now that blocking signals prevents any application code from running
while the last thread is exiting, the cas logic is no longer needed to
prevent decrementing below zero.

the thread count (1+libc.threads_minus_1) must always be greater than
or equal to the number of threads which could have application code
running, even in an async-signal-safe sense. there is at least one
dangerous race condition if this invariant fails to hold: dlopen could
allocate too little TLS for existing threads, and a signal handler
running in the exiting thread could claim the allocated TLS for itself
(via __tls_get_addr), leaving too little for the other threads it was
allocated for and thereby causing out-of-bounds access.

there may be other situations where it's dangerous for the thread
count to be too low, particularly in the case where only one thread
should be left, in which case locking may be omitted. however, all
such code paths seem to arise from undefined behavior, since
async-signal-unsafe functions are not permitted to be called from a
signal handler that interrupts pthread_exit (which is itself
async-signal-unsafe).

this change may also simplify logic in __synccall and improve the
chances of making __synccall async-signal-safe.

for the duration of the vm-sharing clone used by posix_spawn, all
signals are blocked in the parent process, including
implementation-internal signals. since __synccall cannot do anything
until successfully signaling all threads, the fact that signals are
blocked automatically yields the necessary safety.

aside from debloating and general simplification, part of the
motivation for removing the explicit lock is to simplify the
synchronization logic of __synccall in hopes that it can be made
async-signal-safe, which is needed to make setuid and setgid, which
depend on __synccall, conform to the standard. whether this will be
possible remains to be seen.

this caused sigsetjmp not to save the signal mask but instead to
clobber it with whatever happened to be in the sigjmb_buf prior to the
call.

this allows /etc/ld-musl-$(ARCH).path to contain one path per line,
which is much more convenient for users than the :-delimited format,
which was a source of repeated and unnecessary confusion. for
simplicity, \n is also accepted in environment variables, though it
should probably not be used there.

at the same time, issues with overly long paths invoking UB or getting
truncated have been fixed. such issues should not have arisen with the
environment (which is size-limited) but could have been generated by a
path file larger than 2**31 bytes in length.

the standard is clear that the old behavior is conforming: "In this
case, [EILSEQ] shall be stored in errno and the conversion state is
undefined."

however, the specification of mbrtowc has one peculiarity when the
source argument is a null pointer: in this case, it's required to
behave as mbrtowc(NULL, "", 1, ps). no motivation is provided for this
requirement, but the natural one that comes to mind is that the intent
is to reset the mbstate_t object. for stateful encodings, such
behavior is actually specified: "If the corresponding wide character
is the null wide character, the resulting state described shall be the
initial conversion state." but in the case of UTF-8 where the
mbstate_t object contains a partially-decoded character rather than a
shift state, a subsequent '\0' byte indicates that the previous
partial character is incomplete and thus an illegal sequence.

naturally, applications using their own mbstate_t object should clear
it themselves after an error, but the standard presently provides no
way to clear the builtin mbstate_t object used when the ps argument is
a null pointer. I suspect this issue may be addressed in the future by
specifying that a null source argument resets the state, as this seems
to have been the intent all along.

for what it's worth, this change also slightly reduces code size.

the interface contract for mbtowc admits a much faster implementation
than mbrtowc can achieve; wrapping mbrtowc with an extra call frame
only made the situation worse.

since the regex implementation uses mbtowc already, this change should
improve regex performance too. it may be possible to improve
performance in other places internally by switching from mbrtowc to
mbtowc.

this simple change, in my measurements, makes about a 7% performance
improvement. at first glance this change would seem like a
compiler-specific hack, since the modified code is not even used.
however, I suspect the reason is that I'm eliminating a second path
into the main body of the code, allowing the compiler more flexibility
to optimize the normal (hot) path into the main body. so even if it
weren't for the measurable (and quite notable) difference in
performance, I think the change makes sense.

SA and SB are used as the lowest and highest valid starter bytes, but
the value of SB was one-past the last valid starter. this caused
access past the end of the state table when the illegal byte '\xf5'
was encountered in a starter position. the error did not show up in
full-character decoding tests, since the bogus state read from just
past the table was unlikely to admit any continuation bytes as valid,
but would have shown up had we tested feeding '\xf5' to the
byte-at-a-time decoding in mbrtowc: it would cause the funtion to
wrongly return -2 rather than -1.

I may eventually go back and remove all references to SA and SB,
replacing them with the values; this would make the code more
transparent, I think. the original motivation for using macros was to
allow misguided users of the code to redefine them for the purpose of
enlarging the set of accepted sequences past the end of Unicode...

also include fallback code for broken kernels that don't support the
flags. as usual, the fallback has a race condition that can leak file
descriptors.

this is a bit ugly, and the motivation for supporting it is
questionable. however the main factors were:
1. it will be useful to have this for certain internal purposes
anyway -- things like syslog.
2. applications can just save argv[0] in main, but it's hard to fix
non-portable library code that's depending on being able to get the
invocation name without the main application's help.

GNU used several extensions that were incompatible with C99 and POSIX,
so they used alternate names for the standard functions.

The result is that we need these to run standards-conformant programs
that were linked with glibc.

supports ipv4 and ipv6, but not the "extended" usage where
usage statistics and other info are assigned to ifa_data members
of duplicate entries with AF_PACKET family.

since shadow does not yet support enumeration (getspent), the
corresponding FILE-based get and put versions are also subbed out for
now. this is partly out of laziness and partly because it's not clear
how they should work in the presence of TCB shadow files. the stubs
should make it possible to compile some software that expects them to
exist, but such software still may not work properly.

remove redundant headers and comments; this file is completely trivial
now. also, avoid temp var.

remove unneeded headers. this file is utterly trivial now and there's
no sense in having a comment to state that it's in the public domain.

there is no need to zero-fill an mbstate_t object in the caller;
mbsrtowcs will automatically treat a null pointer as the initial
state.

negative values of wchar_t need to be treated in the non-ASCII case so
that they can properly generate EILSEQ rather than getting truncated
to 8bit values and stored in the output.

these changes fix at least two bugs:
- misaligned access to the input as uint32_t for vectorized ASCII test
- incorrect src pointer after stopping on EILSEQ

in addition, the text of the standard makes it unclear whether the
mbstate_t object is to be modified when the destination pointer is
null; previously it was cleared either way; now, it's only cleared
when the destination is non-null. this change may need revisiting, but
it should not affect most applications, since calling mbsrtowcs with
non-zero state can only happen when the head of the string was already
processed with mbrtowc.

finally, these changes shave about 20% size off the function and seem
to improve performance by 1-5%.

this function is mainly (purely?) for obtaining stack address
information, but we also provide the detach state since it's easy to
do anyway.

the issue at hand is that many syscalls require as an argument the
kernel-ABI size of sigset_t, intended to allow the kernel to switch to
a larger sigset_t in the future. previously, each arch was defining
this size in syscall_arch.h, which was redundant with the definition
of _NSIG in bits/signal.h. as it's used in some not-quite-portable
application code as well, _NSIG is much more likely to be recognized
and understood immediately by someone reading the code, and it's also
shorter and less cluttered.

note that _NSIG is actually 65/129, not 64/128, but the division takes
care of throwing away the off-by-one part.

I'm not entirely happy with the amount of ugliness here, but since
F_DUPFD_CLOEXEC is used elsewhere in code that's expected to work on
old kernels (popen), it seems necessary. reportedly even some modern
kernels went back and broke F_DUPFD_CLOEXEC (making it behave like
plain F_DUPFD), so it might be necessary to add some additional fixup
code later to deal with that issue too.

SYS_pipe is not usable directly in general, since mips has a very
broken calling convention for the pipe syscall. instead, just call the
function, so that the mips-specific ugliness is isolated in
mips/pipe.s and not copied elsewhere.

1. as reported by William Haddon, the value returned by snprintf was
wrongly used as a length passed to sendto, despite it possibly
exceeding the buffer length. this could lead to invalid reads and
leaking additional data to syslog.

2. openlog was storing a pointer to the ident string passed by the
caller, rather than copying it. this bug is shared with (and even
documented in) other implementations like glibc, but such behavior
does not seem to meet the requirements of the standard.

3. extremely long ident provided to openlog, or corrupt ident due to
the above issue, could possibly have resulted in buffer overflows.
despite having the potential for smashing the stack, i believe the
impact is low since ident points to a short string literal in typical
application usage (and per the above bug, other usages will break
horribly on other implementations).

4. when used with LOG_NDELAY, openlog was not connecting the
newly-opened socket; sendto was being used instead. this defeated the
main purpose of LOG_NDELAY: preparing for chroot.

5. the default facility was not being used at all, so all messages
without an explicit facility passed to syslog were getting logged at
the kernel facility.

6. setlogmask was not thread-safe; no synchronization was performed
updating the mask. the fix uses atomics rather than locking to avoid
introducing a lock in the fast path for messages whose priority is not
in the mask.

7. in some code paths, the syslog lock was being unlocked twice; this
could result in releasing a lock that was actually held by a different
thread.

some additional enhancements to syslog such as a default identifier
based on argv[0] or similar may still be desired; at this time, only
the above-listed bugs have been fixed.

wctype_t was incorrectly "int" rather than "long" on x86_64. not only
is this an ABI incompatibility; it's also a major design flaw if we
ever wanted wctype_t to be implemented as a pointer, which would be
necessary if locales support custom character classes, since int is
too small to store a converted pointer. this commit fixes wctype_t to
be unsigned long on all archs, matching the LSB ABI; this change does
not matter for C code, but for C++ it affects mangling.

the same issue applied to wctrans_t. glibc/LSB defines this type as
const __int32_t *, but since no such definition is visible, I've just
expanded the definition, int, everywhere.

it would be nice if these types (which don't vary by arch) could be in
wctype.h, but the OB XSI requirement in POSIX that wchar.h expose some
types and functions from wctype.h precludes doing so. glibc works
around this with some hideous hacks, but trying to duplicate that
would go against the intent of musl's headers.