On Wed, Feb 15, 2023 at 03:11:36PM +0100, Laszlo Ersek wrote:
> Add an assert() variant that we may call between fork() and exec*().
>
> Signed-off-by: Laszlo Ersek <[email protected]>
> ---
>
> +++ b/lib/internal.h
> +
> +#ifdef NDEBUG
> +#define NBD_INTERNAL_FORK_SAFE_ASSERT(expression) ((void)0)
May be affected by outcome of side discussion on 01/29 about whether
we want space after cast.
> +#else
> +#define NBD_INTERNAL_FORK_SAFE_ASSERT(expression) \
> + (nbd_internal_fork_safe_assert ((expression) != 0, __FILE__, __LINE__, \
> + __func__, #expression))
> +#endif
> +
> #endif /* LIBNBD_INTERNAL_H */
> +++ b/lib/utils.c
> @@ -431,13 +431,35 @@ nbd_internal_socketpair (int domain, int type, int
> protocol, int *fds)
> if (ret == 0) {
> for (i = 0; i < 2; i++) {
> if (fcntl (fds[i], F_SETFD, FD_CLOEXEC) == -1) {
> close (fds[0]);
> close (fds[1]);
> return -1;
> }
> }
> }
> #endif
>
> return ret;
> }
> +
> +void
> +nbd_internal_fork_safe_assert (int result, const char *file, long line,
> + const char *func, const char *assertion)
> +{
> + const char *line_out;
> + char line_buf[32];
> +
> + if (result)
> + return;
Since no code should ever directly call
nbd_internal_fork_safe_assert(0,...), but instead go through our macro
that has already filtered on expression's value, this conditional is
technically dead code; but I'm okay whether it stays in or gets
removed.
> +
> + xwrite (STDERR_FILENO, file, strlen (file));
> + xwrite (STDERR_FILENO, ":", 1);
Presumably, if our first best-effort xwrite() fails to produce full
output, all later xwrite() will hopefully hit the same error condition
so that we aren't producing even more-mangled output where later
syscalls succeed despite missing context earlier in the overall
output. If it were something we truly wanted to worry about, the
solution would be pre-loading the entire message into a single buffer,
then calling xwrite() just once - but that's far more effort for
something we don't anticipate hitting in normal usage anyways. I'm
happy if you ignore this whole paragraph of mine.
> + line_out = nbd_internal_fork_safe_itoa (line, line_buf, sizeof line_buf);
> + xwrite (STDERR_FILENO, line_out, strlen (line_out));
> + xwrite (STDERR_FILENO, ": ", 2);
> + xwrite (STDERR_FILENO, func, strlen (func));
> + xwrite (STDERR_FILENO, ": Assertion `", 13);
> + xwrite (STDERR_FILENO, assertion, strlen (assertion));
> + xwrite (STDERR_FILENO, "' failed.\n", 10);
These days, the quoting style `' seems to be disappearing in favor of
''. But a quick test showed me that at least glibc 2.36 is still
producing the message with the quotes as you have spelled it here.
> + abort ();
Huh. I checked the source to glibc's assert.c, where it includes a
comment about having to work cleanly even if a second assertion ends
up being raised from within a SIGABRT handler active during the first
attempt to abort(). But POSIX says that abort() shall attempt to
override any SIGABRT handler in place, so I wonder if glibc's
implementation is a bit too defensive. At any rate, your
implementation looks reasonable, and more to the point, satisfies your
desire of being async-signal-safe and thus appropriate between fork()
and exec().
Reviewed-by: Eric Blake <[email protected]>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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