fix: try harder to find a working pdf reader

tools/rdbg4sasa/sasa-rdbg-cmds.ml

@@ -239,7 +239,7 @@ let _ =
 (* split  the vars into returns  (the enab vars, the  activated vars,
    the other vars) nb: in the "Enab" prefix is removed from enab vars
    names; ie we leave only the pid and the action name *)
-type s = (string * string * Data.v)
+type s = (string  string * Data.v)
 let split_data (l:Data.subst list) : s list * s list * s list =
   let l = List.map (fun (x,v) -> Str.split (Str.regexp "_") x, v) l in
   let rec sortv (enab, other) (x,v) =
@@ -438,14 +438,20 @@ let l () =
 (**********************************************************************)
 (* ok, let's start the debugging session! *)
 
-let pdf_viewer =
-  if Sys.command "which zathura" = 0 then "zathura" else
+let pdf_viewer = (* try hard to find a working pdf viewer! *)
+  try
+    let res = Unix.getenv "PDF_VIEWER" in (* may raise Not_found *)
+    if Sys.command (Printf.sprintf "which %s" res) = 0 then res else raise Not_found
+  with Not_found ->
+    if Sys.command "which see" = 0 then "see" else
+    if Sys.command "which zathura" = 0 then "zathura" else
+    if Sys.command "which okular" = 0 then "okular" else
     if Sys.command "which xpdf" = 0 then "xpdf" else
-      if Sys.command "which acroread" = 0 then "acroread" else
-        if Sys.command "which evince" = 0 then "evince" else (
-          Printf.printf "Warning: no pdf viewer is found to visualize %s\n%!" dotfile;
-          "ls"
-        )
+    if Sys.command "which evince" = 0 then "evince" else
+    if Sys.command "which acroread" = 0 then "acroread" else (
+      Printf.printf "Warning: no pdf viewer found to visualize %s\n%!" dotfile;
+      "ls"
+    )
 
 let graph_view () =
   !dot_view ();