Io.copy_channels: make it reentrant (ocaml#11194)

Signed-off-by: Nicolás Ojeda Bär <[email protected]>

doc/changes/11194.md

@@ -0,0 +1,3 @@
+- Fix bug that could result in corrupted file copies by Dune, for example when
+  using the `copy_files#` stanza or the `copy#` action. (@nojb, #11194, fixes
+  #11193)

otherlibs/stdune/src/io.ml

@@ -67,15 +67,25 @@ let input_zero_separated =
 
 let copy_channels =
   let buf_len = 65536 in
-  let buf = Bytes.create buf_len in
-  let rec loop ic oc =
+  let global_buf = Bytes.create buf_len in
+  let rec loop buf ic oc =
     match input ic buf 0 buf_len with
     | 0 -> ()
     | n ->
       output oc buf 0 n;
-      loop ic oc
+      loop buf ic oc
   in
-  loop
+  let busy = ref false in
+  fun ic oc ->
+    if !busy
+    then loop (Bytes.create buf_len) ic oc
+    else (
+      busy := true;
+      match loop global_buf ic oc with
+      | () -> busy := false
+      | exception exn ->
+        busy := false;
+        Exn.reraise exn)
 ;;
 
 let setup_copy ?(chmod = Fun.id) ~src ~dst () =

otherlibs/stdune/src/io.mli

@@ -4,9 +4,6 @@ val close_in : in_channel -> unit
 val close_out : out_channel -> unit
 val close_both : in_channel * out_channel -> unit
 val input_lines : in_channel -> string list
-
-(** This function is not safe to use from multiple threads, even if operating on
-    unrelated channels because it uses a statically-allocated global buffer. *)
 val copy_channels : in_channel -> out_channel -> unit
 
 (** Try to read everything from a channel. Returns [Error ()] if the contents