Suppress unused variable warning in helper_thread.c

[r-barnes]

Allows -Wunused-but-set-variable to pass in opt mode. Adds what appears to be a missing assert.

Please confirm that your commit message(s) follow these guidelines:

1. First line is a commit title, a descriptive one-liner for the change
2. Empty second line
3. Commit message body that explains why the change is useful. Break lines that
   aren't something like a URL at 72-74 chars.
4. Empty line
5. Signed-off-by: Real Name real@email.com

Reminders:

1. If you modify struct thread_options, also make corresponding changes in
   cconv.c and bump FIO_SERVER_VER in server.h
2. If you change the ioengine interface (hooks, flags, etc), remember to bump
   FIO_IOOPS_VERSION in ioengines.h.

[vincentkfu]

Is this line necessary with the assert added below?

[r-barnes]

It is. A more comprehensive take would be:

#if defined(NDEBUG)
#define USED_FOR_DEBUG(x) (void)0;
#else
#define USED_FOR_DEBUG(x) (void)x;
#endif

That way the warning is suppressed during debug mode, but not during opt mode.

But this seems simpler for a smaller codebase.

[vincentkfu]

I've built with ./configure --extra-cflags=Wall and confirmed that CONFIG_PTHREAD_SIGMASK is defined but cannot trigger the warning that you report.

Our CI builds with -Werror and this warning does not appear.

What did you do to trigger this warning?

[r-barnes]

@vincentkfu Our builds use a different build system with LLVM-15 with -Wunused-but-set-variable enabled, and -Werror. That triggers a build failure for the instance in question.

[vincentkfu]

Ok. The convention in our codebase is to use /* ... */ for comments. Please make this change to your patch.

[r-barnes]

@vincentkfu - Thanks! I've made the change. My apologies for not noticing your convention the first time.

[vincentkfu]

These are small changes but I can't let them go in this way.

How about this:

Patch 1: add missing assert only
Patch 2: add (void) ret; to suppress unused variable warning

Please make sure the commit messages for each patch accurately describe the purpose of the change.