libc/resolv: attempt to fix the test under WARNS=6

[kevans]

Summary

In a side-change that I'm working on to start defaulting src builds to WARNS=6 where WARNS isn't otherwise specified, GCC6 (and clang, to a lesser extent) pointed out a number of issues with the resolv tests:

• Global method variable that gets shadowed in run_tests()
• Pointer comparison to char ('\0')
• Signed/unsigned comparison between i in run_tests() and hosts->sl_cur

The shadowed variable looks like it might actually be bogus as written, as we pass it to RUN_TESTS -> run_tests, but other parts use the global method instead. This change is mainly geared towards correcting that by removing the global and plumbing the method through from run_tests -> run into the new thread.

I assume from the context of ptr == '\0' that it was intended to check for an empty string, so it's adjusted to ptr[0] == '\0'.

For signed/unsigned comparison, there's no compelling reason to not just switch i/nthreads/nhosts to size_t.

[ngie]

Thank you!

[pstef]

lib/libc/tests/resolv/resolv_test.c

84

I think this was checking for a null pointer, in which case you risk a null pointer dereference after this change.

[kevans]

lib/libc/tests/resolv/resolv_test.c

84

ptr is already checked for null as part of the loop invariant here.

[pstef]

lib/libc/tests/resolv/resolv_test.c

84

I admit I was wrong about the risk. But I'm not convinced it wasn't a bells and whistles... sorry, belts and suspenders kind of safety guarantee. I know I've written such reduntant checks. It's difficult for me to imagine strtok() returning an empty string, but I'm not sure.