Test for absence of select: introduce open_1024 in tests

We'll check statically that we are not using Unix.select, but it is good to have a runtime check too,
in case some library (C or OCaml) indirectly uses it.
Open 1024 file descriptors at the beginning of the test, so that we would hit an exception on the first use of Unix.select with any newly created
file descriptor as their value would be >1024 (the limit in `select`).

Unconditionally call these in the stdext-unix, quicktest, and xapi tests.
Set the default ulimit in 'make test' to 2048.

If you are running tests with `dune runtest` directly then you might need to raise this limit,
if your shell has a lower limit by default.
If needed we could add a better error message or a check for ulimit before attempting to open the 1024 files.

No functional change for the product.

Signed-off-by: Edwin Török <[email protected]>

Makefile

@@ -62,6 +62,7 @@ TEST_TIMEOUT=600
 TEST_TIMEOUT2=1200
 test:
 	ulimit -S -t $(TEST_TIMEOUT); \
+	 ulimit -n 2048; \
 	 (sleep $(TEST_TIMEOUT) && ps -ewwlyF --forest)& \
 	 PSTREE_SLEEP_PID=$$!; \
 	 trap "kill $${PSTREE_SLEEP_PID}" SIGINT SIGTERM EXIT; \

ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/test/unixext_test.ml

@@ -300,4 +300,5 @@ let tests =
 let () =
   (* avoid SIGPIPE *)
   let (_ : Sys.signal_behavior) = Sys.signal Sys.sigpipe Sys.Signal_ignore in
+  Xapi_stdext_unix.Unixext.test_open 1024;
   QCheck_base_runner.run_tests_main tests

ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/unixext.ml

@@ -913,6 +913,23 @@ let domain_of_addr str =
     Some (Unix.domain_of_sockaddr (Unix.ADDR_INET (addr, 1)))
   with _ -> None
 
+let test_open_called = Atomic.make false
+
+let test_open n =
+  if not (Atomic.compare_and_set test_open_called false true) then
+    invalid_arg "test_open can only be called once" ;
+  (* we could make this conditional on whether ulimit was increased or not,
+     but that could hide bugs if we think the CI has tested this, but due to ulimit it hasn't.
+  *)
+  if n > 0 then (
+    let socket = Unix.socket Unix.PF_UNIX Unix.SOCK_STREAM 0 in
+    at_exit (fun () -> Unix.close socket) ;
+    for _ = 2 to n do
+      let fd = Unix.dup socket in
+      at_exit (fun () -> Unix.close fd)
+    done
+  )
+
 module Direct = struct
   type t = Unix.file_descr
 

ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/unixext.mli

@@ -287,6 +287,17 @@ val statvfs : string -> statvfs_t
 val domain_of_addr : string -> Unix.socket_domain option
 (** Returns Some Unix.PF_INET or Some Unix.PF_INET6 if passed a valid IP address, otherwise returns None. *)
 
+val test_open : int -> unit
+(** [test_open n] opens n file descriptors. This is useful for testing that the application makes no calls
+  to [Unix.select] that use file descriptors, because such calls will then immediately fail.
+
+  This assumes that [ulimit -n] has been suitably increased in the test environment.  
+  
+  Can only be called once in a program, and will raise an exception otherwise.
+
+  The file descriptors will stay open until the program exits.
+ *)
+
 module Direct : sig
   (** Perform I/O in O_DIRECT mode using 4KiB page-aligned buffers *)
 

ocaml/quicktest/quicktest.ml

@@ -16,6 +16,7 @@
 
 let () =
   Quicktest_args.parse () ;
+  Xapi_stdext_unix.Unixext.test_open 1024;
   Qt_filter.wrap (fun () ->
       let suite =
         [

ocaml/tests/common/suite_init.ml

@@ -1,4 +1,6 @@
 let harness_init () =
+  (* before any calls to XAPI code, to catch early uses of Unix.select *)
+  Xapi_stdext_unix.Unixext.test_open 1024;
   Xapi_stdext_unix.Unixext.mkdir_safe Test_common.working_area 0o755 ;
   (* Alcotest hides the standard output of successful tests,
      so we will probably not exceed the 4MB limit in Travis *)