Make it possible to log kprintf() to file

It's now possible to compile Emacs using cosmocc. However we need to
troubleshoot why it's event loop isn't working correctly at runtime.

libc/log/backtrace2.c

@@ -148,11 +148,11 @@ static int PrintBacktraceUsingAddr2line(int fd, const struct StackFrame *bp) {
                        strlen(" (discriminator ") - 1)) &&
           (p3 = memchr(p2, '\n', got - (p2 - p1)))) {
         if (p3 > p2 && p3[-1] == '\r') --p3;
-        sys_write(2, p1, p2 - p1);
+        _klog(p1, p2 - p1);
         got -= p3 - p1;
         p1 += p3 - p1;
       } else {
-        sys_write(2, p1, got);
+        _klog(p1, got);
         break;
       }
     }

libc/log/log.h

@@ -40,6 +40,7 @@ void _meminfo(int);                    /* shows malloc statistics &c. */
 void _memsummary(int);                 /* light version of same thing */
 bool IsTerminalInarticulate(void) nosideeffect;
 const char *commandvenv(const char *, const char *);
+int LogKprintfToFile(const char *);
 const char *GetAddr2linePath(void);
 const char *GetGdbPath(void);
 void ShowCrashReports(void);

libc/log/logkprintftofile.c

@@ -0,0 +1,57 @@
+/*-*- 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 2023 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/calls.h"
+#include "libc/calls/internal.h"
+#include "libc/dce.h"
+#include "libc/log/log.h"
+#include "libc/sysv/consts/f.h"
+#include "libc/sysv/consts/o.h"
+
+static int __klog_fd;
+extern _Hide long __klog_handle;
+
+/**
+ * Redirects kprintf(), `--strace`, etc. output to file.
+ *
+ * @param path is filename to append to; if null is specified then
+ *     this file logging facility will be disabled; when the empty
+ *     string is specified, then the default path shall be used
+ * @return 0 on success, or -1 w/ errno
+ */
+int LogKprintfToFile(const char *path) {
+  int fd, dd;
+  if (!path) {
+    if (__klog_fd) {
+      __klog_handle = 0;
+      close(__klog_fd);
+      __klog_fd = 0;
+    }
+    return 0;
+  }
+  if (!*path) path = "/tmp/kprintf.log";
+  fd = open(path, O_WRONLY | O_APPEND | O_CREAT, 0644);
+  if (fd == -1) return -1;
+  dd = fcntl(fd, F_DUPFD_CLOEXEC, 100);
+  close(fd);
+  if (dd == -1) return -1;
+  if (__klog_fd) close(__klog_fd);
+  __klog_fd = dd;
+  __klog_handle = IsWindows() ? g_fds.p[dd].handle : dd;
+  return 0;
+}

libc/log/oncrash_amd64.c

@@ -235,11 +235,11 @@ relegated void ShowCrashReport(int err, int sig, struct siginfo *si,
     p = ShowGeneralRegisters(p, ctx);
     p = ShowSseRegisters(p, ctx);
     *p++ = '\n';
-    write(2, buf, p - buf);
+    _klog(buf, p - buf);
     ShowFunctionCalls(ctx);
   } else {
     *p++ = '\n';
-    write(2, buf, p - buf);
+    _klog(buf, p - buf);
   }
   kprintf("\n");
   if (!IsWindows()) __print_maps();