test: demonstrate overflow with sendfile (ocaml#10334)

See ocaml#10333

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

test/blackbox-tests/test-cases/dune

@@ -43,6 +43,13 @@
  (applies_to fdo)
  (enabled_if false))
 
+; This test copies a large file, which is too slow to be enabled all the time.
+; The source file is actually sparse, but sendfile seems to do a full copy
+; nevertheless.
+(cram
+ (applies_to sendfile-large-file)
+ (enabled_if false))
+
 ; This test is flaky
 
 (cram

test/blackbox-tests/test-cases/sendfile-large-file.t

@@ -0,0 +1,22 @@
+We create a large file and check that it is copied completely.
+
+  $ cat > dune-project << EOF
+  > (lang dune 1.0)
+  > EOF
+
+  $ cat > create.ml << EOF
+  > let () = Unix.truncate "file.dat" 0x1_00_00_00_03
+  > EOF
+  $ touch file.dat
+  $ ocaml unix.cma create.ml
+  $ rm create.ml
+
+  $ dune build file.dat
+
+  $ dune_cmd stat size file.dat
+  4294967299
+
+  $ dune_cmd stat size _build/default/file.dat
+  3
+
+(3 indicates that the file size is taken modulo 2**32)