Improve system calls

- Wrap clock_getres()
- Wrap sched_setscheduler()
- Make sleep() api conformant
- Polyfill sleep() using select()
- Improve clock_gettime() polyfill
- Make nanosleep() POSIX conformant
- Slightly improve some DNS functions
- Further strengthen pledge() sandboxing
- Improve rounding of timeval / timespec
- Allow layering of pledge() calls on Linux
- Polyfill sched_yield() using select() on XNU
- Delete more system constants we probably don't need

test/libc/calls/sched_setscheduler_test.c

@@ -0,0 +1,66 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2022 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/bits/midpoint.h"
+#include "libc/calls/calls.h"
+#include "libc/calls/struct/sched_param.h"
+#include "libc/dce.h"
+#include "libc/errno.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/runtime/runtime.h"
+#include "libc/sysv/consts/sched.h"
+#include "libc/testlib/testlib.h"
+
+#define DEFAULT_POLICY SCHED_OTHER
+
+void SetUp(void) {
+  if (IsXnu() || IsWindows() || IsOpenbsd() || IsWindows()) {
+    exit(0);
+  }
+  int oldpolicy = sched_getscheduler(0);
+  struct sched_param p = {sched_get_priority_min(DEFAULT_POLICY)};
+  EXPECT_SYS(0, oldpolicy, sched_setscheduler(0, DEFAULT_POLICY, &p));
+}
+
+bool CanTuneRealtimeSchedulers(void) {
+  int e = errno;
+  int policy = SCHED_FIFO;
+  struct sched_param p = {sched_get_priority_min(policy)};
+  if (sched_setscheduler(0, policy, &p) != -1) {
+    struct sched_param p = {sched_get_priority_min(DEFAULT_POLICY)};
+    EXPECT_SYS(0, policy, sched_setscheduler(0, DEFAULT_POLICY, &p));
+    return true;
+  } else if (errno == EPERM) {
+    errno = e;
+    return false;
+  } else {
+    abort();
+  }
+}
+
+TEST(sched_setscheduler, test) {
+  struct sched_param p = {sched_get_priority_min(SCHED_OTHER)};
+  EXPECT_SYS(0, DEFAULT_POLICY, sched_setscheduler(0, SCHED_OTHER, &p));
+}
+
+TEST(sched_setscheduler, testMidpoint) {
+  if (!CanTuneRealtimeSchedulers()) return;
+  struct sched_param p = {_midpoint(sched_get_priority_min(SCHED_FIFO),
+                                    sched_get_priority_max(SCHED_FIFO))};
+  EXPECT_SYS(0, DEFAULT_POLICY, sched_setscheduler(0, SCHED_FIFO, &p));
+}