callbacks.go

package libvirt

// Helpers functions to register a Go callback function to a C
// function. For a simple example, look at how SetErrorFunc works in
// error.go.
//
// - Create a struct that will contain at least the Go callback to
//   invoke (errorContext).
//
// - Create an exported Golang function whose job will be to retrieve
//   the context and execute the callback in it
//   (connErrCallback). Such a function should receive a callback ID
//   and will use it to retrive the context.
//
// - Create a CGO function similar to the above function but with the
//   appropriate signature to be registered as a callback in C code
//   (connErrCallback_cgo). Notably, it will have a void* argument
//   that should be cast to long to retrieve the callback ID. It
//   should be just a thin wrapper to transform the opaque argument to
//   a callback ID.
//
// - Create a CGO function which will be a wrapper around the C
//   function to register the callback (virConnSetErrorFunc_cgo). Its
//   only role is to transform a callback ID (long) to an opaque (void*)
//   and call the C function.
//
// - When setting up a callback (SetErrorFunc), register the struct from first step
//   with registerCallbackId and invoke the CGO function from the
//   previous step with the appropriate ID.
//
// - When unregistering the callback, don't forget to call freecallbackId.
//
// If you need to associate some additional data with the connection,
// look at saveConnectionData, getConnectionData and
// releaseConnectionData.

import "C"

import (
	"sync"
)

const firstGoCallbackId int = 100 // help catch some additional errors during test
var goCallbackLock sync.RWMutex
var goCallbacks = make(map[int]interface{})
var nextGoCallbackId int = firstGoCallbackId

//export freeCallbackId
func freeCallbackId(goCallbackId int) {
	goCallbackLock.Lock()
	delete(goCallbacks, goCallbackId)
	goCallbackLock.Unlock()
}

func getCallbackId(goCallbackId int) interface{} {
	goCallbackLock.RLock()
	ctx := goCallbacks[goCallbackId]
	goCallbackLock.RUnlock()
	if ctx == nil {
		// If this happens there must be a bug in libvirt
		panic("Callback arrived after freeCallbackId was called")
	}
	return ctx
}

func registerCallbackId(ctx interface{}) int {
	goCallbackLock.Lock()
	goCallBackId := nextGoCallbackId
	nextGoCallbackId++
	for goCallbacks[nextGoCallbackId] != nil {
		nextGoCallbackId++
	}
	goCallbacks[goCallBackId] = ctx
	goCallbackLock.Unlock()
	return goCallBackId
}