Harden windows_get_shell

In Windows native containers, the Filename.chop_suffix call in
windows_get_shell was occasionally failing. OpamStubs.getProcessAncestry
can return empty strings. windows_get_shell now explictly handles the
empty string case and .exe is now _optionally_ stripped.

master_changes.md

@@ -105,6 +105,7 @@ users)
 ## Internal
 
 ## Internal: Windows
+  * Fix sporadic crash in shell detection (seen in native containers) [#5714 @dra27]
 
 ## Test
 

src/core/opamStd.ml

@@ -1073,6 +1073,9 @@ module OpamSys = struct
     else
       fun x -> x
 
+  let chop_exe_suffix name =
+    Option.default name (Filename.chop_suffix_opt name ~suffix:".exe")
+
   let windows_process_ancestry = Lazy.from_fun OpamStubs.getProcessAncestry
 
   type shell_choice = Accept of shell
@@ -1085,10 +1088,11 @@ module OpamSys = struct
       | "pwsh.exe" -> Some (Accept (SH_pwsh Powershell_pwsh))
       | "cmd.exe" -> Some (Accept SH_cmd)
       | "env.exe" -> Some (Accept SH_sh)
+      | "" -> None
       | name ->
         Option.map
           (fun shell -> Accept shell)
-          (shell_of_string (Filename.chop_suffix name ".exe"))
+          (shell_of_string (chop_exe_suffix name))
     in
     lazy (
       let lazy ancestors = windows_process_ancestry in

src/core/opamStd.mli

@@ -503,6 +503,9 @@ module Sys : sig
   (** Append .exe (only if missing) to executable filenames on Windows *)
   val executable_name : string -> string
 
+  (** Remove .exe (if present) from an executable filename on Windows *)
+  val chop_exe_suffix : string -> string
+
   (** The different families of shells we know about *)
   type powershell_host = Powershell_pwsh | Powershell
   type shell = SH_sh | SH_bash | SH_zsh | SH_csh | SH_fish