ACTRIS COQ DEVELOPMENT

This directory contains the artifact for the paper "Actris: Session Type Based Reasoning in Separation Logic".

It has been built and tested with the following dependencies

• Coq 8.10
• std++ at commit 9041e6d8.
• iris at commit 1f83451a.

In order to build, install the above dependencies and then run make -j [num CPU cores] to compile Actris.

Theory of Actris

The theory of Actris (semantics of channels, the CPS model, and the proof rules) can be found in the directory theories/channel. The files correspond to the following parts of the paper:

• theories/channel/channel.v: The definitional semantics of bidirectional channels in terms of Iris's HeapLang language.
• theories/channel/proto_model.v: The CPS model of Dependent Separation Protocols.
• theories/channel/proto_channel.v: The definition of the connective ↣ for channel ownership, and lemmas corresponding to the Actris proof rules. The relevant proof rules are as follows:
  • new_chan_proto_spec: proof rule for new_chan.
  • send_proto_spec and send_proto_spec_packed: proof rules for send, the first version is more convenient to use in Coq, but otherwise the same as the latter, which is the rule in the paper.
  • recv_proto_spec and recv_proto_spec_packed: proof rules for recv, the first version is more convenient to use in Coq, but otherwise the same as the latter, which is the rule in the paper.
  • select_spec: proof rule for select.
  • branch_spec: proof rule for branch.

Weakest preconditions and Coq tactics

The presentation of Actris logic in the paper makes use of Hoare triples. In Coq, we make use of weakest preconditions because these are more convenient for interactive theorem proving using the the proof mode tactics. To state concise program specifications, we use the notion of Texan Triples from Iris, which provides a convenient "Hoare triple"-like syntax around weakest preconditions:

{{{ P }}} e {{{ x .. y, RET v; Q }}} :=
  □ ∀ Φ, P -∗ ▷ (∀ x .. y, Q -∗ Φ v) -∗ WP e {{ Φ }}

In order to prove programs using Actris, one can make use of a combination of Iris's symbolic execution tactics for HeapLang programs and Actris's symbolic execution tactics for message passing. The Actris tactics are as follows:

• wp_send (t1 .. tn) with "selpat": symbolically execute send c v by looking up ownership of a send protocol H : c ↣ <!> y1 .. yn, MSG v; {{ P }}; prot in the proof mode context. The tactic instantiates the variables y1 .. yn using the terms t1 .. tn and uses selpat to prove P. If fewer terms t are given than variables y, they will be instantiated using existential variables (evars). The tactic will put H : c ↣ prot back into the context.
• wp_recv (x1 .. xn) as "ipat": symbolically execute recv c by looking up H : c ↣ <?> y1 .. yn, MSG v; {{ P }}; prot in the proof mode context. The variables y1 .. yn are introduced as x1 .. xn, and the predicate P is introduced using the introduction pattern ipat. The tactic will put H : c ↣ prot back into the context.
• wp_select with "selpat": symbolically execute select c b by looking up H : c ↣ prot1 {Q1}<+>{Q2} prot2 in the proof mode context. The selection pattern selpat is used to resolve either Q1 or Q2, based on the chosen branch b. The tactic will put H : c ↣ prot1 or H : c ↣ prot2 back into the context based on the chosen branch b.
• wp_branch as ipat1 | ipat2: symbolically execute branch c e1 e2 by looking up H : c ↣ prot1 {Q1}<&>{Q2} prot2 in the proof mode context. The result of the tactic involves two subgoals, in which Q1 and Q2 are introduced using the introduction patterns ipat1 and ipat2, respectively. The tactic will put H : c ↣ prot1 and H : c ↣ prot2 back into the contexts of the two respectively goals.

The above tactics implicitly perform normalization of the protocol prot in the hypothesis H : c ↣ prot. For example, wp_send also works if there is a channel with the protocol iProto_dual ((<?> y1 .. yn, MSG v; {{ P }}; END) <++> prot). Concretely, the normalization performs the following actions:

• It re-associates appends (<++>), and removes left-identities (END) of it.
• It moves appends (<++>) into sends (<!>), receives (<?>), selections (<+>) and branches (<&>).
• It distributes duals (iProto_dual) over append (<++>).
• It unfolds prot1 into prot2 if there is an instance of the type class ProtoUnfold prot1 prot2. When defining a recursive protocol, it is useful to define a ProtoUnfold instance to obtain automatic unfolding of the recursive protocol. For example, see sort_protocol_br_unfold in theories/examples/sort_br_del.v.

Examples

The examples can be found in the direction theories/examples.

The following list gives a mapping between the examples in the paper and their mechanization in Coq:

1. Introduction: theories/examples/basics.v
2. Tour of Actris
   • 2.3 Basic: theories/examples/sort.v
   • 2.4 Higher-Order Functions: theories/examples/sort.v
   • 2.5 Branching: theories/examples/sort_br_del.v
   • 2.6 Recursion: theories/examples/sort_br_del.v
   • 2.7 Delegation: theories/examples/sort_br_del.v
   • 2.8 Dependent: theories/examples/sort_fg.v
3. Manifest sharing via locks
   • 3.1 Sample program: theories/examples/basics.v
   • 3.2 Distributed mapper: theories/examples/map.v
4. Case study: map reduce:
   • Utilities for shuffling/grouping: theories/utils/group.v
   • Implementation and verification: theories/examples/map_reduce.v

Differences between the formalization and the paper

There are a number of small differences between the paper presentation of Actris and the formalization in Coq, that are briefly discussed here.

• Weakest preconditions versus Hoare triples

  See the section "Weakest preconditions and Coq tactics" above.

• Connectives for physical ownership of channels

  In the paper, physical ownership of a channel is formalized using a single connective (c1,c2) ↣ (vs1,vs2), while the mechanization has two connectives for the endpoints and one for connecting them, namely:

  • chan_own γ Left vs1 and chan_own γ Right vs1
  • is_chan N γ c1 c2

  Here, γ is a ghost name and N an invariant name. This setup is less intuitive but gives rise to a more practical Jacobs/Piessens-style spec of recv that does not need a closing view shift (to handle the case that the buffer is empty).

• Later modalities in primitive rules for channels

  The primitive rules for send and recv (send_spec and recv_spec in theories/channel/channel.v) contain three later (▷) modalities, which are omitted for brevity's sake in the paper. These later modalities expose that these operations perform at least three steps in the operational semantics, and are needed to deal with the three levels of indirection in the invariant for protocols:

  1. the ▶ in the CPS encoding of protocols,
  2. the higher-order ghost state used for ownership of protocols, and
  3. the opening of the protocol invariant.