[musl] Potential bug in __res_msend_rc() wrt to union initialization.

[Mike Cui <cuicui@gmail.com>]

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I recently upgraded to clang-18, and after compiling musl with it, I
noticed that all my getaddrinfo() calls are failing. I tracked this to be
an issue in __res_msend_rc(), where the 'sin6' member of union 'sa' is
initialized to garbage, rather than 0. Then later bind() fails
with EADDRNOTAVAIL.

I reported this bug on clang discourse:
https://discourse.llvm.org/t/union-initialization-and-aliasing-clang-18-seems-to-miscompile-musl/77724,
and the discussion seems to suggest that there is potentially a bug in musl
as well. TL;DR:

- According to strict interpretation of the C standard, initializing a
union with '{0}', only initializes the first member of the union to 0 (in
this case, sin4), and "default" initializes the rest. This interpretation
is still up for debate. The proper way to initialize the entire union is '{
}' not '{ 0 }'.
- There is currently a bug in clang-18 that treats '{ }' to be the same as
'{ 0 }'. The proposed fix is to just zero out the entire union for both "{
0 }" and "{ }". However we cannot rely on "{ 0 }" to always zero out the
entire union in the future.

musl should be fixed to use "{ }" for initialization. And to work around
the current buggy release of clang-18, perhaps flip the order to make sin6
the first member of the struct? I've attached a patch that works for me.
There may be other instances of the same bug in the musl code base.

--- a/src/network/res_msend.c
+++ b/src/network/res_msend.c
@@ -83,9 +83,9 @@ int __res_msend_rc(int nqueries, const unsigned char
*const *queries,
        int fd;
        int timeout, attempts, retry_interval, servfail_retry;
        union {
-               struct sockaddr_in sin;
                struct sockaddr_in6 sin6;
-       } sa = {0}, ns[MAXNS] = {{0}};
+               struct sockaddr_in sin;
+       } sa = {}, ns[MAXNS] = {{}};
        socklen_t sl = sizeof sa.sin;
        int nns = 0;
        int family = AF_INET;

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