[Libguestfs] Re: [PATCH libnbd] lib: Add nbd_get_subprocess_pid to return h->pid

On Sun, Aug 25, 2024 at 02:13:39PM GMT, Richard W.M. Jones wrote: > While debugging an ugly nbdkit / OCaml crash[1] which only happened > when nbdkit was run as a subprocess of libnbd, I needed a way to start > gdb attached to nbdkit. It's not possible to do this by running > "gdb --args nbdkit -s ..." as a command for various reasons. > > My solution to this was (error checking removed for clarity): > > nbd_connect_command (nbd, { "nbdkit", "-s" /* ... */}); > char pid[100]; > snprintf (pid, sizeof pid, "%d", nbd->pid); > if (fork () > 0) { > execl ("/usr/bin/gdb", "gdb", "../server/nbdkit", pid, NULL); > _exit (1); > } > while (access ("/tmp/continue", F_OK) != 0) { > sleep (1); > } > /* Test case continues after the file is created. */ > > In this case I was fishing directly into the private nbd_handle struct > to get the pid field. However this seems like a genuine (if rare) > case for being able to get the process ID directly. > > The documentation is written to be clear that this is only useful for > debugging cases, only works on some platforms, and shouldn't be relied > on more generally. Since we have given the appropriate disclaimer about it being mainly useful to debugging sessions, I don't see a problem with including this. > > [1] https://discuss.ocaml.org/t/free-uninitalized-data-when-calling-caml-c-th... Quite a debug session you went through there.

> + > + "get_subprocess_pid", { > + default_call with > + args = []; ret = RInt64; > + shortdesc = "get the process ID of the subprocess"; > + longdesc = "\ > +For connections which create a subprocess such as > +L<nbd_connect_command(3)>, this returns the process ID (PID) > +of the subprocess. This is only supported on some platforms. > + > +This is mainly useful in debugging cases. For example we used > +this to attach L<gdb(1)> to an nbdkit subprocess that was crashing > +inside a plugin."; The first-person past-tense tone of the sentence is a bit out of character with the rest of the documentation. Maybe simplify it to just: For example, this could be used to learn where to attach L<gdb(1)> to diagnose a crash in an nbdkit subprocess.

> +++ b/tests/get-subprocess-pid.c > +int > +main (int argc, char *argv[]) > +{ > + struct nbd_handle *nbd; > + int64_t pid; > + const char *cmd[] = { NBDKIT, "-s", "--exit-with-parent", > + "memory", "size=1m", NULL }; Is there any (easy?) way to get nbdkit to confess its own pid, to compare that it matches with what we queried from libnbd?

Reviewed-by: Eric Blake <eblake(a)redhat.com&gt;