Add lazy mode to build_deps

Signed-off-by: HasanA <mhmd_alameen1023@outlook.com>

bin/describe/describe_pp.ml

@@ -74,7 +74,9 @@ let execute_pp_action ~sctx file pp_file dump_file =
   in
   let ok =
     let open Fiber.O in
-    let build_deps deps = Build_system.build_deps deps |> Memo.run in
+    let build_deps build_mode deps =
+      Build_system.build_deps ~build_mode deps |> Memo.run
+    in
     let* result = Dune_engine.Action_exec.exec input ~build_deps in
     Dune_engine.Action_exec.Exec_result.ok_exn result >>| ignore
   in

src/dune_engine/action_exec.ml

@@ -97,6 +97,7 @@ module Exec_result = struct
       | Unix.Unix_error (err, call, args) -> Unix (err, call, args)
       | Memo.Non_reproducible Scheduler.Run.Build_cancelled ->
         Nonreproducible_build_cancelled
+      | Memo.Error.E e -> reraise (Memo.Error.get e)
       | Memo.Cycle_error.E _ as e ->
         (* [Memo.Cycle_error.t] is hard to serialize and can only be raised during action
            execution with the dynamic dependencies plugin, which is not production-ready yet.
@@ -139,6 +140,11 @@ module Exec_result = struct
   ;;
 end
 
+(** mode for calling build_deps. in lazy mode, any attempt to do some build will fail.*)
+type build_mode =
+  | Lazy
+  | Eager
+
 module Action_res = struct
   type t =
     { done_or_more_deps : Action_plugin.done_or_more_deps
@@ -165,7 +171,7 @@ type exec_context =
   ; context : Build_context.t option
   ; metadata : Process.metadata
   ; rule_loc : Loc.t
-  ; build_deps : Dep.Set.t -> Dep.Facts.t Fiber.t
+  ; build_deps : build_mode -> Dep.Set.t -> Dep.Facts.t Fiber.t
   }
 
 type exec_environment =
@@ -318,6 +324,7 @@ let bash_exn =
    kind of dynamic build dependency functions or prepped dependencies, etc,
    which should be handled here instead. *)
 let restrict_ctx { targets; context; metadata; rule_loc; build_deps } =
+  let build_deps = build_deps Eager in
   { Action.Ext.targets; context; purpose = metadata.purpose; rule_loc; build_deps }
 ;;
 
@@ -635,12 +642,12 @@ let exec_until_all_deps_ready ~display ~ectx ~eenv t =
     let* result = exec ~display ~ectx ~eenv t in
     match result with
     | { done_or_more_deps = Done; needed_deps = deps } ->
-      let* fact_map = Produce.of_fiber @@ ectx.build_deps deps in
+      let* fact_map = Produce.of_fiber @@ ectx.build_deps Lazy deps in
       Produce.return (stages, (deps, fact_map))
     | { done_or_more_deps = Need_more_deps (relative_deps, deps_to_build)
       ; needed_deps = _
       } ->
-      let* fact_map = Produce.of_fiber @@ ectx.build_deps deps_to_build in
+      let* fact_map = Produce.of_fiber @@ ectx.build_deps Eager deps_to_build in
       let stages = (deps_to_build, fact_map) :: stages in
       let eenv =
         { eenv with

src/dune_engine/action_exec.mli

@@ -27,6 +27,11 @@ module Exec_result : sig
   val ok_exn : t -> ok Fiber.t
 end
 
+(** mode for calling build_deps. in lazy mode, any attempt to do some build will fail.*)
+type build_mode =
+  | Lazy
+  | Eager
+
 type input =
   { targets : Targets.Validated.t option (* Some Jane Street actions use [None] *)
   ; root : Path.t
@@ -41,5 +46,5 @@ type input =
 
 val exec
   :  input
-  -> build_deps:(Dep.Set.t -> Dep.Fact.t Dep.Map.t Fiber.t)
+  -> build_deps:(build_mode -> Dep.Set.t -> Dep.Fact.t Dep.Map.t Fiber.t)
   -> Exec_result.t Fiber.t

src/dune_engine/action_runner.ml

@@ -345,7 +345,7 @@ let create server ~name =
 
 module Worker = struct
   let exec_action =
-    let build_deps _ = Code_error.raise "no dynamic actions yet" [] in
+    let build_deps _ _ = Code_error.raise "no dynamic actions yet" [] in
     fun (action : Action_exec.input) ->
       Log.info
         [ Pp.text "action runner executing action:"