[Libguestfs] [PATCH libguestfs 3/4] ocaml: Conditionally acquire the lock in callbacks

This fix was originally suggested by Jürgen Hötzel (link below) which I have lightly modified so it works with OCaml <= 4 too. Link: https://listman.redhat.com/archives/libguestfs/2023-May/031640.html Link: https://discuss.ocaml.org/t/test-caml-state-and-conditionally-caml-acquir... --- ocaml/guestfs-c.c | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) diff --git a/ocaml/guestfs-c.c b/ocaml/guestfs-c.c index a1865a726a..67dc354721 100644 --- a/ocaml/guestfs-c.c +++ b/ocaml/guestfs-c.c @@ -19,6 +19,7 @@ #include <config.h> #include <stdio.h> #include <stdlib.h> +#include <stdbool.h> #include <string.h> #include <errno.h> @@ -36,6 +37,7 @@ #include <caml/signals.h> #include <caml/threads.h> #include <caml/unixsupport.h> +#include <caml/version.h> #include "guestfs-c.h" @@ -397,13 +399,32 @@ event_callback_wrapper (guestfs_h *g, { /* Ensure we are holding the GC lock before any GC operations are * possible. (RHBZ#725824) + * + * There are many paths where we already hold the OCaml lock before + * this function, for example "non-blocking" calls, and the + * libguestfs global atexit path (which calls guestfs_close). To + * avoid double acquisition we need to check if we already hold the + * lock. OCaml 5 is strict about this. In earlier OCaml versions + * there is no way to check, but they did not implement the lock as + * a mutex and so it didn't cause problems. + * + * See also: + * https://discuss.ocaml.org/t/test-caml-state-and-conditionally-caml-acquir... */ - caml_acquire_runtime_system (); +#if OCAML_VERSION_MAJOR >= 5 + bool acquired = caml_state != NULL; +#else + const bool acquired = false; +#endif + + if (!acquired) + caml_acquire_runtime_system (); event_callback_wrapper_locked (g, data, event, event_handle, flags, buf, buf_len, array, array_len); - caml_release_runtime_system (); + if (!acquired) + caml_release_runtime_system (); } value -- 2.41.0