Do not allow actions to create dirs outside of build dir

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

CHANGES.md

@@ -30,7 +30,7 @@ unreleased
 
 - Add a 'cookies' option to ppx_rewriter/deriver flags in library stanzas
   (#2106, @mlasson @diml). This allow to specify cookie requests from
-  variables expanded at each invocation of the preprocessor. 
+  variables expanded at each invocation of the preprocessor.
 
 - Add more opam metadata and use it to generate `.opam` files. In particular, a
   `package` field has been added to specify package specific information.
@@ -65,6 +65,10 @@ unreleased
 - Make `dune subst` add a `(version ...)` field to the `dune-project`
   file (#2148, @diml)
 
+- Fine tune directory creation in dune. Dune will never complain if the
+  directory exists but forbid the user from creating source or external
+  directories with the `mkdir` action. (#2159, fix #2158, @rgrinberg)
+
 1.9.3 (06/05/2019)
 ------------------
 

src/action_exec.ml

@@ -108,7 +108,14 @@ let rec exec t ~ectx ~dir ~env ~stdout_to ~stderr_to =
     Path.rm_rf path;
     Fiber.return ()
   | Mkdir path ->
-    Path.mkdir_p path;
+    begin
+      if Path.is_in_build_dir path then
+        Path.mkdir_p path
+      else if Path.exists path then
+        ()
+      else
+        die "action attempted to create %a outside the build dir" Path.pp path
+    end;
     Fiber.return ()
   | Digest_files paths ->
     let s =