Expose OpamStd.Sys.resolve_in_path

Exposes the portion of OpamStd.Sys.resolve_command which searches the
PATH environment for a given basename.

master_changes.md

@@ -177,3 +177,4 @@ users)
   * `OpamStubs.readRegistry`: on Windows, complements `OpamStubs.writeRegistry` [#5963 @dra27]
   * `OpamStubs.get_initial_environment`: on Windows, returns the pristine environment for new shells [#5963 @dra27]
   * `OpamStd.Sys.resolve_command`: extracted the logic from `OpamSystem.resolve_command`, without the default environment handling from OpamProcess. [#5991 @dra27]
+  * `OpamStd.Sys.resolve_in_path`: split the logic of `OpamStd.Sys.resolve_command` to allow searching for an arbitrary file in the search path [#5991 @dra27]

src/core/opamStd.ml

@@ -1220,19 +1220,31 @@ module OpamSys = struct
     in
     aux 0
 
+  let is_external_cmd name =
+    let forward_to_back =
+      if Sys.win32 then
+        String.map (function '/' -> '\\' | c -> c)
+      else
+        fun x -> x
+    in
+    let name = forward_to_back name in
+    OpamString.contains_char name Filename.dir_sep.[0]
+
+  let resolve_in_path env name =
+    if not (Filename.is_relative name) || is_external_cmd name then
+      invalid_arg "OpamStd.Sys.resolve_in_path: bare command expected"
+    else
+      let path = split_path_variable (env_var env "PATH") in
+      List.filter_map (fun path ->
+          let candidate = Filename.concat path name in
+          match Sys.is_directory candidate with
+          | false -> Some candidate
+          | true | exception (Sys_error _) -> None)
+        path
+
   (* OCaml 4.05.0 no longer follows the updated PATH to resolve commands. This
      makes unqualified commands absolute as a workaround. *)
   let resolve_command =
-    let is_external_cmd name =
-      let forward_to_back =
-        if Sys.win32 then
-          String.map (function '/' -> '\\' | c -> c)
-        else
-          fun x -> x
-      in
-      let name = forward_to_back name in
-      OpamString.contains_char name Filename.dir_sep.[0]
-    in
     let check_perms =
       if Sys.win32 then fun f ->
         try (Unix.stat f).Unix.st_kind = Unix.S_REG
@@ -1277,24 +1289,16 @@ module OpamSys = struct
         else `Cmd cmd
       end else
       (* bare command, lookup in PATH *)
-      (* Following the shell sematics for looking up PATH, programs with the
-         expected name but not the right permissions are skipped silently.
-         Therefore, only two outcomes are possible in that case, [`Cmd ..] or
-         [`Not_found]. *)
-      let path = split_path_variable (env_var env "PATH") in
       let name =
         if Sys.win32 && not (Filename.check_suffix name ".exe") then
           name ^ ".exe"
         else name
       in
-      let possibles =
-        List.filter_map (fun path ->
-            let candidate = Filename.concat path name in
-            match Sys.is_directory candidate with
-            | false -> Some candidate
-            | true | exception (Sys_error _) -> None)
-          path
-      in
+      let possibles = resolve_in_path env name in
+      (* Following the shell sematics for looking up PATH, programs with the
+         expected name but not the right permissions are skipped silently.
+         Therefore, only two outcomes are possible in that case, [`Cmd ..] or
+         [`Not_found]. *)
       match List.find check_perms possibles with
       | cmdname -> `Cmd cmdname
       | exception Not_found ->
@@ -1307,6 +1311,12 @@ module OpamSys = struct
       let env = match env with None -> Env.raw_env () | Some e -> e in
       resolve env ?dir name
 
+  let resolve_in_path ?env name =
+    let env = match env with None -> Env.raw_env () | Some e -> e in
+    match resolve_in_path env name with
+    | result::_ -> Some result
+    | [] -> None
+
   let get_windows_executable_variant =
     if Sys.win32 then
       let results = Hashtbl.create 17 in

src/core/opamStd.mli

@@ -553,6 +553,12 @@ module Sys : sig
   val resolve_command: ?env:string array -> ?dir:string -> string ->
     [ `Cmd of string | `Denied | `Not_found ]
 
+  (** Search for an arbitrary file in PATH. Unlike {!resolve_command}, no
+      transformations take place on the name in Windows (i.e. .exe, etc. is
+      never appended) and no executable check takes place. The name passed
+      must be a basename (no directory component). *)
+  val resolve_in_path: ?env:string array -> string -> string option
+
   (** For native Windows builds, returns [`Cygwin] if the command is a Cygwin-
       compiled executable, [`Msys2] if the command is a MSYS2-compiled
       executable, and [`Tainted of [ `Msys2 | `Cygwin ]] if the command links