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

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Jun 18, 2021, at 7:28 PM, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
> 
> 
> 
>> On May 24, 2021, at 9:36 PM, Nick Rosbrook <rosbrookn@xxxxxxxxx> 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@xxxxxxxxxxxx>
>> ---
>> 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