[Libguestfs] [PATCH libnbd] generator: Fix race condition when validating h->state.

[Richard W.M. Jones]

The code for (eg) nbd_aio_pread starts by checking this outside
the lock:

  if (!(nbd_aio_is_ready (h) || nbd_aio_is_processing (h))) {

However there can still be a race condition even though h->state is
atomic:

  Thread A                         Thread B
  (in a call that holds h->lock)   (calling nbd_aio_pread)
  --------------------------------------------------------------------
  h->state is processing
                                   checks nbd_aio_is_ready
                                     (it's false)
  h->state is moved to READY
                                   checks nbd_aio_is_processing
                                     (it's false)
                                   validation check fails

However the state was valid so the validation check should have
succeeded.

Fixes commit e63a11736930c381a79a8cc2d03844cfff5db3ef.

Thanks: Eric Blake for identifying the race condition above.
---
 generator/generator | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/generator/generator b/generator/generator
index ea6f739..8068762 100755
--- a/generator/generator
+++ b/generator/generator
@@ -2820,10 +2820,9 @@ let generate_lib_api_c () =
       pr "  nbd_internal_reset_error (\"nbd_%s\");\n" name;
       pr "\n"
     );
+    if is_locked then
+      pr "  pthread_mutex_lock (&h->lock);\n";
     if permitted_states <> [] then (
-      pr "  /* We can check the state outside the handle lock because the\n";
-      pr "   * the state is atomic.\n";
-      pr "   */\n";
       let tests =
         List.map (
           function
@@ -2842,8 +2841,6 @@ let generate_lib_api_c () =
       pr "  }\n";
       pr "\n"
     );
-    if is_locked then
-      pr "  pthread_mutex_lock (&h->lock);\n";
     pr "  ret = nbd_unlocked_%s (h" name;
     let argnames = List.flatten (List.map name_of_arg args) in
     List.iter (pr ", %s") argnames;
-- 
2.21.0