pkg: defer evaluating platform vars in commands

src/dune_lang/package_variable_name.ml

@@ -53,6 +53,10 @@ let post = of_string "post"
 let one_of t xs = List.mem xs ~equal t
 let dev = of_string "dev"
 
+let platform_specific =
+  Set.of_list [ arch; os; os_version; os_distribution; os_family; sys_ocaml_version ]
+;;
+
 module Project = struct
   let encode name = Dune_sexp.Encoder.string (":" ^ to_string name)
 

src/dune_lang/package_variable_name.mli

@@ -32,6 +32,10 @@ val build : t
 val dev : t
 val one_of : t -> t list -> bool
 
+(** The set of variable names whose values are expected to differ depending on
+    the current platform. *)
+val platform_specific : Set.t
+
 module Project : sig
   val encode : t Dune_sexp.Encoder.t
   val decode : t Dune_sexp.Decoder.t

src/dune_pkg/opam_solver.ml

@@ -1736,9 +1736,18 @@ let opam_package_to_lock_file_pkg
     | [] -> action
     | env_update -> Action.Withenv (env_update, action)
   in
-  let get_solver_var variable_name =
-    Solver_stats.Updater.expand_variable stats_updater variable_name;
-    Solver_env.get solver_env variable_name
+  let get_solver_var =
+    (* Remove platform-specific variables from the solver env so that the
+       generated commands are portable rather than specialized to the computer
+       performing the solve. *)
+    let portable_solver_env =
+      Solver_env.unset_multi solver_env Package_variable_name.platform_specific
+    in
+    fun variable_name ->
+      let value = Solver_env.get portable_solver_env variable_name in
+      if Option.is_some value
+      then Solver_stats.Updater.expand_variable stats_updater variable_name;
+      value
   in
   let build_command =
     if Resolved_package.dune_build resolved_package

src/dune_pkg/solver_env.mli

@@ -21,5 +21,8 @@ val extend : t -> t -> t
 val with_defaults : t
 
 val pp : t -> 'a Pp.t
+
+(** Remove a set of bindings from the env *)
 val unset_multi : t -> Package_variable_name.Set.t -> t
+
 val to_env : t -> OpamFilter.env

test/blackbox-tests/test-cases/pkg/solver-vars-in-lockdir-metadata.t

@@ -140,11 +140,18 @@ stored in the lockdir metadata:
   (version 0.0.1)
 
   (install
-   (run echo qux))
+   (when
+    (= %{arch} arm)
+    (run echo qux)))
 
   (build
    (progn
-    (run echo foo)
+    (when
+     (= %{os} linux)
+     (run echo foo))
+    (when
+     (= %{os} macos)
+     (run echo bar))
     (run echo baz)))
   $ cat dune.lock/lock.dune
   (lang package 0.1)
@@ -154,9 +161,3 @@ stored in the lockdir metadata:
   (repositories
    (complete false)
    (used))
-
-  (expanded_solver_variable_bindings
-   (variable_values
-    (os linux)
-    (arch arm))
-   (unset_variables x))