move rsyscall out of pthread_create module

this is something of a tradeoff, as now set*id() functions, rather
than pthread_create, are what pull in the code overhead for dealing
with linux's refusal to implement proper POSIX thread-vs-process
semantics. my motivations are:

1. it's cleaner this way, especially cleaner to optimize out the
rsyscall locking overhead from pthread_create when it's not needed.
2. it's expected that only a tiny number of core system programs will
ever use set*id() functions, whereas many programs may want to use
threads, and making thread overhead tiny is an incentive for "light"
programs to try threads.

src/thread/__rsyscall.c

@@ -0,0 +1,113 @@
+#include "pthread_impl.h"
+
+/* "rsyscall" is a mechanism by which a thread can synchronously force all
+ * other threads to perform an arbitrary syscall. It is necessary to work
+ * around the non-conformant implementation of setuid() et al on Linux,
+ * which affect only the calling thread and not the whole process. This
+ * implementation performs some tricks with signal delivery to work around
+ * the fact that it does not keep any list of threads in userspace. */
+
+static struct {
+	volatile int lock, hold, blocks, cnt;
+	unsigned long arg[6];
+	int nr;
+	int err;
+	int init;
+} rs;
+
+static void rsyscall_handler(int sig, siginfo_t *si, void *ctx)
+{
+	struct pthread *self = __pthread_self();
+	long r;
+
+	if (!rs.hold || rs.cnt == libc.threads_minus_1) return;
+
+	/* Threads which have already decremented themselves from the
+	 * thread count must not increment rs.cnt or otherwise act. */
+	if (self->dead) {
+		sigfillset(&((ucontext_t *)ctx)->uc_sigmask);
+		return;
+	}
+
+	r = __syscall(rs.nr, rs.arg[0], rs.arg[1],
+		rs.arg[2], rs.arg[3], rs.arg[4], rs.arg[5]);
+	if (r < 0) rs.err=-r;
+
+	a_inc(&rs.cnt);
+	__wake(&rs.cnt, 1, 1);
+	while(rs.hold)
+		__wait(&rs.hold, 0, 1, 1);
+	a_dec(&rs.cnt);
+	if (!rs.cnt) __wake(&rs.cnt, 1, 1);
+}
+
+int __rsyscall(int nr, long a, long b, long c, long d, long e, long f)
+{
+	int i, ret;
+	sigset_t set = { 0 };
+	struct pthread *self;
+
+	if (!libc.threads_minus_1)
+		return syscall(nr, a, b, c, d, e, f);
+
+	self = __pthread_self();
+
+	LOCK(&rs.lock);
+	while ((i=rs.blocks))
+		__wait(&rs.blocks, 0, i, 1);
+
+	sigfillset(&set);
+	__libc_sigprocmask(SIG_BLOCK, &set, &set);
+
+	if (!rs.init) {
+		struct sigaction sa = {
+			.sa_sigaction = rsyscall_handler,
+			.sa_mask = set
+		};
+		sigfillset(&sa.sa_mask);
+		sa.sa_sigaction = rsyscall_handler;
+		__libc_sigaction(SIGSYSCALL, &sa, 0);
+	}
+
+	rs.nr = nr;
+	rs.arg[0] = a; rs.arg[1] = b;
+	rs.arg[2] = c; rs.arg[3] = d;
+	rs.arg[4] = d; rs.arg[5] = f;
+	rs.err = 0;
+	rs.cnt = 0;
+	rs.hold = 1;
+
+	/* Dispatch signals until all threads respond */
+	for (i=libc.threads_minus_1; i; i--)
+		sigqueue(self->pid, SIGSYSCALL, (union sigval){0});
+	while ((i=rs.cnt) < libc.threads_minus_1) {
+		sigqueue(self->pid, SIGSYSCALL, (union sigval){0});
+		__wait(&rs.cnt, 0, i, 1);
+	}
+
+	/* Handle any lingering signals with no-op */
+	__libc_sigprocmask(SIG_UNBLOCK, &set, &set);
+
+	/* Resume other threads' signal handlers and wait for them */
+	rs.hold = 0;
+	__wake(&rs.hold, -1, 0);
+	while((i=rs.cnt)) __wait(&rs.cnt, 0, i, 1);
+
+	if (rs.err) errno = rs.err, ret = -1;
+	else ret = syscall(nr, a, b, c, d, e, f);
+
+	UNLOCK(&rs.lock);
+	return ret;
+}
+
+void __rsyscall_lock()
+{
+	a_inc(&rs.blocks);
+	while (rs.lock) __wait(&rs.lock, 0, 1, 1);
+}
+
+void __rsyscall_unlock()
+{
+	a_dec(&rs.blocks);
+	if (rs.lock) __wake(&rs.blocks, 1, 1);
+}