Merge pull request digitalocean#158 from digitalocean/wait-for-signal-block

qemu/block: fixes deadlock in waitForSignal

Author: benlemasurier

hypervisor/example_test.go

@@ -23,10 +23,6 @@ import (
 	"github.com/digitalocean/go-qemu/qmp/qmptest"
 )
 
-// ExampleStub exists to prevent godoc from treating this as a "whole-file"
-// example, and showing the code for the unexported types.
-func ExampleStub() {}
-
 func ExampleNew() {
 	// Create a hypervisor.Hypervisor using an in-memory hypervisor.Driver,
 	// that returns a fixed list of three domains.

internal/qmp-gen/main.go

@@ -22,7 +22,7 @@ import (
 	"io/ioutil"
 )
 
-const specURL = `https://raw.githubusercontent.com/qemu/qemu/master/qapi-schema.json`
+const specURL = `https://raw.githubusercontent.com/qemu/qemu/stable-2.11/qapi-schema.json`
 
 var (
 	inputSpec  = flag.String("input", specURL, "Input spec")

qemu/block.go

@@ -225,21 +225,25 @@ func (bd BlockDevice) Snapshot(d *Domain, overlay string) error {
 // waitForSignal opens a domain's QMP event stream and invokes an input
 // closure to send commands which would create results on that event stream.
 func waitForSignal(d *Domain, signal string, timeout time.Duration, fn func() error) error {
-	// "done" signal must be sent in both the error and non-error case, to
-	// avoid leaking goroutines.
+	// "done" signal must always be sent to avoid leaking goroutines.
 	events, done, err := d.Events()
 	if err != nil {
 		return err
 	}
+	defer func() { done <- struct{}{} }()
 
-	if err = fn(); err != nil {
-		done <- struct{}{}
+	// start listening for events prior to command execution. QMP events
+	// may emit before the command returns.
+	jobErr := make(chan error)
+	go func() {
+		jobErr <- waitForJob(events, signal, timeout)
+	}()
+
+	if err := fn(); err != nil {
 		return err
 	}
 
-	err = waitForJob(events, signal, timeout)
-	done <- struct{}{}
-	return err
+	return <-jobErr
 }
 
 // waitForJob monitors the domain's QMP event stream, waiting

qemu/example_test.go

@@ -23,10 +23,6 @@ import (
 	"github.com/digitalocean/go-qemu/qmp/qmptest"
 )
 
-// ExampleStub exists to prevent godoc from treating this as a "whole-file"
-// example, and showing the code for the unexported exampleMonitor type.
-func ExampleStub() {}
-
 // This example demonstrates how to use qemu.NewDomain with a qmp.Monitor to
 // perform actions on a Domain.
 //

qmp/raw/spec.txt

@@ -1,5 +1,5 @@
 
-##QEMU SPECIFICATION VERSION:  2.11.50
+##QEMU SPECIFICATION VERSION:  2.11.2
 
 
 # Whitelists to permit QAPI rule violations; think twice before you