Improve cancellations, randomness, and time

- Exhaustively document cancellation points
- Rename SIGCANCEL to SIGTHR just like BSDs
- Further improve POSIX thread cancellations
- Ensure asynchronous cancellations work correctly
- Elevate the quality of getrandom() and getentropy()
- Make futexes cancel correctly on OpenBSD 6.x and 7.x
- Add reboot.com and shutdown.com to examples directory
- Remove underscore prefix from awesome timespec_*() APIs
- Create assertions that help verify our cancellation points
- Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)

test/libc/stdio/getentropy_test.c

@@ -0,0 +1,95 @@
+/*-*- 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/atomic.h"
+#include "libc/calls/calls.h"
+#include "libc/calls/struct/sigaction.h"
+#include "libc/calls/struct/sigset.h"
+#include "libc/dce.h"
+#include "libc/errno.h"
+#include "libc/mem/gc.h"
+#include "libc/mem/mem.h"
+#include "libc/runtime/runtime.h"
+#include "libc/stdio/rand.h"
+#include "libc/stdio/stdio.h"
+#include "libc/str/tab.internal.h"
+#include "libc/sysv/consts/sig.h"
+#include "libc/testlib/testlib.h"
+#include "libc/thread/thread.h"
+
+// TODO(jart): Why can EINTR happen on Windows?
+
+atomic_int done;
+atomic_int ready;
+pthread_t parent;
+atomic_int gotsome;
+
+void OnSig(int sig) {
+  ++gotsome;
+}
+
+void *TortureWorker(void *arg) {
+  sigset_t ss;
+  sigfillset(&ss);
+  ASSERT_SYS(0, 0, sigprocmask(SIG_SETMASK, &ss, 0));
+  ready = true;
+  while (!done) {
+    if (!IsWindows()) pthread_kill(parent, SIGUSR1);
+    usleep(3);
+    if (!IsWindows()) pthread_kill(parent, SIGUSR2);
+    usleep(3);
+  }
+  return 0;
+}
+
+TEST(getentropy, test) {
+  pthread_t child;
+  double e, w = 7.7;
+  struct sigaction sa;
+  int i, j, k, n = 999;
+  char *buf = _gc(calloc(1, n));
+  sa.sa_flags = 0;
+  sa.sa_handler = OnSig;
+  sigemptyset(&sa.sa_mask);
+  ASSERT_SYS(0, 0, sigaction(SIGUSR1, &sa, 0));
+  ASSERT_SYS(0, 0, sigaction(SIGUSR2, &sa, 0));
+  parent = pthread_self();
+  ASSERT_EQ(0, pthread_create(&child, 0, TortureWorker, 0));
+  while (!ready) pthread_yield();
+  for (k = 0; k < 200; ++k) {
+    ASSERT_SYS(0, 0, getrandom(0, 0, 0));
+    ASSERT_SYS(0, n, getrandom(buf, n, 0));
+    ASSERT_SYS(EFAULT, -1, getrandom(0, n, 0));
+    ASSERT_SYS(EINVAL, -1, getrandom(buf, n, -1));
+    if ((e = MeasureEntropy(buf, n)) < w) {
+      fprintf(stderr, "error: entropy suspect! got %g but want >=%g\n", e, w);
+      for (i = 0; i < n;) {
+        if (!(i % 16)) fprintf(stderr, "%6x ", i);
+        fprintf(stderr, "%lc", kCp437[buf[i] & 255]);
+        if (!(++i % 16)) fprintf(stderr, "\n");
+      }
+      fprintf(stderr, "\n");
+      done = true;
+      pthread_join(child, 0);
+      exit(1);
+    }
+  }
+  done = true;
+  ASSERT_EQ(0, pthread_join(child, 0));
+  if (!IsWindows()) ASSERT_GT(gotsome, 0);
+}