event.go

// +build windows

package winlog

import (
	"bytes"
	"fmt"
	"syscall"
	"unsafe"
)

/* Get a handle to a render context which will render properties from the System element.
   Wraps EvtCreateRenderContext() with Flags = EvtRenderContextSystem. The resulting
   handle must be closed with CloseEventHandle. */
func GetSystemRenderContext() (SysRenderContext, error) {
	context, err := EvtCreateRenderContext(0, 0, EvtRenderContextSystem)
	if err != nil {
		return 0, err
	}
	return SysRenderContext(context), nil
}

/* Get a handle for a event log subscription on the given channel.
   `query` is an XPath expression to filter the events on the channel - "*" allows all events.
   The resulting handle must be closed with CloseEventHandle. */
func CreateListener(channel, query string, startpos EVT_SUBSCRIBE_FLAGS, watcher *LogEventCallbackWrapper) (ListenerHandle, error) {
	wideChan, err := syscall.UTF16PtrFromString(channel)
	if err != nil {
		return 0, err
	}
	wideQuery, err := syscall.UTF16PtrFromString(query)
	if err != nil {
		return 0, err
	}
	listenerHandle, err := EvtSubscribe(0, 0, wideChan, wideQuery, 0, uintptr(unsafe.Pointer(watcher)), uintptr(syscall.NewCallback(eventCallback)), uint32(startpos))
	if err != nil {
		return 0, err
	}
	return ListenerHandle(listenerHandle), nil
}

/* Get a handle for an event log subscription on the given channel. Will begin at the
   bookmarked event, or the closest possible event if the log has been truncated.
   `query` is an XPath expression to filter the events on the channel - "*" allows all events.
   The resulting handle must be closed with CloseEventHandle. */
func CreateListenerFromBookmark(channel, query string, watcher *LogEventCallbackWrapper, bookmarkHandle BookmarkHandle) (ListenerHandle, error) {
	wideChan, err := syscall.UTF16PtrFromString(channel)
	if err != nil {
		return 0, err
	}
	wideQuery, err := syscall.UTF16PtrFromString(query)
	if err != nil {
		return 0, err
	}
	listenerHandle, err := EvtSubscribe(0, 0, wideChan, wideQuery, syscall.Handle(bookmarkHandle), uintptr(unsafe.Pointer(watcher)), syscall.NewCallback(eventCallback), uint32(EvtSubscribeStartAfterBookmark))
	if err != nil {
		return 0, err
	}
	return ListenerHandle(listenerHandle), nil
}

/* Get the formatted string that represents this message. This method wraps EvtFormatMessage. */
func FormatMessage(eventPublisherHandle PublisherHandle, eventHandle EventHandle, format EVT_FORMAT_MESSAGE_FLAGS) (string, error) {
	var size uint32 = 0
	err := EvtFormatMessage(syscall.Handle(eventPublisherHandle), syscall.Handle(eventHandle), 0, 0, nil, uint32(format), 0, nil, &size)
	if err != nil {
		if errno, ok := err.(syscall.Errno); !ok || errno != 122 {
			// Check if the error is ERR_INSUFICIENT_BUFFER
			return "", err
		}
	}
	buf := make([]uint16, size)
	err = EvtFormatMessage(syscall.Handle(eventPublisherHandle), syscall.Handle(eventHandle), 0, 0, nil, uint32(format), uint32(len(buf)), &buf[0], &size)
	if err != nil {
		return "", err
	}
	return syscall.UTF16ToString(buf), nil
}

/* Get the formatted string for the last error which occurred. Wraps GetLastError and FormatMessage. */
func GetLastError() error {
	return syscall.GetLastError()
}

/* Render the system properties from the event and returns an array of properties.
   Properties can be accessed using RenderStringField, RenderIntField, RenderFileTimeField,
   or RenderUIntField depending on type. This buffer must be freed after use. */
func RenderEventValues(renderContext SysRenderContext, eventHandle EventHandle) (EvtVariant, error) {
	var bufferUsed uint32 = 0
	var propertyCount uint32 = 0
	err := EvtRender(syscall.Handle(renderContext), syscall.Handle(eventHandle), EvtRenderEventValues, 0, nil, &bufferUsed, &propertyCount)
	if bufferUsed == 0 {
		return nil, err
	}
	buffer := make([]byte, bufferUsed)
	bufSize := bufferUsed
	err = EvtRender(syscall.Handle(renderContext), syscall.Handle(eventHandle), EvtRenderEventValues, bufSize, (*uint16)(unsafe.Pointer(&buffer[0])), &bufferUsed, &propertyCount)
	if err != nil {
		return nil, err
	}
	return NewEvtVariant(buffer), nil
}

// Render the event as XML.
func RenderEventXML(eventHandle EventHandle) (string, error) {
	var bufferUsed, propertyCount uint32

	err := EvtRender(0, syscall.Handle(eventHandle), EvtRenderEventXml, 0, nil, &bufferUsed, &propertyCount)

	if bufferUsed == 0 {
		return "", err
	}

	buffer := make([]byte, bufferUsed)
	bufSize := bufferUsed

	err = EvtRender(0, syscall.Handle(eventHandle), EvtRenderEventXml, bufSize, (*uint16)(unsafe.Pointer(&buffer[0])), &bufferUsed, &propertyCount)
	if err != nil {
		return err.Error(), err
	}

	//Remove null bytes
	xml := bytes.Replace(buffer, []byte("\x00"), []byte{}, -1)

	return string(xml), nil
}

/* Get a handle that represents the publisher of the event, given the rendered event values. */
func GetEventPublisherHandle(renderedFields EvtVariant) (PublisherHandle, error) {
	publisher, err := renderedFields.String(EvtSystemProviderName)
	if err != nil {
		return 0, err
	}
	widePublisher, err := syscall.UTF16PtrFromString(publisher)
	if err != nil {
		return 0, err
	}
	handle, err := EvtOpenPublisherMetadata(0, widePublisher, nil, 0, 0)
	if err != nil {
		return 0, err
	}
	return PublisherHandle(handle), nil
}

/* Close an event handle. */
func CloseEventHandle(handle uint64) error {
	return EvtClose(syscall.Handle(handle))
}

/* Cancel pending actions on the event handle. */
func CancelEventHandle(handle uint64) error {
	err := EvtCancel(syscall.Handle(handle))
	if err != nil {
		return err
	}
	return nil
}

/* Get the first event in the log, for testing */
func getTestEventHandle() (EventHandle, error) {
	wideQuery, _ := syscall.UTF16PtrFromString("*")
	wideChannel, _ := syscall.UTF16PtrFromString("Application")
	handle, err := EvtQuery(0, wideChannel, wideQuery, EvtQueryChannelPath)
	if err != nil {
		return 0, err
	}

	var record syscall.Handle
	var recordsReturned uint32
	err = EvtNext(handle, 1, &record, 500, 0, &recordsReturned)
	if err != nil {
		EvtClose(handle)
		return 0, nil
	}
	EvtClose(handle)
	return EventHandle(record), nil
}

func eventCallback(Action uint32, Context unsafe.Pointer, handle syscall.Handle) uintptr {
	cbWrap := (*LogEventCallbackWrapper)(Context)
	if Action == 0 {
		cbWrap.callback.PublishError(fmt.Errorf("Event log callback got error: %v", GetLastError()))
	} else {
		cbWrap.callback.PublishEvent(EventHandle(handle), cbWrap.subscribedChannel)
	}
	return 0
}