Upstream repo.parallel fix

We encountered an issue on an app using 12.12.x where we needed the
payloads from `repo.parallel` when it completed. This commit upstreams
that change.

packages/microcosm/docs/api/microcosm.md

@@ -333,16 +333,18 @@ fork.push(getPeople)
 
 Create a new "group" action bound to the resolution of a list of
 actions. If all actions resolve or cancel, the group action will
-resolve. If any action is rejected, the group action fails:
++resolve. If any action is rejected, the group action fails. If all
+actions resolve, their respective payloads will be available in the
+`onDone` callback:
 
 ```javascript
 let group = repo.parallel([
   repo.push(actionOne),
   repo.push(actionTwo)
 ])
 
-group.onDone(function () {
-  console.log('hurrah!')
+group.onDone(function ([answer1, answer2]) {
+  console.log('hurrah!', answer1, answer2)
 })
 ```
 

packages/microcosm/src/action.js

@@ -189,23 +189,27 @@ class Action extends Emitter {
    */
   link(actions: Action[]): this {
     let outstanding = actions.length
+    let answers = []
 
-    const onResolve = () => {
-      if (outstanding <= 1) {
-        this.resolve()
-      } else {
-        outstanding -= 1
-      }
+    if (actions.length === 0) {
+      return this.resolve(answers)
     }
 
     actions.forEach(action => {
+      let onResolve = answer => {
+        answers[actions.indexOf(action)] = answer
+        outstanding -= 1
+
+        if (outstanding <= 0) {
+          this.resolve(answers)
+        }
+      }
+
       action.onDone(onResolve)
       action.onCancel(onResolve)
       action.onError(this.reject)
     })
 
-    onResolve()
-
     return this
   }
 

packages/microcosm/test/unit/action/link.test.js

@@ -0,0 +1,63 @@
+import { Microcosm } from 'microcosm'
+
+function delay(payload, n) {
+  return new Promise(resolve => setTimeout(() => resolve(payload), n))
+}
+
+const cancels = () => {
+  return action => action.cancel()
+}
+
+const rejects = () => {
+  return action => action.reject()
+}
+
+describe('Action link', function() {
+  it('remembers and returns the results of each child action', async function() {
+    const repo = new Microcosm()
+
+    const action = repo.parallel([
+      repo.push(() => delay(1, 20)),
+      repo.push(() => delay(2, 10)),
+      repo.push(() => delay(3, 15))
+    ])
+
+    let payload = await action
+
+    expect(payload).toEqual([1, 2, 3])
+  })
+
+  it('resolves empty', async function() {
+    const repo = new Microcosm()
+
+    const action = repo.parallel([])
+
+    let payload = await action
+
+    expect(payload).toEqual([])
+  })
+
+  it('handles cancelled actions by just returning undefined in that position', function() {
+    const repo = new Microcosm()
+
+    const action = repo.parallel([
+      repo.push(() => 1),
+      repo.push(cancels),
+      repo.push(() => 3)
+    ])
+
+    expect(action.payload).toEqual([1, undefined, 3])
+  })
+
+  it('rejects if any of the actions reject', function() {
+    const repo = new Microcosm()
+
+    const action = repo.parallel([
+      repo.push(rejects),
+      repo.push(() => 2),
+      repo.push(() => 3)
+    ])
+
+    expect(action.status).toEqual('reject')
+  })
+})