Windows: use GetSystemTimePreciseAsFileTime if available

PostgreSQL on Windows 8 or Windows Server 2012 will now
get high-resolution timestamps by dynamically loading the
GetSystemTimePreciseAsFileTime function. It'll fall back to
to GetSystemTimeAsFileTime if the higher precision variant
isn't found, so the same binaries without problems on older
Windows releases.

No attempt is made to detect the Windows version.  Only the
presence or absence of the desired function is considered.

Craig Ringer

src/backend/main/main.c

@@ -260,6 +260,12 @@ startup_hacks(const char *progname)
 
 		/* In case of general protection fault, don't show GUI popup box */
 		SetErrorMode(SEM_FAILCRITICALERRORS | SEM_NOGPFAULTERRORBOX);
+
+#ifndef HAVE_GETTIMEOFDAY
+		/* Figure out which syscall to use to capture timestamp information */
+		init_win32_gettimeofday();
+#endif
+
 	}
 #endif   /* WIN32 */
 

src/include/port.h

@@ -328,6 +328,8 @@ extern FILE *pgwin32_popen(const char *command, const char *type);
 #ifndef HAVE_GETTIMEOFDAY
 /* Last parameter not used */
 extern int	gettimeofday(struct timeval * tp, struct timezone * tzp);
+/* On windows we need to call some backend start setup for accurate timing */
+extern void init_win32_gettimeofday(void);
 #endif
 #else							/* !WIN32 */
 

src/port/gettimeofday.c

@@ -41,10 +41,59 @@ static const unsigned __int64 epoch = UINT64CONST(116444736000000000);
 #define FILETIME_UNITS_PER_SEC	10000000L
 #define FILETIME_UNITS_PER_USEC	10
 
+/*
+ * Both GetSystemTimeAsFileTime and GetSystemTimePreciseAsFileTime share a
+ * signature, so we can just store a pointer to whichever we find. This
+ * is the pointer's type.
+ */
+typedef VOID (WINAPI *PgGetSystemTimeFn)(LPFILETIME);
+
+/* Storage for the function we pick at runtime */
+static PgGetSystemTimeFn pg_get_system_time = NULL;
+
+/*
+ * During backend startup, determine if GetSystemTimePreciseAsFileTime is
+ * available and use it; if not, fall back to GetSystemTimeAsFileTime.
+ */
+void
+init_win32_gettimeofday(void)
+{
+	/*
+	 * Because it's guaranteed that kernel32.dll will be linked into our
+	 * address space already, we don't need to LoadLibrary it and worry about
+	 * closing it afterwards, so we're not using Pg's dlopen/dlsym() wrapper.
+	 *
+	 * We'll just look up the address of GetSystemTimePreciseAsFileTime if
+	 * present.
+	 *
+	 * While we could look up the Windows version and skip this on Windows
+	 * versions below Windows 8 / Windows Server 2012 there isn't much point,
+	 * and determining the windows version is its self somewhat Windows version
+	 * and development SDK specific...
+	 */
+	pg_get_system_time = (PgGetSystemTimeFn) GetProcAddress(
+			GetModuleHandle(TEXT("kernel32.dll")),
+				"GetSystemTimePreciseAsFileTime");
+	if (pg_get_system_time == NULL)
+	{
+		/*
+		 * The expected error from GetLastError() is ERROR_PROC_NOT_FOUND, if
+		 * the function isn't present. No other error should occur.
+		 *
+		 * It's too early in startup to elog(...) if we get some unexpected
+		 * error, and not serious enough to warrant a fprintf to stderr about
+		 * it or save the error and report it later. So silently fall back to
+		 * GetSystemTimeAsFileTime irrespective of why the failure occurred.
+		 */
+		pg_get_system_time = &GetSystemTimeAsFileTime;
+	}
+
+}
+
 /*
  * timezone information is stored outside the kernel so tzp isn't used anymore.
  *
- * Note: this function is not for Win32 high precision timing purpose. See
+ * Note: this function is not for Win32 high precision timing purposes. See
  * elapsed_time().
  */
 int
@@ -53,7 +102,7 @@ gettimeofday(struct timeval * tp, struct timezone * tzp)
 	FILETIME	file_time;
 	ULARGE_INTEGER ularge;
 
-	GetSystemTimeAsFileTime(&file_time);
+	(*pg_get_system_time)(&file_time);
 	ularge.LowPart = file_time.dwLowDateTime;
 	ularge.HighPart = file_time.dwHighDateTime;