  Message passing is a useful abstraction for implementing concurrent programs.
For real-world systems, however, it is often combined with other programming
and concurrency paradigms, such as higher-order functions, mutable state,
shared-memory concurrency, and locks. We present Actris: a logic for proving
functional correctness of programs that use a combination of the aforementioned
features. Actris combines the power of modern concurrent separation logics with
a first-class protocol mechanism -- based on session types -- for reasoning
about message passing in the presence of other concurrency paradigms. We show
that Actris provides a suitable level of abstraction by proving functional
correctness of a variety of examples, including a channel-based merge sort, a
channel-based load-balancing mapper, and a variant of the map-reduce model,
using concise specifications. While Actris was already presented in a
conference paper (POPL'20), this paper expands the prior presentation
significantly. Moreover, it extends Actris to Actris 2.0 with a notion of
subprotocols -- based on session-type subtyping -- that permits additional
flexibility when composing channel endpoints, and that takes full advantage of
the asynchronous semantics of message passing in Actris. Soundness of Actris
2.0 is proven using a model of its protocol mechanism in the Iris framework. We
have mechanised the theory of Actris, together with custom tactics, as well as
all examples in the paper, in the Coq proof assistant.
