std: Serialize resolution of deferred functions

We want the js author to be able to rely on deterministic order of promise
resolution to avoid surprises and generate non-determinitic configuration.

We do this by making the order of promise resolution be the same as promise
resolution, serializing the deferred goroutines before they re-enter v8 to
resolve the promises.

Fixes: #110

pkg/deferred/deferred.go

@@ -18,9 +18,17 @@ import (
 )
 
 var (
-	globalDeferreds = &deferreds{}
+	globalDeferreds = newDeferreds()
 )
 
+func newDeferreds() *deferreds {
+	d := &deferreds{
+		serial: 1,
+	}
+	d.serialCond = sync.NewCond(&d.serialMu)
+	return d
+}
+
 // Register schedules an action to be performed later, with the result
 // sent to `resolver`, using the global deferred scheduler.
 func Register(p performFunc, r resolver) Serial {
@@ -37,13 +45,38 @@ func Wait() {
 // JavaScript.
 type Serial uint64
 
+// To enforce determinism, we resolve deferred in the same order they are
+// created. This is done through resolvedSerial that stores what was the last
+// deferred resolved and we use a sync.Cond to handle synchronization between
+// goroutines servicing the deferred.
 type deferreds struct {
-	serialMu sync.Mutex
-	serial   Serial
+	serialMu       sync.Mutex
+	serial         Serial
+	serialCond     *sync.Cond
+	resolvedSerial Serial
 
 	outstanding sync.WaitGroup
 }
 
+func (d *deferreds) waitForSerial(s Serial) {
+	d.serialMu.Lock()
+	defer d.serialMu.Unlock()
+
+	for {
+		if d.resolvedSerial == s {
+			return
+		}
+		d.serialCond.Wait()
+	}
+}
+
+func (d *deferreds) serialResolved(s Serial) {
+	d.serialMu.Lock()
+	d.resolvedSerial = s
+	d.serialMu.Unlock()
+	d.serialCond.Broadcast()
+}
+
 // responder is the interface for a deferred request to use to send
 // its response.
 type resolver interface {
@@ -63,8 +96,16 @@ func (d *deferreds) Register(perform performFunc, r resolver) Serial {
 	d.serialMu.Unlock()
 	d.outstanding.Add(1)
 	go func(s Serial) {
-		defer d.outstanding.Done()
+		defer func() {
+			d.serialResolved(s)
+			d.outstanding.Done()
+		}()
+
 		b, err := perform()
+
+		// Wait for the serial-1 goroutine to be resolved.
+		d.waitForSerial(s - 1)
+
 		if err != nil {
 			r.Error(s, err)
 			return