Re: [RESEND PATCH 12/12] golang/xenlight: add NotifyDomainDeath method to Context

[George Dunlap <George.Dunlap@citrix.com>]

> On Jun 18, 2021, at 7:28 PM, George Dunlap <george.dunlap@citrix.com> wrote:
> 
> 
> 
>> On May 24, 2021, at 9:36 PM, Nick Rosbrook <rosbrookn@gmail.com> wrote:
>> 
>> Add a helper function to wait for domain death events, and then write
>> the events to a provided channel. This handles the enabling/disabling of
>> the event type, freeing the event, and converting it to a Go type. The
>> caller can then handle the event however they need to. This function
>> will run until a provided context.Context is cancelled.
>> 
>> NotifyDomainDeath spawns two goroutines that return when the
>> context.Context is done. The first will make sure that the domain death
>> event is disabled, and that the corresponding event queue is cleared.
>> The second calls libxl_event_wait, and writes the event to the provided
>> channel.
>> 
>> With this, callers should be able to manage a full domain life cycle.
>> Add to the comment of DomainCreateNew so that package uses know they
>> should use this method in conjunction with DomainCreateNew.
>> 
>> Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
>> ---
>> tools/golang/xenlight/xenlight.go | 83 ++++++++++++++++++++++++++++++-
>> 1 file changed, 82 insertions(+), 1 deletion(-)
>> 
>> diff --git a/tools/golang/xenlight/xenlight.go b/tools/golang/xenlight/xenlight.go
>> index 6fb22665cc..8406883433 100644
>> --- a/tools/golang/xenlight/xenlight.go
>> +++ b/tools/golang/xenlight/xenlight.go
>> @@ -53,6 +53,7 @@ import "C"
>> */
>> 
>> import (
>> +	"context"
>> 	"fmt"
>> 	"os"
>> 	"os/signal"
>> @@ -1340,7 +1341,9 @@ func (ctx *Context) DeviceUsbdevRemove(domid Domid, usbdev *DeviceUsbdev) error
>> 	return nil
>> }
>> 
>> -// DomainCreateNew creates a new domain.
>> +// DomainCreateNew creates a new domain. Callers of DomainCreateNew are
>> +// responsible for handling the death of the resulting domain. This should be
>> +// done using NotifyDomainDeath.
>> func (ctx *Context) DomainCreateNew(config *DomainConfig) (Domid, error) {
>> 	var cdomid C.uint32_t
>> 	var cconfig C.libxl_domain_config
>> @@ -1358,6 +1361,84 @@ func (ctx *Context) DomainCreateNew(config *DomainConfig) (Domid, error) {
>> 	return Domid(cdomid), nil
>> }
>> 
>> +// NotifyDomainDeath registers an event handler for domain death events for a
>> +// given domnid, and writes events received to ec. NotifyDomainDeath returns an
>> +// error if it cannot register the event handler, but other errors encountered
>> +// are just logged. The goroutine spawned by calling NotifyDomainDeath runs
>> +// until the provided context.Context's Done channel is closed.
>> +func (ctx *Context) NotifyDomainDeath(c context.Context, domid Domid, ec chan<- Event) error {
>> +	var deathw *C.libxl_evgen_domain_death
>> +
>> +	ret := C.libxl_evenable_domain_death(ctx.ctx, C.uint32_t(domid), 0, &deathw)
>> +	if ret != 0 {
>> +		return Error(ret)
>> +	}
>> +
>> +	// Spawn a goroutine that is responsible for cleaning up when the
>> +	// passed context.Context's Done channel is closed.
>> +	go func() {
>> +		<-c.Done()
>> +
>> +		ctx.logd("cleaning up domain death event handler for domain %d", domid)
>> +
>> +		// Disable the event generation.
>> +		C.libxl_evdisable_domain_death(ctx.ctx, deathw)
>> +
>> +		// Make sure any events that were generated get cleaned up so they
>> +		// do not linger in the libxl event queue.
>> +		var evc *C.libxl_event
>> +		for {
>> +			ret := C.libxl_event_check(ctx.ctx, &evc, C.LIBXL_EVENTMASK_ALL, nil, nil)
>> +			if ret != 0 {
>> +				return
>> +			}
>> +			C.libxl_event_free(ctx.ctx, evc)
> 
> I have to admit, I don’t really understand how the libxl event stuff is supposed to work.  But it looks like this will drain all events of any type, for any domain, associated with this context?
> 
> So if you had two domains, and called NotifyDomainDeath() on both with different contexts, and you closed the one context, you might miss events from the other context?
> 
> Or, suppose you did this:
> * ctx.NotifyDomainDeath(ctx1, dom1, ec1)
> * ctx.NotifyDiskEject(ctx2, dom1, ec2)
> * ctx1CancelFunc()
> 
> Wouldn’t there be a risk that the disk eject message would get lost?
> 
> Ian, any suggestions for the right way to use these functions in this scenario?

It looks like one option would be to add a “predicate” function filter, to filter by type and domid.

It looks like the other option would be to try to use libxl_event_register_callbacks().  We could have the C callback pass all the events to a goroutine which would act as a dispatcher.

 -George