Cache actions that generate symlinks (#4405)

* Fix a bug where dune always re-runs all actions that produce symlinks

Signed-off-by: Arseniy Alekseyev <aalekseyev@janestreet.com>

Author: aalekseyev

CHANGES.md

@@ -111,6 +111,9 @@ Unreleased
   Previously, Dune would have re-executed the action again at the last
   line. Now it remembers the result of the first execution.
 
+- Fix a bug where dune would always re-run all actions that produce symlinks,
+  even if their dependencies did not change. (#4405, @aalekseyev)
+
 2.8.2 (21/01/2021)
 ------------------
 

src/dune_engine/cached_digest.ml

@@ -130,6 +130,11 @@ let refresh_and_chmod fn =
     if Cache.cachable stats.st_kind then
       Path.chmod ~stats:(Some stats) ~mode:0o222 ~op:`Remove fn
   in
+  let stats =
+    match stats.st_kind with
+    | S_LNK -> Path.stat fn
+    | _ -> stats
+  in
   refresh_ stats fn
 
 let peek_file fn =

test/blackbox-tests/test-cases/symlink-targets.t/run.t

@@ -0,0 +1,22 @@
+Test demonstrating the handling of actions that produce symlinks.
+
+  $ echo "(lang dune 2.8)" > dune-project
+  $ cat >dune <<EOF
+  > (rule (targets b) (deps a) (action (bash "ln -s a b")))
+  > EOF
+  $ echo a > a
+  $ dune build ./b --display=short
+          bash b
+  $ readlink _build/default/b
+  a
+  $ cat _build/default/b
+  a
+
+  $ dune build ./b --display=short
+
+
+  $ echo a-v2 > a
+  $ dune build ./b --display=short
+          bash b
+  $ cat _build/default/b
+  a-v2