Introduce --strace flag for system call tracing

This is similar to the --ftrace (c function call trace) flag, except
it's less noisy since it only logs system calls to stderr. Having this
flag is valuable because (1) system call tracing tells us a lot about
the behavior of complex programs and (2) it's usually very hard to get
system call tracing on various operating systems, e.g. strace, ktrace,
dtruss, truss, nttrace, etc. Especially on Apple platforms where even
with the special boot trick, debuggers still aren't guaranteed to work.

    make -j8 o//examples
    o//examples/hello.com --strace

This is enabled by default in MODE=, MODE=opt, and MODE=dbg. In MODE=dbg
extra information will be printed.

    make -j8 MODE=dbg o/dbg/examples
    o/dbg/examples/hello.com --strace |& less

This change also changes:

- Rename IsText() → _istext()
- Rename IsUtf8() → _isutf8()
- Fix madvise() on Windows NT
- Fix empty string case of inet_ntop()
- vfork() wrapper now saves and restores errno
- Update xsigaction() to yoink syscall support

libc/sock/sockdebug.c

@@ -0,0 +1,114 @@
+/*-*- 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 2021 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/calls/struct/sockaddr6.h"
+#include "libc/errno.h"
+#include "libc/fmt/itoa.h"
+#include "libc/macros.internal.h"
+#include "libc/sock/sock.h"
+#include "libc/sock/sockdebug.h"
+#include "libc/str/str.h"
+#include "libc/sysv/consts/af.h"
+#include "libc/sysv/consts/ipproto.h"
+#include "libc/sysv/consts/sock.h"
+
+const char *__describe_socket_family(int family) {
+  static char buf[12];
+  if (family == AF_UNIX) return "AF_UNIX";
+  if (family == AF_INET) return "AF_INET";
+  if (family == AF_INET6) return "AF_INET6";
+  FormatInt32(buf, family);
+  return buf;
+}
+
+const char *__describe_socket_type(int type) {
+  int x;
+  char *p;
+  static char buf[12 + 1 + 12 + 1 + 13 + 1];
+  p = buf;
+  x = type & ~(SOCK_CLOEXEC | SOCK_NONBLOCK);
+  if (x == SOCK_STREAM) {
+    p = stpcpy(p, "SOCK_STREAM");
+  } else if (x == SOCK_DGRAM) {
+    p = stpcpy(p, "SOCK_DGRAM");
+  } else if (x == SOCK_RAW) {
+    p = stpcpy(p, "SOCK_RAW");
+  } else {
+    p = FormatInt32(p, x);
+  }
+  if (type & SOCK_CLOEXEC) p = stpcpy(p, "|SOCK_CLOEXEC");
+  if (type & SOCK_NONBLOCK) p = stpcpy(p, "|SOCK_NONBLOCK");
+  return buf;
+}
+
+const char *__describe_socket_protocol(int family) {
+  static char buf[12];
+  if (family == IPPROTO_IP) return "IPPROTO_IP";
+  if (family == IPPROTO_ICMP) return "IPPROTO_ICMP";
+  if (family == IPPROTO_TCP) return "IPPROTO_TCP";
+  if (family == IPPROTO_UDP) return "IPPROTO_UDP";
+  if (family == IPPROTO_RAW) return "IPPROTO_RAW";
+  if (family == IPPROTO_IPV6) return "IPPROTO_IPv6";
+  FormatInt32(buf, family);
+  return buf;
+}
+
+const char *__describe_sockaddr(const struct sockaddr *sa, size_t sasize) {
+  int e;
+  size_t n;
+  uint16_t port;
+  char *p, ip[72];
+  static char buf[128];
+  e = errno;
+  stpcpy(buf, "NULL");
+  if (sa && sasize >= sizeof(sa->sa_family)) {
+    stpcpy(buf, __describe_socket_family(sa->sa_family));
+    if (sa->sa_family == AF_INET && sasize >= sizeof(struct sockaddr_in)) {
+      const struct sockaddr_in *in;
+      in = (const struct sockaddr_in *)sa;
+      if (inet_ntop(AF_INET, &in->sin_addr, ip, sizeof(ip))) {
+        p = buf;
+        p = stpcpy(p, ip);
+        *p++ = ':';
+        p = FormatUint32(p, in->sin_port);
+      }
+    } else if (sa->sa_family == AF_INET6 &&
+               sasize >= sizeof(struct sockaddr_in6)) {
+      const struct sockaddr_in6 *in6;
+      in6 = (const struct sockaddr_in6 *)sa;
+      if (inet_ntop(AF_INET6, &in6->sin6_addr, ip, sizeof(ip))) {
+        p = buf;
+        *p++ = '[';
+        p = stpcpy(p, ip);
+        *p++ = ']';
+        *p++ = ':';
+        p = FormatUint32(p, in6->sin6_port);
+      }
+    } else if (sa->sa_family == AF_UNIX &&
+               sasize >= sizeof(struct sockaddr_un)) {
+      const struct sockaddr_un *unix;
+      unix = (const struct sockaddr_un *)sa;
+      n = strnlen(unix->sun_path, sizeof(unix->sun_path));
+      n = MIN(n, sizeof(buf) - 1);
+      memcpy(buf, unix->sun_path, n);
+      buf[n] = 0;
+    }
+  }
+  errno = e;
+  return buf;
+}