provide Nix depexts with shell environment variables

src/client/opamSolution.ml

@@ -1181,7 +1181,7 @@ let install_depexts ?(force_depext=false) ?(confirm=true) t packages =
     let answer =
       let pkgman =
         OpamConsole.colorise `yellow
-          (OpamSysInteract.package_manager_name ~env config)
+          (OpamSysInteract.package_manager_name ~env t.switch config)
       in
       OpamConsole.menu ~unsafe_yes:`Yes ~default:`Yes ~no:`Quit
         "opam believes some required external dependencies are missing. opam \
@@ -1213,7 +1213,7 @@ let install_depexts ?(force_depext=false) ?(confirm=true) t packages =
     | `Quit -> give_up_msg (); OpamStd.Sys.exit_because `Aborted
   and print_command sys_packages =
     let commands =
-      OpamSysInteract.install_packages_commands ~env config sys_packages
+      OpamSysInteract.install_packages_commands ~env t.switch config sys_packages
       |> List.map (fun ((`AsAdmin c | `AsUser c), a) -> c::a)
     in
     OpamConsole.formatted_msg
@@ -1242,7 +1242,7 @@ let install_depexts ?(force_depext=false) ?(confirm=true) t packages =
     | `Quit -> give_up ()
   and auto_install t sys_packages =
     try
-      OpamSysInteract.install ~env config sys_packages; (* handles dry_run *)
+      OpamSysInteract.install ~env t.switch config sys_packages; (* handles dry_run *)
       map_sysmap (fun _ -> OpamSysPkg.Set.empty) t
     with Failure msg ->
       OpamConsole.error "%s" msg;

src/state/opamEnv.ml

@@ -540,7 +540,14 @@ let compute_updates ?(force_path=false) st =
     in
     List.map resolve_separator_and_format updates
   in
-  switch_env @ pkg_env @ man_path @ [path]
+  let nix_env =
+    let open OpamFilename in
+    create (OpamPath.Switch.meta OpamStateConfig.(!r.root_dir) st.switch) (basename (raw "nix.env"))
+    |> OpamFile.make
+    |> OpamFile.Environment.read_opt
+    |> Option.fold ~none:[] ~some:(List.map resolve_separator_and_format)
+  in
+  switch_env @ pkg_env @ man_path @ [path] @ nix_env
 
 let updates_common ~set_opamroot ~set_opamswitch root switch =
   let root =

src/state/opamSysInteract.ml

@@ -134,6 +134,7 @@ type families =
   | Macports
   | Msys2
   | Netbsd
+  | Nix
   | Openbsd
   | Suse
 
@@ -198,6 +199,7 @@ let family ~env () =
     | "gentoo" -> Gentoo
     | "homebrew" -> Homebrew
     | "macports" -> Macports
+    | "nixos" -> Nix
     | "macos" ->
       failwith
         "External dependency handling for macOS requires either \
@@ -896,6 +898,16 @@ let packages_status ?(env=OpamVariable.Map.empty) config packages =
       |> package_set_of_pkgpath
     in
     compute_sets sys_installed
+  | Nix ->
+      (* We say all requested packages are available but uninstalled.
+         We could check that these packages are available in Nixpkgs,
+         but that would involve an expensive Nixpkgs evaluation.
+         Saying no packages are installed results in a warning that
+         conf packages depend on a 'system package that can no longer
+         be found.' But omitting them will mean that they won't be
+         added to the Nix derivation.
+      *)
+      packages, OpamSysPkg.Set.empty;
   | Openbsd ->
     let sys_installed =
       run_query_command "pkg_info" ["-mqP"]
@@ -931,7 +943,7 @@ let packages_status ?(env=OpamVariable.Map.empty) config packages =
 
 (* Install *)
 
-let install_packages_commands_t ?(env=OpamVariable.Map.empty) config sys_packages =
+let install_packages_commands_t ?(env=OpamVariable.Map.empty) switch config sys_packages =
   let unsafe_yes = OpamCoreConfig.answer_is `unsafe_yes in
   let yes ?(no=[]) yes r =
     if unsafe_yes then
@@ -1009,14 +1021,43 @@ let install_packages_commands_t ?(env=OpamVariable.Map.empty) config sys_package
     [`AsUser (Commands.msys2 config),
      "-Su"::"--noconfirm"::packages], None
   | Netbsd -> [`AsAdmin "pkgin", yes ["-y"] ("install" :: packages)], None
+  | Nix ->
+    let open OpamFilename in
+    let dir = OpamPath.Switch.meta OpamStateConfig.(!r.root_dir) switch in
+    let drvFile = create dir (basename (raw "env.nix")) in
+    let packages = String.concat " " (List.rev (OpamSysPkg.Set.fold (fun p l -> OpamSysPkg.to_string p :: l) sys_packages [])) in
+    let contents =
+{|{ pkgs ? import <nixpkgs> {} }:
+with pkgs;
+stdenv.mkDerivation {
+  name = "opam-nix-env";
+  nativeBuildInputs = with buildPackages; [ |} ^ packages ^ {| ];
+
+  phases = [ "buildPhase" ];
+
+  buildPhase = ''
+vars=("NIX_CC" "NIX_CC_FLAGS" "NIX_CFLAGS_COMPILE" "NIX_CC_WRAPPER_TARGET_HOST_x86_64_unknown_linux_gnu" "NIX_LDFLAGS" "PKG_CONFIG_PATH")
+for var in "''${vars[@]}"; do
+  escaped="$(echo "''${!var}" | sed -e 's/^$/@/' -e 's/ /\\ /g')"
+  echo "$var	=	$escaped	Nix" >> $out
+done
+echo "PATH	+=	$PATH	Nix" >> $out
+  '';
+
+  preferLocalBuild = true;
+}
+|} in
+    write drvFile contents;
+    let envFile = create dir (basename (raw "nix.env")) |> OpamFilename.to_string in
+    [`AsUser "nix-build", [ OpamFilename.to_string drvFile; "--out-link"; envFile ] ], None
   | Openbsd -> [`AsAdmin "pkg_add", yes ~no:["-i"] ["-I"] packages], None
   | Suse -> [`AsAdmin "zypper", yes ["--non-interactive"] ("install"::packages)], None
 
-let install_packages_commands ?env config sys_packages =
-  fst (install_packages_commands_t ?env config sys_packages)
+let install_packages_commands ?env switch config sys_packages =
+  fst (install_packages_commands_t ?env switch config sys_packages)
 
-let package_manager_name ?env config =
-  match install_packages_commands ?env config OpamSysPkg.Set.empty with
+let package_manager_name ?env switch config =
+  match install_packages_commands ?env switch config OpamSysPkg.Set.empty with
   | ((`AsAdmin pkgman | `AsUser pkgman), _) :: _ -> pkgman
   | [] -> assert false
 
@@ -1041,11 +1082,11 @@ let sudo_run_command ?(env=OpamVariable.Map.empty) ?vars cmd args =
       "failed with exit code %d at command:\n    %s"
       code (String.concat " " (cmd::args))
 
-let install ?env config packages =
+let install ?env switch config packages =
   if OpamSysPkg.Set.is_empty packages then
     log "Nothing to install"
   else
-    let commands, vars = install_packages_commands_t ?env config packages in
+    let commands, vars = install_packages_commands_t ?env switch config packages in
     let vars = OpamStd.Option.map (List.map (fun x -> `add, x)) vars in
     List.iter
       (fun (cmd, args) ->
@@ -1069,6 +1110,7 @@ let update ?(env=OpamVariable.Map.empty) config =
     | Macports -> Some (`AsAdmin "port", ["sync"])
     | Msys2 -> Some (`AsUser (Commands.msys2 config), ["-Sy"])
     | Netbsd -> None
+    | Nix -> None
     | Openbsd -> None
     | Suse -> Some (`AsAdmin "zypper", ["--non-interactive"; "refresh"])
   in

src/state/opamSysInteract.mli

@@ -23,16 +23,16 @@ val packages_status:
 (* Return the commands to run to install given system packages.
    [env] is used to determine host specification. *)
 val install_packages_commands:
-  ?env:gt_variables -> OpamFile.Config.t -> OpamSysPkg.Set.t ->
+  ?env:gt_variables -> OpamSwitch.t -> OpamFile.Config.t -> OpamSysPkg.Set.t ->
   ([`AsAdmin of string | `AsUser of string] * string list) list
 
 (* Install given system packages, by calling local system package manager.
    [env] is used to determine host specification. *)
-val install: ?env:gt_variables -> OpamFile.Config.t -> OpamSysPkg.Set.t -> unit
+val install: ?env:gt_variables -> OpamSwitch.t -> OpamFile.Config.t -> OpamSysPkg.Set.t -> unit
 
 val update: ?env:gt_variables -> OpamFile.Config.t -> unit
 
-val package_manager_name: ?env:gt_variables -> OpamFile.Config.t -> string
+val package_manager_name: ?env:gt_variables -> OpamSwitch.t -> OpamFile.Config.t -> string
 
 (* Determine if special packages may need installing to enable other
    repositories.