Windows: Implement ReadMemInfo()

Signed-off-by: John Howard <jhoward@microsoft.com>

system/meminfo_unsupported.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build !linux,!windows
 
 package system
 

system/meminfo_windows.go

@@ -0,0 +1,44 @@
+package system
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var (
+	modkernel32 = syscall.NewLazyDLL("kernel32.dll")
+
+	procGlobalMemoryStatusEx = modkernel32.NewProc("GlobalMemoryStatusEx")
+)
+
+// https://msdn.microsoft.com/en-us/library/windows/desktop/aa366589(v=vs.85).aspx
+// https://msdn.microsoft.com/en-us/library/windows/desktop/aa366770(v=vs.85).aspx
+type memorystatusex struct {
+	dwLength                uint32
+	dwMemoryLoad            uint32
+	ullTotalPhys            uint64
+	ullAvailPhys            uint64
+	ullTotalPageFile        uint64
+	ullAvailPageFile        uint64
+	ullTotalVirtual         uint64
+	ullAvailVirtual         uint64
+	ullAvailExtendedVirtual uint64
+}
+
+// ReadMemInfo retrieves memory statistics of the host system and returns a
+//  MemInfo type.
+func ReadMemInfo() (*MemInfo, error) {
+	msi := &memorystatusex{
+		dwLength: 64,
+	}
+	r1, _, _ := procGlobalMemoryStatusEx.Call(uintptr(unsafe.Pointer(msi)))
+	if r1 == 0 {
+		return &MemInfo{}, nil
+	}
+	return &MemInfo{
+		MemTotal:  int64(msi.ullTotalPhys),
+		MemFree:   int64(msi.ullAvailPhys),
+		SwapTotal: int64(msi.ullTotalPageFile),
+		SwapFree:  int64(msi.ullAvailPageFile),
+	}, nil
+}