darwin: work around clock jumping back in time

It was reported that mach_absolute_time() can jump backward in time when
the machine is suspended. Use mach_continuous_time() when available to
work around that (macOS 10.12 and up.)

Fixes: libuv#2891
PR-URL: libuv#2894
Reviewed-By: Phil Willoughby <philwill@fb.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

src/unix/darwin.c

@@ -25,13 +25,18 @@
 #include <stdint.h>
 #include <errno.h>
 
+#include <dlfcn.h>
 #include <mach/mach.h>
 #include <mach/mach_time.h>
 #include <mach-o/dyld.h> /* _NSGetExecutablePath */
 #include <sys/resource.h>
 #include <sys/sysctl.h>
 #include <unistd.h>  /* sysconf */
 
+static uv_once_t once = UV_ONCE_INIT;
+static uint64_t (*time_func)(void);
+static mach_timebase_info_data_t timebase;
+
 
 int uv__platform_loop_init(uv_loop_t* loop) {
   loop->cf_state = NULL;
@@ -48,15 +53,19 @@ void uv__platform_loop_delete(uv_loop_t* loop) {
 }
 
 
-uint64_t uv__hrtime(uv_clocktype_t type) {
-  static mach_timebase_info_data_t info;
-
-  if ((ACCESS_ONCE(uint32_t, info.numer) == 0 ||
-       ACCESS_ONCE(uint32_t, info.denom) == 0) &&
-      mach_timebase_info(&info) != KERN_SUCCESS)
+static void uv__hrtime_init_once(void) {
+  if (KERN_SUCCESS != mach_timebase_info(&timebase))
     abort();
 
-  return mach_absolute_time() * info.numer / info.denom;
+  time_func = (uint64_t (*)(void)) dlsym(RTLD_DEFAULT, "mach_continuous_time");
+  if (time_func == NULL)
+    time_func = mach_absolute_time;
+}
+
+
+uint64_t uv__hrtime(uv_clocktype_t type) {
+  uv_once(&once, uv__hrtime_init_once);
+  return time_func() * timebase.numer / timebase.denom;
 }