DONE: {ConnectionClosed: CLOSED},
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MUST_CLOSE: {ConnectionClosed: CLOSED},
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CLOSED: {ConnectionClosed: CLOSED},
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SWITCHED_PROTOCOL: {},
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ERROR: {},
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},
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}
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StateTransitionType = Dict[
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Tuple[Type[Sentinel], Type[Sentinel]], Dict[Type[Sentinel], Type[Sentinel]]
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]
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# NB: there are also some special-case state-triggered transitions hard-coded
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# into _fire_state_triggered_transitions below.
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STATE_TRIGGERED_TRANSITIONS: StateTransitionType = {
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# (Client state, Server state) -> new states
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# Protocol negotiation
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(MIGHT_SWITCH_PROTOCOL, SWITCHED_PROTOCOL): {CLIENT: SWITCHED_PROTOCOL},
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# Socket shutdown
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(CLOSED, DONE): {SERVER: MUST_CLOSE},
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(CLOSED, IDLE): {SERVER: MUST_CLOSE},
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(ERROR, DONE): {SERVER: MUST_CLOSE},
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(DONE, CLOSED): {CLIENT: MUST_CLOSE},
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(IDLE, CLOSED): {CLIENT: MUST_CLOSE},
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(DONE, ERROR): {CLIENT: MUST_CLOSE},
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}
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class ConnectionState:
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def __init__(self) -> None:
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# Extra bits of state that don't quite fit into the state model.
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# If this is False then it enables the automatic DONE -> MUST_CLOSE
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# transition. Don't set this directly; call .keep_alive_disabled()
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self.keep_alive = True
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# This is a subset of {UPGRADE, CONNECT}, containing the proposals
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# made by the client for switching protocols.
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self.pending_switch_proposals: Set[Type[Sentinel]] = set()
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self.states: Dict[Type[Sentinel], Type[Sentinel]] = {CLIENT: IDLE, SERVER: IDLE}
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def process_error(self, role: Type[Sentinel]) -> None:
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self.states[role] = ERROR
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self._fire_state_triggered_transitions()
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def process_keep_alive_disabled(self) -> None:
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self.keep_alive = False
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self._fire_state_triggered_transitions()
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def process_client_switch_proposal(self, switch_event: Type[Sentinel]) -> None:
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self.pending_switch_proposals.add(switch_event)
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self._fire_state_triggered_transitions()
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def process_event(
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self,
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role: Type[Sentinel],
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event_type: Type[Event],
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server_switch_event: Optional[Type[Sentinel]] = None,
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) -> None:
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_event_type: Union[Type[Event], Tuple[Type[Event], Type[Sentinel]]] = event_type
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if server_switch_event is not None:
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assert role is SERVER
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if server_switch_event not in self.pending_switch_proposals:
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raise LocalProtocolError(
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"Received server {} event without a pending proposal".format(
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server_switch_event
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)
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)
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_event_type = (event_type, server_switch_event)
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if server_switch_event is None and _event_type is Response:
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self.pending_switch_proposals = set()
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self._fire_event_triggered_transitions(role, _event_type)
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# Special case: the server state does get to see Request
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# events.
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if _event_type is Request:
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assert role is CLIENT
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self._fire_event_triggered_transitions(SERVER, (Request, CLIENT))
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self._fire_state_triggered_transitions()
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def _fire_event_triggered_transitions(
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self,
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role: Type[Sentinel],
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event_type: Union[Type[Event], Tuple[Type[Event], Type[Sentinel]]],
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) -> None:
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state = self.states[role]
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try:
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new_state = EVENT_TRIGGERED_TRANSITIONS[role][state][event_type]
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except KeyError:
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event_type = cast(Type[Event], event_type)
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raise LocalProtocolError(
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"can't handle event type {} when role={} and state={}".format(
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event_type.__name__, role, self.states[role]
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)
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) from None
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self.states[role] = new_state
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def _fire_state_triggered_transitions(self) -> None:
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# We apply these rules repeatedly until converging on a fixed point
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while True:
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start_states = dict(self.states)
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# It could happen that both these special-case transitions are
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# enabled at the same time:
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#
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# DONE -> MIGHT_SWITCH_PROTOCOL
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# DONE -> MUST_CLOSE
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#
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# For example, this will always be true of a HTTP/1.0 client
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# requesting CONNECT. If this happens, the protocol switch takes
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# priority. From there the client will either go to
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# SWITCHED_PROTOCOL, in which case it's none of our business when
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# they close the connection, or else the server will deny the
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# request, in which case the client will go back to DONE and then
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# from there to MUST_CLOSE.
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if self.pending_switch_proposals:
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if self.states[CLIENT] is DONE:
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self.states[CLIENT] = MIGHT_SWITCH_PROTOCOL
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if not self.pending_switch_proposals:
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if self.states[CLIENT] is MIGHT_SWITCH_PROTOCOL:
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self.states[CLIENT] = DONE
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if not self.keep_alive:
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for role in (CLIENT, SERVER):
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if self.states[role] is DONE:
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self.states[role] = MUST_CLOSE
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# Tabular state-triggered transitions
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joint_state = (self.states[CLIENT], self.states[SERVER])
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changes = STATE_TRIGGERED_TRANSITIONS.get(joint_state, {})
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self.states.update(changes)
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if self.states == start_states:
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# Fixed point reached
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return
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def start_next_cycle(self) -> None:
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if self.states != {CLIENT: DONE, SERVER: DONE}:
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raise LocalProtocolError(
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"not in a reusable state. self.states={}".format(self.states)
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)
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# Can't reach DONE/DONE with any of these active, but still, let's be
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# sure.
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assert self.keep_alive
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assert not self.pending_switch_proposals
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self.states = {CLIENT: IDLE, SERVER: IDLE}
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