Presence of .merlin.skip-if-not-cwd skips config in dir

Mitigation for https://github.com/ocaml/merlin/issues 1869

Author: jonahbeckford

src/kernel/mconfig_dot.ml

@@ -404,25 +404,38 @@ let find_project_context start_dir =
       then Some dir
       else None
   in
+  let cwd = Sys.getcwd () in
+  let cwd = Misc.canonicalize_filename ~cwd cwd in
 
   let rec loop workdir dir =
     try
       Some
-        (List.find_map [ ".merlin"; "dune-project"; "dune-workspace" ]
-           ~f:(fun f ->
-             let fname = Filename.concat dir f in
-             if Sys.file_exists fname && not (Sys.is_directory fname) then
-               (* When starting [dot-merlin-reader] from [dir]
+        (List.find_map [
+            ".merlin.skip-if-not-cwd";
+            ".merlin"; "dune-project"; "dune-workspace"
+          ]
+          ~f:(fun f ->
+            let fname = Filename.concat dir f in
+            if Sys.file_exists fname && not (Sys.is_directory fname)
+            then (
+              (* Special case:
+                 1. exists .merlin.skip-if-not-cwd
+                 2. not cwd (aka. `cwd <> dir`) *)
+              if f = ".merlin.skip-if-not-cwd" then (
+                if cwd <> Misc.canonicalize_filename ~cwd dir then
+                  raise Not_found
+                else None)
+              else
+                (* When starting [dot-merlin-reader] from [dir]
                   the workdir is always [dir] *)
-               let workdir = if f = ".merlin" then None else workdir in
-               let workdir = Option.value ~default:dir workdir in
-               Some
-                 ( { workdir;
-                     process_dir = dir;
-                     configurator = Option.get (Configurator.of_string_opt f)
-                   },
-                   fname )
-             else None))
+                let workdir = if f = ".merlin" then None else workdir in
+                let workdir = Option.value ~default:dir workdir in
+                Some ({
+                  workdir;
+                  process_dir = dir;
+                  configurator = Option.get (Configurator.of_string_opt f)
+                }, fname))
+            else None))
     with Not_found ->
       let parent = Filename.dirname dir in
       if parent <> dir then

src/kernel/mconfig_dot.mli

@@ -80,6 +80,14 @@ val get_config : context -> string -> config * string list
     - dune-project
     - dune-workspace
 
-    They are detected in that order. [dune] and [jbuild] file do not need to be taken into account because any project using a recent version of dune should have a dune-project file which is even auto-generated when it is missing. And only recent versions of dune will stop writing .merlin files.
+    They are detected in that order. [dune] and [jbuild] file do not need to
+    be taken into account because any project using a recent version of dune
+    should have a dune-project file which is even auto-generated when it is
+    missing. And only recent versions of dune will stop writing .merlin files.
+
+    The presence of the file [".merlin.skip-if-not-cwd"] in a directory means
+    that the three (3) project configuration files are {b not} checked if the
+    directory containing [".merlin.skip-if-not-cwd"] is not the current
+    working directory.
 *)
 val find_project_context : string -> (context * string) option