this is necessary to avoid build errors if feature test macros are not
properly defined when including ucontext.h

this race is fundamentally due to linux's bogus requirement that
userspace, rather than kernelspace, fill in the siginfo structure. an
intervening signal handler that calls fork could cause both the parent
and child process to send signals claiming to be from the parent,
which could in turn have harmful effects depending on what the
recipient does with the signal. we simply block all signals for the
interval between getuid and sigqueue syscalls (much like what raise()
does already) to prevent the race and make the getuid/sigqueue pair
atomic.

this will be a non-issue if linux is fixed to validate the siginfo
structure or fill it in from kernelspace.

it's nicer for the function that doesn't use errno to be independent,
and have the other one call it. saves some time and avoids clobbering
errno.

setrlimit is supposed to be per-process, not per-thread, but again
linux gets it wrong. work around this in userspace. not only is it
needed for correctness; setxid also depends on the resource limits for
all threads being the same to avoid situations where temporarily
unlimiting the limit succeeds in some threads but fails in others.

previously, stdio used spinlocks, which would be unacceptable if we
ever add support for thread priorities, and which yielded
pathologically bad performance if an application attempted to use
flockfile on a key file as a major/primary locking mechanism.

i had held off on making this change for fear that it would hurt
performance in the non-threaded case, but actually support for
recursive locking had already inflicted that cost. by having the
internal locking functions store a flag indicating whether they need
to perform unlocking, rather than using the actual recursive lock
counter, i was able to combine the conditionals at unlock time,
eliminating any additional cost, and also avoid a nasty corner case
where a huge number of calls to ftrylockfile could cause deadlock
later at the point of internal locking.

this commit also fixes some issues with usage of pthread_self
conflicting with __attribute__((const)) which resulted in crashes with
some compiler versions/optimizations, mainly in flockfile prior to
pthread_create.

changing credentials in a multi-threaded program is extremely
difficult on linux because it requires synchronizing the change
between all threads, which have their own thread-local credentials on
the kernel side. this is further complicated by the fact that changing
the real uid can fail due to exceeding RLIMIT_NPROC, making it
possible that the syscall will succeed in some threads but fail in
others.

the old __rsyscall approach being replaced was robust in that it would
report failure if any one thread failed, but in this case, the program
would be left in an inconsistent state where individual threads might
have different uid. (this was not as bad as glibc, which would
sometimes even fail to report the failure entirely!)

the new approach being committed refuses to change real user id when
it cannot temporarily set the rlimit to infinity. this is completely
POSIX conformant since POSIX does not require an implementation to
allow real-user-id changes for non-privileged processes whatsoever.
still, setting the real uid can fail due to memory allocation in the
kernel, but this can only happen if there is not already a cached
object for the target user. thus, we forcibly serialize the syscalls
attempts, and fail the entire operation on the first failure. this
*should* lead to an all-or-nothing success/failure result, but it's
still fragile and highly dependent on kernel developers not breaking
things worse than they're already broken.

ideally linux will eventually add a CLONE_USERCRED flag that would
give POSIX conformant credential changes without any hacks from
userspace, and all of this code would become redundant and could be
removed ~10 years down the line when everyone has abandoned the old
broken kernels. i'm not holding my breath...

this helps some tiny programs be even more tiny, and barly increases
code size even if both are used.

thanks to mikachu

per POSIX:

The setenv() function shall fail if:

[EINVAL] The name argument is a null pointer, points to an empty
string, or points to a string containing an '=' character.

instead of creating temp dso objects on the stack and moving them to
the heap if dlopen/dlsym are used, use static objects to begin with,
and just donate them to malloc if we no longer need them.

these changes also make it so clock_gettime(CLOCK_REALTIME, &ts) works
even on pre-2.6 kernels, emulated via the gettimeofday syscall. there
is no cost for the fallback check, as it falls under the error case
that already must be checked for storing the error code in errno, but
which would normally be hidden inside __syscall_ret.

we cannot report failure after forking, so the idea is to ensure prior
to fork that fd 0,1,2 exist. this will prevent dup2 from possibly
hitting a resource limit and failing in the child process. fcntl
rather than dup2 is used prior to forking to avoid race conditions.

-1, not 0, indicates failure

some program was undefining AF_NETLINK and thereby breaking AF_ROUTE...

this resolves an issue reported by Vasiliy Kulikov

fread was calling f->read without checking that the file was in
reading mode. this could:
1. crash, if f->read was a null pointer
2. cause unwanted blocking on a terminal already at eof
3. allow reading on a write-only file

1. my interpretation of subject sequence definition was wrong. adjust
parser to conform to the standard.

2. some code for handling tail overflow case was missing (forgot to
finish writing it).

3. typo (= instead of ==) caused ERANGE to wrongly behave like EINVAL

stopping without letting the parser see a stop character prevented
getting a result. so treat all high chars as the null character and
pass them into the parser.

also eliminated ugly tmp var using compound literals.

this fixes a number of bugs in integer parsing due to lazy haphazard
wrapping, as well as some misinterpretations of the standard. the new
parser is able to work character-at-a-time or on whole strings, making
it easy to support the wide functions without unbounded space for
conversion. it will also be possible to update scanf to use the new
parser.

also support (and restrict to subsets) older chinese sets, and
explicitly refuse to convert to cjk (since there's no code for it yet)

these are useless legacy functions but some old software contains
cruft that expects them to exist...

this broke most uses of iconv in real-world programs, especially
glib's iconv wrappers.