[cache] Fix dune freezing on MacOS with cache enabled

Summary:
On MacOS shutdown on a socket raises `ENOTCONN` even when only one
side of the duplex communication is closed. This doesn't make much
sense to me (also, man pages are rather fuzzy around this, and on
Linux it behaves differently), but it is what it is.

Since cache/client calls `Unix.shutdown client.fd Unix.SHUTDOWN_SEND`
during teardown, `cache_daemon.ml` gets an exception on its shutdown,
which it swallows and therefore never closes `client.fd`.

Test plan:
1. Cache trimming tests work (and now enabled for all platforms),
2. Freezing described in ocaml#3233 is now gone.
3. Still works fine on Linux.

Fixes ocaml#3233 ocaml#2973

Signed-off-by: Artem Pyanykh <artem.pyanykh@gmail.com>

src/cache_daemon/cache_daemon.ml

@@ -181,10 +181,9 @@ let client_thread (events, (client : client)) =
       in
       handle client
     and finally () =
-      ( try
-          Unix.shutdown client.fd Unix.SHUTDOWN_ALL;
-          Unix.close client.fd
+      ( try Unix.shutdown client.fd Unix.SHUTDOWN_ALL
         with Unix.Unix_error (Unix.ENOTCONN, _, _) -> () );
+      Unix.close client.fd;
       Evt.sync (Evt.send events (Client_left client.fd))
     in
     try Exn.protect ~f ~finally with

test/blackbox-tests/dune.inc

@@ -358,8 +358,7 @@
  (action
   (chdir
    test-cases/dune-cache/trim
-   (progn (run dune-cram run run.t) (diff? run.t run.t.corrected))))
- (enabled_if (<> %{ocaml-config:system} macosx)))
+   (progn (run dune-cram run run.t) (diff? run.t run.t.corrected)))))
 
 (rule
  (alias dune-init)

test/blackbox-tests/gen_tests.ml

@@ -221,7 +221,7 @@ let exclusions =
   ; make "merlin/merlin-tests" ~external_deps:true
   ; make "use-meta" ~external_deps:true
   ; make "output-obj" ~skip_platforms:[ Mac; Win ] ~only_ocaml:(">=", "4.06.0")
-  ; make "dune-cache/trim" ~skip_platforms:[ Mac ]
+  ; make "dune-cache/trim"
   ; make "github644" ~external_deps:true
   ; make "private-public-overlap" ~external_deps:true
   ; make "reason" ~external_deps:true