<sys/extattr.h>: use designated initializers for EXTATTR_NAMESPACE_NAMES

[kevans]

Summary

This is not a functional change, but it makes it more clear upon
inspection of the definition that the mapping property described is
preserved. Maybe more importantly, if one ends up getting an index
wrong or punching a hole in the name array unexpectedly, then it'll
hopefully manifest more clearly as a (null) or nullptr deref rather than
potentially just emitting the wrong namespace name.

Sponsored by: Klara, Inc.

[kib]

AFAIR this is not a valid C++, if anybody cares.

[kevans]

In D55323#1265205, @kib wrote:

AFAIR this is not a valid C++, if anybody cares.

Yeah, seems to be true- FWIW, though, apparently clang and GCC both just accept it?

root@ifrit:/tmp # c++ test.cc
test.cc:5:30: warning: array designators are a C99 extension [-Wc99-designator]
    5 | const char * const names[] = EXTATTR_NAMESPACE_NAMES;
      |                              ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/sys/extattr.h:60:2: note: expanded from macro 'EXTATTR_NAMESPACE_NAMES'
   60 |         [EXTATTR_NAMESPACE_EMPTY] = EXTATTR_NAMESPACE_EMPTY_STRING, \
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
root@ifrit:/tmp # g++13 -Wall -Wextra test.cc

I'm a little surprised that GCC doesn't emit any kind of complaint as well.

[kib]

gcc might need -pedantic or something like this.

[rmacklem]

Thanks for doing this. I think it does clarify
the relationship between EXTATTR_NAMESPACE_EMPTY
and EXTATTR_NAMESPACE_EMPTY_STRING for your
other patch.

[mckusick]

This adds clarity for me.