fix: pass O_SHARE_DELETE when digesting files

Fixes #8268

This flag corresponds to what `Stdlib.open_in` does. When passed, the
files can be removed while they are still open. This condition can
happen when background digests are enabled. This is a no-op everywhere
else.

This allows enabling background digests on Windows.

Signed-off-by: Etienne Millon <me@emillon.org>

CHANGES.md

@@ -24,9 +24,9 @@ Unreleased
 - Fix permission errors when `sendfile` is not available (#8234, fixes #8120,
   @emillon)
 
-- Disable background digests on Windows. This prevents an issue where
+- Fix background file digests on Windows. This prevents an issue where
   unremovable files would make dune crash when the shared cache is enabled.
-  (#8243, fixes #8228, @emillon)
+  (#8243, #8262, fixes #8228, fixes #8268, @emillon)
 
 3.9.1 (2023-07-06)
 ------------------

src/dune_config/config.ml

@@ -116,10 +116,7 @@ let background_digests =
   let t =
     { name = "background_digests"
     ; of_string = Toggle.of_string
-    ; value =
-        (match Platform.OS.value with
-        | Linux -> `Enabled
-        | _ -> `Disabled)
+    ; value = background_default
     }
   in
   register t;

src/dune_digest/dune_digest.ml

@@ -18,7 +18,7 @@ end
 module Direct_impl : Digest_impl = struct
   let file file =
     let fd =
-      match Unix.openfile file [ Unix.O_RDONLY ] 0 with
+      match Unix.openfile file [ Unix.O_RDONLY; O_SHARE_DELETE ] 0 with
       | fd -> fd
       | exception Unix.Unix_error (Unix.EACCES, _, _) ->
         raise (Sys_error (sprintf "%s: Permission denied" file))