fix(sendfile): Flush the out_channel used in the fallback path (#8288)

Fixes #8284

Signed-off-by: Alan Hu <alanh@ccs.neu.edu>

CHANGES.md

@@ -1,6 +1,8 @@
 Unreleased
 ----------
 
+- Fix flushing when using `sendfile` fallback (#8288, @alan-j-hu)
+
 - Add `dune show rules` as alias of the `dune rules` command. (#8000, @Alizter)
 
 - Fix `%{deps}` to expand properly in `(cat ...)` when containing 2 or more

otherlibs/stdune/src/io.ml

@@ -145,17 +145,23 @@ module Copyfile = struct
       | Error `Src_missing ->
         let message = Printf.sprintf "%s: No such file or directory" src in
         raise (Sys_error message)
-      | Ok (src, dst, src_size) ->
-        Exn.protect
-          ~f:(fun () ->
-            try sendfile ~src ~dst src_size
-            with Unix.Unix_error (EINVAL, "sendfile", _) ->
-              let ic = Unix.in_channel_of_descr src in
-              let oc = Unix.out_channel_of_descr dst in
-              copy_channels ic oc)
-          ~finally:(fun () ->
-            Unix.close src;
-            Unix.close dst)
+      | Ok (src, dst, src_size) -> (
+        let close_fds () =
+          Unix.close src;
+          Unix.close dst
+        in
+        match sendfile ~src ~dst src_size with
+        | exception Unix.Unix_error (EINVAL, "sendfile", _) ->
+          let ic = Unix.in_channel_of_descr src in
+          let oc = Unix.out_channel_of_descr dst in
+          Exn.protect
+            ~f:(fun () -> copy_channels ic oc)
+            ~finally:(fun () ->
+              (* we make sure to close the fd's with the channel api to make
+                 sure everything has been flushed *)
+              close_both (ic, oc))
+        | () -> close_fds ()
+        | exception _ -> close_fds ())
 
   let copyfile ?chmod ~src ~dst () =
     let src_stats =

test/blackbox-tests/test-cases/github8041.t

@@ -30,4 +30,4 @@ larger files.
   > else
   >   echo "File copied incorrectly"
   > fi
-  File copied incorrectly
+  File copied correctly