Merge pull request #143 from jkcfg/2019-03-05-deterministic-promise-r…

…esolution

Deterministic promise resolution

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

tests/test-deterministic-promises.js

@@ -0,0 +1,9 @@
+import std from '@jkcfg/std';
+
+const resolvedOrder = [];
+const promises = [];
+for (let i = 0; i < 100; i += 1) {
+  promises.push(std.read('success.json').then(() => resolvedOrder.push(i)));
+}
+
+Promise.all(promises).then(() => std.log(resolvedOrder.join(' ')));

tests/test-deterministic-promises.js.expected

@@ -0,0 +1 @@
+0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99