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

[Nick Rosbrook <rosbrookn@gmail.com>]

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)
+		}
+	}()
+
+	go func() {
+		var (
+			ev  Event
+			evc *C.libxl_event
+		)
+
+		for {
+			select {
+			case <-c.Done():
+				return
+			default:
+				// Go on and check for an event...
+			}
+
+			ret := C.libxl_event_wait(ctx.ctx, &evc, C.LIBXL_EVENTMASK_ALL, nil, nil)
+			if ret != 0 {
+				ctx.logw("unexpected error waiting for event: %s", Error(ret))
+				continue
+			}
+
+			// Try to convert the event to Go, and then free the
+			// C.libxl_event no matter what.
+			err := ev.fromC(evc)
+			C.libxl_event_free(ctx.ctx, evc)
+			if err != nil {
+				ctx.logw("error converting event from C: %v", err)
+				continue
+			}
+
+			ctx.logd("received domain death event (domid=%v, type=%v)", ev.Domid, ev.Type)
+
+			// Write the event to the channel
+			select {
+			case ec <- ev:
+			case <-c.Done():
+				return
+			}
+		}
+	}()
+
+	return nil
+}
+
 // DomainDestroy destroys a domain given a domid.
 func (ctx *Context) DomainDestroy(domid Domid) error {
 	ret := C.libxl_domain_destroy(ctx.ctx, C.uint32_t(domid), nil)
-- 
2.17.1