Snapshot internal observers before Subscriber iteration (#214)

Author: domfarolino

spec.bs

@@ -205,7 +205,13 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
     1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
        Document=] is not [=Document/fully active=], then return.
 
-    1. [=set/For each=] |observer| of [=this=]'s [=Subscriber/internal observers=]:
+    1. Let |internal observers copy| be a copy of [=this=]'s [=Subscriber/internal observers=].
+
+       Note: We make a copy of the [=Subscriber/internal observers=] list and iterate over it so
+       that if one of the [=internal observer=]'s [=internal observer/next steps=] below causes
+       another subscription, the observer list does not mutate during iteration.
+
+    1. [=set/For each=] |observer| of |internal observers copy|:
 
        1. Run |observer|'s [=internal observer/next steps=] given |value|.
 
@@ -234,7 +240,9 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
 
     1. [=close a subscription|Close=] [=this=].
 
-    1. [=set/For each=] |observer| of [=this=]'s [=Subscriber/internal observers=]:
+    1. Let |internal observers copy| be a copy of [=this=]'s [=Subscriber/internal observers=].
+
+    1. [=set/For each=] |observer| of |internal observers copy|:
 
        1. Run |observer|'s [=internal observer/error steps=] given |error|.
 
@@ -253,7 +261,9 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
 
     1. [=close a subscription|Close=] [=this=].
 
-    1. [=set/For each=] |observer| of [=this=]'s [=Subscriber/internal observers=]:
+    1. Let |internal observers copy| be a copy of [=this=]'s [=Subscriber/internal observers=].
+
+    1. [=set/For each=] |observer| of |internal observers copy|:
 
        1. Run |observer|'s [=internal observer/complete steps=].