Clocksource: use a better clock if available.

clock_gettime() is a far better clock than gettimeofday().

Even better than clock_gettime() is that you can select either
CLOCK_MONOTONIC, or even CLOCK_MONOTONIC_RAW. These clocks offer
high precision time. And the _RAW variant will never roll back
due to NTP drift or daylight savings, or otherwise.

I've adjusted this code to select the right clock method auto-
matically based on what's available in the OS. This means that
if you're running a very old linux version, MacOS or other,
you will automatically get the best clocksource available.

I've tested all Linux clocksources by selectively compiling and
running a 10k+ timer test suite. In all cases I confirmed that
the 3 POSIX Linux clocksources worked properly, and were
selected properly.

I've modified the OS X compile path to use the high-res clock
source for all time functions, but I can't confirm it works or
that it compiles.

As for WIN32, I confirmed that the used clocksource is indeed
a Monotonic clocksource, so good news: that code section appears
to be exactly what it should be.

2 files changed, 35 insertions, 22 deletions

diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index e7c1b7dd5..7255852e0 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -1727,7 +1727,7 @@ Helper functions
 * `minetest.is_yes(arg)`
     * returns whether `arg` can be interpreted as yes
 * `minetest.get_us_time()`
-    * returns time with microsecond precision
+    * returns time with microsecond precision. May not return wall time.
 * `table.copy(table)`: returns a table
     * returns a deep copy of `table`
 
diff --git a/src/porting.h b/src/porting.h
index 5da32607c..5fe81a440 100644
--- a/src/porting.h
+++ b/src/porting.h
@@ -211,43 +211,56 @@ void initIrrlicht(irr::IrrlichtDevice * );
 	}
 
 #else // Posix
-
-	inline u32 getTimeS()
+	inline void _os_get_clock(struct timespec *ts)
 	{
+#if defined(__MACH__) && defined(__APPLE__)
+	// from http://stackoverflow.com/questions/5167269/clock-gettime-alternative-in-mac-os-x
+	// OS X does not have clock_gettime, use clock_get_time
+		clock_serv_t cclock;
+		mach_timespec_t mts;
+		host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
+		clock_get_time(cclock, &mts);
+		mach_port_deallocate(mach_task_self(), cclock);
+		ts.tv_sec = mts.tv_sec;
+		ts.tv_nsec = mts.tv_nsec;
+#elif defined(CLOCK_MONOTONIC_RAW)
+		clock_gettime(CLOCK_MONOTONIC_RAW, ts);
+#elif defined(_POSIX_MONOTONIC_CLOCK)
+		clock_gettime(CLOCK_MONOTONIC, ts);
+#else
 		struct timeval tv;
 		gettimeofday(&tv, NULL);
-		return tv.tv_sec;
+		TIMEVAL_TO_TIMESPEC(&tv, ts);
+#endif // defined(__MACH__) && defined(__APPLE__)
+	}
+
+	// Note: these clock functions do not return wall time, but
+	// generally a clock that starts at 0 when the process starts.
+	inline u32 getTimeS()
+	{
+		struct timespec ts;
+		_os_get_clock(&ts);
+		return ts.tv_sec;
 	}
 
 	inline u32 getTimeMs()
 	{
-		struct timeval tv;
-		gettimeofday(&tv, NULL);
-		return tv.tv_sec * 1000 + tv.tv_usec / 1000;
+		struct timespec ts;
+		_os_get_clock(&ts);
+		return ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
 	}
 
 	inline u32 getTimeUs()
 	{
-		struct timeval tv;
-		gettimeofday(&tv, NULL);
-		return tv.tv_sec * 1000000 + tv.tv_usec;
+		struct timespec ts;
+		_os_get_clock(&ts);
+		return ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
 	}
 
 	inline u32 getTimeNs()
 	{
 		struct timespec ts;
-		// from http://stackoverflow.com/questions/5167269/clock-gettime-alternative-in-mac-os-x
-#if defined(__MACH__) && defined(__APPLE__) // OS X does not have clock_gettime, use clock_get_time
-		clock_serv_t cclock;
-		mach_timespec_t mts;
-		host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
-		clock_get_time(cclock, &mts);
-		mach_port_deallocate(mach_task_self(), cclock);
-		ts.tv_sec = mts.tv_sec;
-		ts.tv_nsec = mts.tv_nsec;
-#else
-		clock_gettime(CLOCK_REALTIME, &ts);
-#endif
+		_os_get_clock(&ts);
 		return ts.tv_sec * 1000000000 + ts.tv_nsec;
 	}