Add a cache system

The response of a request is cached and it is invalidated every time one
of then packages of the new request is involve between the new opam
commit and the old one (cached opam commit).

Anytime the cached response is the same as the new response, we keep the
old one for the oldest opam commit.

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "scache"]
+	path = scache
+	url = https://github.com/moyodiallo/scache.git
+	branch = master

bin/main.ml

@@ -75,24 +75,25 @@ let start_server ~service vat_config =
   let+ vat = Capnp_rpc_unix.serve vat_config ~restore in
   Capnp_rpc_unix.Vat.sturdy_uri vat service_id
 
-let main_service () solver cap_file vat_config =
-  let uri = start_server ~service:(Solver_service.Service.v solver) vat_config in
+let main_service () solver cacheable cap_file vat_config =
+  let uri = start_server ~service:(Solver_service.Service.v ~cacheable solver) vat_config in
   Capnp_rpc_unix.Cap_file.save_uri uri cap_file |> or_die;
   Fmt.pr "Wrote solver service's address to %S@." cap_file;
   Fiber.await_cancel ()
 
-let main_service_pipe () solver =
+let main_service_pipe () solver cacheable =
   let socket = Lwt_unix.stdin in
   (* Run locally reading from socket *)
-  export (Solver_service.Service.v solver) ~on:socket
+  export (Solver_service.Service.v ~cacheable solver) ~on:socket
 
-let main_cluster () solver name capacity register_addr =
+let main_cluster () solver cacheable name capacity register_addr =
   let vat = Capnp_rpc_unix.client_only_vat () in
   let sr = Capnp_rpc_unix.Vat.import_exn vat register_addr in
   let `Cancelled =
     Solver_worker.run solver sr
       ~name
       ~capacity
+      ~cacheable
   in
   ()
 
@@ -128,6 +129,11 @@ let capacity =
   @@ Arg.info ~doc:"The number of cluster jobs that can run in parallel" ~docv:"N"
     [ "capacity" ]
 
+let cacheable =
+  Arg.value
+  @@ Arg.flag
+  @@ Arg.info ~doc:"Activate the cache system" [ "activate-cache"; "cache" ]
+
 let cap_file =
   Arg.required
   @@ Arg.opt Arg.(some string) None
@@ -168,6 +174,7 @@ let () =
         const main_service
         $ setup_log
         $ solver
+        $ cacheable
         $ cap_file
         $ Capnp_rpc_unix.Vat_config.cmd
       )
@@ -179,6 +186,7 @@ let () =
         const main_service_pipe
         $ setup_log
         $ solver
+        $ cacheable
       )
   in
   let run_agent =
@@ -188,6 +196,7 @@ let () =
         const main_cluster
         $ setup_log
         $ solver
+        $ cacheable
         $ worker_name
         $ capacity
         $ register_addr

service/dune

@@ -7,5 +7,8 @@
   solver-service-api
   opam-0install
   capnp-rpc-net
+  opam-file-format
   git-unix
-  ocaml-version))
+  scache
+  ocaml-version)
+ (preprocess (pps ppx_deriving_yojson)))

service/service.ml

@@ -1,6 +1,6 @@
 module Worker = Solver_service_api.Worker
 
-let v t =
+let v ?cacheable t =
   let open Capnp_rpc_lwt in
   let module X = Solver_service_api.Raw.Service.Solver in
   X.local
@@ -27,7 +27,7 @@ let v t =
                (Capnp_rpc.Error.exn "Bad JSON in request: %s" msg))
         | Ok request ->
           Lwt_eio.run_eio @@ fun () ->
-          let selections = Solver.solve t ~log request in
+          let selections = Solver.solve ?cacheable t ~log request in
           let json =
             Yojson.Safe.to_string
               (Worker.Solve_response.to_yojson selections)

service/service.mli

@@ -1,2 +1,2 @@
-val v : Solver.t -> Solver_service_api.Solver.t
+val v : ?cacheable:bool -> Solver.t -> Solver_service_api.Solver.t
 (** [capnp_service t] is a Cap'n Proto service that handles requests using [t]. *)