Clients.get: block on reserved clients. (#1323)

To reflect the decision that we don't expose reserved clients (#1216),
this change introduces the client's discarded flag, and uses that flag and the
execution ready flag to resolve the promise when the client reaches one of those
states instead of capturing and exposing the reserved client.

Follow-up to 07e9487 (#1259).

docs/index.bs

@@ -222,6 +222,13 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
 
     A <dfn export id="dfn-service-worker-client" for="">service worker client</dfn> is an [=environment=].
 
+    A [=/service worker client=] has an associated <dfn export>discarded flag</dfn>. It is initially unset.
+
+   Each [=/service worker client=] has the following [=environment discarding steps=]:
+        1. Set |client|'s [=discarded flag=].
+
+    Note: Implementations can discard clients whose [=discarded flag=] is set.
+
     A [=/service worker client=] has an algorithm defined as the <dfn export for="service worker client">origin</dfn> that returns the [=/service worker client=]'s [=environment settings object/origin=] if the [=/service worker client=] is an [=environment settings object=], and the [=/service worker client=]'s <a>creation URL</a>'s [=url/origin=] otherwise.
 
     A <dfn export id="dfn-window-client">window client</dfn> is a [=/service worker client=] whose [=environment settings object/global object=] is a {{Window}} object.
@@ -1147,28 +1154,9 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
         1. Let |promise| be a new <a>promise</a>.
         1. Run these substeps <a>in parallel</a>:
             1. For each [=/service worker client=] |client| whose [=service worker client/origin=] is the <a lt="same origin">same</a> as the associated [=ServiceWorkerGlobalScope/service worker=]'s [=environment settings object/origin=]:
-                1. If |client|'s [=environment/id=] is not |id|, continue to the next iteration of the loop.
-                1. If |client| is an [=environment settings object=], then:
-                    1. If |client| is not a <a>secure context</a>, reject |promise| with a "{{SecurityError}}" {{DOMException}} and abort these steps.
-                1. Else:
-                    1. If |client|’s <a>creation URL</a> is not a <a>potentially trustworthy URL</a>, reject |promise| with a "{{SecurityError}}" {{DOMException}} and abort these steps.
-                1. If |client| is an [=environment settings object=] and is not a [=window client=], then:
-                    1. Let |clientObject| be the result of running <a>Create Client</a> algorithm with |client| as the argument.
-                    1. Resolve |promise| with |clientObject| and abort these steps.
-                1. Else:
-                    1. Let |browsingContext| be null.
-                    1. Let |visibilityState| be null.
-                    1. Let |focusState| be false.
-                    1. Let |ancestorOriginsList| be the empty list.
-                    1. If |client| is an [=environment settings object=], set |browsingContext| to |client|'s [=environment settings object/global object=]'s [=/browsing context=].
-                    1. Else, set |browsingContext| to |client|’s [=environment/target browsing context=].
-                    1. <a>Queue a task</a> |task| to run the following substeps on |browsingContext|'s <a>event loop</a> using the <a>user interaction task source</a>:
-                        1. Set |visibilityState| to |browsingContext|'s <a>active document</a>'s {{Document/visibilityState}} attribute value.
-                        1. Set |focusState| to the result of running the <a>has focus steps</a> with |browsingContext|'s <a>active document</a> as the argument.
-                        1. If |client| is a <a>window client</a>, set |ancestorOriginsList| to |browsingContext|'s <a>active document</a>'s <a>relevant global object</a>'s {{Location}} object's [=Location/ancestor origins list=]'s associated list.
-                    1. Wait for |task| to have executed.
-                    1. Let |windowClient| be the result of running <a>Create Window Client</a> algorithm with |client|, |visibilityState|, |focusState|, and |ancestorOriginsList| as the arguments.
-                    1. Resolve |promise| with |windowClient| and abort these steps.
+                1. If |client|'s [=environment/id=] is not |id|, [=continue=].
+                1. Wait for either |client|'s [=environment/execution ready flag=] to be set or for |client|'s [=discarded flag=] to be set.
+                1. If |client|'s [=environment/execution ready flag=] is set, then invoke [=Resolve Get Client Promise=] with |client| and |promise|, and abort these steps.
             1. Resolve |promise| with undefined.
         1. Return |promise|.
     </section>
@@ -3216,6 +3204,41 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
 
           Note: When an exception is [=throw|thrown=], the implementation does undo (roll back) any changes made to the cache storage during the batch operation job.
   </section>
+
+  <section algorithm>
+    <h3 id="resolve-get-client-promise-algorithm"><dfn>Resolve Get Client Promise</dfn></h3>
+
+      : Input
+      :: |client|, a [=/service worker client=]
+      :: |promise|, a [=promise=]
+
+      : Output
+      :: none
+
+      1. If |client| is an [=environment settings object=], then:
+          1. If |client| is not a [=secure context=], reject |promise| with a "{{SecurityError}}" {{DOMException}} and abort these steps.
+      1. Else:
+          1. If |client|’s [=creation URL=] is not a [=potentially trustworthy URL=], reject |promise| with a "{{SecurityError}}" {{DOMException}} and abort these steps.
+      1. If |client| is an [=environment settings object=] and is not a [=window client=], then:
+          1. Let |clientObject| be the result of running [=Create Client=] algorithm with |client| as the argument.
+          1. Resolve |promise| with |clientObject| and abort these steps.
+      1. Else:
+          1. Let |browsingContext| be null.
+          1. Let |visibilityState| be null.
+          1. Let |focusState| be false.
+          1. Let |ancestorOriginsList| be the empty list.
+          1. If |client| is an [=environment settings object=], set |browsingContext| to |client|'s [=environment settings object/global object=]'s [=/browsing context=].
+          1. Else, set |browsingContext| to |client|’s [=environment/target browsing context=].
+          1. [=Queue a task=] |task| to run the following substeps on |browsingContext|'s [=event loop=] using the [=user interaction task source=]:
+              1. Set |visibilityState| to |browsingContext|'s [=active document=]'s {{Document/visibilityState}} attribute value.
+              1. Set |focusState| to the result of running the [=has focus steps=] with |browsingContext|'s [=active document=] as the argument.
+              1. If |client| is a [=window client=], set |ancestorOriginsList| to |browsingContext|'s [=active document=]'s [=relevant global object=]'s {{Location}} object's [=Location/ancestor origins list=]'s associated list.
+          1. Wait for |task| to have executed.
+          1. If |client|'s [=discarded flag=] is set, resolve |promise| with undefined and abort these steps.
+          1. Let |windowClient| be the result of running [=Create Window Client=] algorithm with |client|, |visibilityState|, |focusState|, and |ancestorOriginsList| as the arguments.
+          1. Resolve |promise| with |windowClient|.
+  </section>
+
 </section>
 
 <section>