[Kernel] remove reliance on 'PWD' environment variable

The absence of Unix.realpath in OCaml < 4.13 led us to rely on the PWD
environment variable to avoid issues with symbolic links in directories.
Now that Frama-C requires OCaml >= 4.13, we can use it instead and avoid
issues with Dune and Node.js related to out-of-date values for PWD.

src/libraries/utils/filepath.ml

@@ -125,22 +125,15 @@ let insert base path_name =
       Array.set cache (hash land 255) (Some (path_name, path));
       path
 
-(* TODO: we currently use PWD instead of Sys.getcwd () because OCaml has
-   no function in its stdlib to resolve symbolic links (e.g. realpath)
-   for a given path. 'getcwd' always resolves them, but if the user
-   supplies a path with symbolic links, this may cause issues.
-   Instead of forcing the user to always provide resolved paths, we
-   currently choose to never resolve them.
-   Note that, in rare situations (e.g. some Docker images), PWD does not
-   exist in the environment, so in that case, we fallback to Sys.getcwd.
-
-   REMARK[LC]: when the Frama-C binary is directly invoked by Node without
-   going through a shell script wrapper like ./bin/frama-c, the environment
-   variable "PWD" is _no more_ synchronized with Sys.getcwd.
-   This problem has been solved in `Dome.spawn()` by forcing the `PWD`
-   environement variable accordingly.
+(* Note: the call to Unix.realpath prevents some issues with symbolic links
+   in directory names. If you have problems with this, please contact us.
+   For the same reason, Sys.getcwd should _not_ be called directly, but only
+   via this function, to avoid conflicting results in case the user forgot
+   to call Unix.realpath.
 *)
-let cwd = insert dummy (try Sys.getenv "PWD" with Not_found -> Sys.getcwd ())
+let pwd () = Unix.(realpath (getcwd ()))
+
+let cwd = insert dummy (pwd ())
 
 type existence =
   | Must_exist
@@ -336,8 +329,6 @@ type position =
 let pp_pos fmt pos =
   Format.fprintf fmt "%a:%d" Normalized.pretty pos.pos_path pos.pos_lnum
 
-let pwd () = try Unix.getenv "PWD" with Not_found -> Sys.getcwd ()
-
 (*
 Local Variables:
 compile-command: "make -C ../../.."

src/libraries/utils/filepath.mli

@@ -209,15 +209,8 @@ type position =
 val pp_pos : Format.formatter -> position -> unit
 
 (** Return the current working directory.
-    Currently uses the environment's PWD instead of Sys.getcwd () because OCaml
-    has no function in its stdlib to resolve symbolic links (e.g. realpath)
-    for a given path. 'getcwd' always resolves them, but if the user
-    supplies a path with symbolic links, this may cause issues.
-    Instead of forcing the user to always provide resolved paths, we
-    currently choose to never resolve them.
-    We only resort to getcwd() to avoid issues when PWD does not exist.
-    Note that this function does not validate that PWD has not been tampered
-    with.
+    Implicitly uses {!Unix.realpath} to normalize paths and avoid issues with
+    symbolic links in directory names.
 
     @since 25.0-Manganese
 *)