diff --git a/dom/bindings/Bindings.conf b/dom/bindings/Bindings.conf
index 0907a0faf7..fcd84e28b6 100644
--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -116,11 +116,13 @@ DOMInterfaces = {
 },
 
 'Cache': {
-    'implicitJSContext': [ 'add', 'addAll' ],
+    'implicitJSContext': [ 'add', 'addAll', 'match', 'matchAll', 'put',
+                           'delete', 'keys' ],
     'nativeType': 'mozilla::dom::cache::Cache',
 },
 
 'CacheStorage': {
+    'implicitJSContext': [ 'match' ],
     'nativeType': 'mozilla::dom::cache::CacheStorage',
 },
 
diff --git a/dom/cache/AutoUtils.cpp b/dom/cache/AutoUtils.cpp
index d1f354336e..bfa4cffd23 100644
--- a/dom/cache/AutoUtils.cpp
+++ b/dom/cache/AutoUtils.cpp
@@ -289,9 +289,9 @@ MatchInPutList(InternalRequest* aRequest,
 } // namespace
 
 void
-AutoChildOpArgs::Add(InternalRequest* aRequest, BodyAction aBodyAction,
-                     SchemeAction aSchemeAction, Response& aResponse,
-                     ErrorResult& aRv)
+AutoChildOpArgs::Add(JSContext* aCx, InternalRequest* aRequest,
+                     BodyAction aBodyAction, SchemeAction aSchemeAction,
+                     Response& aResponse, ErrorResult& aRv)
 {
   MOZ_DIAGNOSTIC_ASSERT(!mSent);
 
@@ -329,7 +329,7 @@ AutoChildOpArgs::Add(InternalRequest* aRequest, BodyAction aBodyAction,
       mTypeUtils->ToCacheRequest(pair.request(), aRequest, aBodyAction,
                                  aSchemeAction, mStreamCleanupList, aRv);
       if (!aRv.Failed()) {
-        mTypeUtils->ToCacheResponse(pair.response(), aResponse,
+        mTypeUtils->ToCacheResponse(aCx, pair.response(), aResponse,
                                     mStreamCleanupList, aRv);
       }
 
diff --git a/dom/cache/AutoUtils.h b/dom/cache/AutoUtils.h
index 244639f7c0..188c9d74e3 100644
--- a/dom/cache/AutoUtils.h
+++ b/dom/cache/AutoUtils.h
@@ -55,7 +55,7 @@ public:
 
   void Add(InternalRequest* aRequest, BodyAction aBodyAction,
            SchemeAction aSchemeAction, ErrorResult& aRv);
-  void Add(InternalRequest* aRequest, BodyAction aBodyAction,
+  void Add(JSContext* aCx, InternalRequest* aRequest, BodyAction aBodyAction,
            SchemeAction aSchemeAction, Response& aResponse, ErrorResult& aRv);
 
   const CacheOpArgs& SendAsOpArgs();
diff --git a/dom/cache/Cache.cpp b/dom/cache/Cache.cpp
index 60e4f76b92..dd142881ec 100644
--- a/dom/cache/Cache.cpp
+++ b/dom/cache/Cache.cpp
@@ -184,7 +184,10 @@ public:
 
     // Now store the unwrapped Response list in the Cache.
     ErrorResult result;
-    RefPtr<Promise> put = mCache->PutAll(mRequestList, responseList, result);
+    // TODO: Here we use the JSContext as received by the ResolvedCallback, and
+    // its state could be the wrong one. The spec doesn't say anything
+    // about it, yet (bug 1384006)
+    RefPtr<Promise> put = mCache->PutAll(aCx, mRequestList, responseList, result);
     if (NS_WARN_IF(result.Failed())) {
       // TODO: abort the fetch requests we have running (bug 1157434)
       mPromise->MaybeReject(result);
@@ -245,7 +248,7 @@ Cache::Cache(nsIGlobalObject* aGlobal, CacheChild* aActor)
 }
 
 already_AddRefed<Promise>
-Cache::Match(const RequestOrUSVString& aRequest,
+Cache::Match(JSContext* aCx, const RequestOrUSVString& aRequest,
              const CacheQueryOptions& aOptions, ErrorResult& aRv)
 {
   if (NS_WARN_IF(!mActor)) {
@@ -255,7 +258,8 @@ Cache::Match(const RequestOrUSVString& aRequest,
 
   CacheChild::AutoLock actorLock(mActor);
 
-  RefPtr<InternalRequest> ir = ToInternalRequest(aRequest, IgnoreBody, aRv);
+  RefPtr<InternalRequest> ir =
+    ToInternalRequest(aCx, aRequest, IgnoreBody, aRv);
   if (NS_WARN_IF(aRv.Failed())) {
     return nullptr;
   }
@@ -274,7 +278,7 @@ Cache::Match(const RequestOrUSVString& aRequest,
 }
 
 already_AddRefed<Promise>
-Cache::MatchAll(const Optional<RequestOrUSVString>& aRequest,
+Cache::MatchAll(JSContext* aCx, const Optional<RequestOrUSVString>& aRequest,
                 const CacheQueryOptions& aOptions, ErrorResult& aRv)
 {
   if (NS_WARN_IF(!mActor)) {
@@ -290,8 +294,8 @@ Cache::MatchAll(const Optional<RequestOrUSVString>& aRequest,
   AutoChildOpArgs args(this, CacheMatchAllArgs(void_t(), params), 1);
 
   if (aRequest.WasPassed()) {
-    RefPtr<InternalRequest> ir = ToInternalRequest(aRequest.Value(),
-                                                     IgnoreBody, aRv);
+    RefPtr<InternalRequest> ir = ToInternalRequest(aCx, aRequest.Value(),
+                                                   IgnoreBody, aRv);
     if (aRv.Failed()) {
       return nullptr;
     }
@@ -390,8 +394,8 @@ Cache::AddAll(JSContext* aContext,
 }
 
 already_AddRefed<Promise>
-Cache::Put(const RequestOrUSVString& aRequest, Response& aResponse,
-           ErrorResult& aRv)
+Cache::Put(JSContext* aCx, const RequestOrUSVString& aRequest,
+           Response& aResponse, ErrorResult& aRv)
 {
   if (NS_WARN_IF(!mActor)) {
     aRv.Throw(NS_ERROR_UNEXPECTED);
@@ -404,14 +408,14 @@ Cache::Put(const RequestOrUSVString& aRequest, Response& aResponse,
     return nullptr;
   }
 
-  RefPtr<InternalRequest> ir = ToInternalRequest(aRequest, ReadBody, aRv);
+  RefPtr<InternalRequest> ir = ToInternalRequest(aCx, aRequest, ReadBody, aRv);
   if (NS_WARN_IF(aRv.Failed())) {
     return nullptr;
   }
 
   AutoChildOpArgs args(this, CachePutAllArgs(), 1);
 
-  args.Add(ir, ReadBody, TypeErrorOnInvalidScheme,
+  args.Add(aCx, ir, ReadBody, TypeErrorOnInvalidScheme,
            aResponse, aRv);
   if (NS_WARN_IF(aRv.Failed())) {
     return nullptr;
@@ -421,7 +425,7 @@ Cache::Put(const RequestOrUSVString& aRequest, Response& aResponse,
 }
 
 already_AddRefed<Promise>
-Cache::Delete(const RequestOrUSVString& aRequest,
+Cache::Delete(JSContext* aCx, const RequestOrUSVString& aRequest,
               const CacheQueryOptions& aOptions, ErrorResult& aRv)
 {
   if (NS_WARN_IF(!mActor)) {
@@ -431,7 +435,8 @@ Cache::Delete(const RequestOrUSVString& aRequest,
 
   CacheChild::AutoLock actorLock(mActor);
 
-  RefPtr<InternalRequest> ir = ToInternalRequest(aRequest, IgnoreBody, aRv);
+  RefPtr<InternalRequest> ir =
+    ToInternalRequest(aCx, aRequest, IgnoreBody, aRv);
   if (NS_WARN_IF(aRv.Failed())) {
     return nullptr;
   }
@@ -450,7 +455,7 @@ Cache::Delete(const RequestOrUSVString& aRequest,
 }
 
 already_AddRefed<Promise>
-Cache::Keys(const Optional<RequestOrUSVString>& aRequest,
+Cache::Keys(JSContext* aCx, const Optional<RequestOrUSVString>& aRequest,
             const CacheQueryOptions& aOptions, ErrorResult& aRv)
 {
   if (NS_WARN_IF(!mActor)) {
@@ -466,8 +471,8 @@ Cache::Keys(const Optional<RequestOrUSVString>& aRequest,
   AutoChildOpArgs args(this, CacheKeysArgs(void_t(), params), 1);
 
   if (aRequest.WasPassed()) {
-    RefPtr<InternalRequest> ir = ToInternalRequest(aRequest.Value(),
-                                                     IgnoreBody, aRv);
+    RefPtr<InternalRequest> ir =
+      ToInternalRequest(aCx, aRequest.Value(), IgnoreBody, aRv);
     if (NS_WARN_IF(aRv.Failed())) {
       return nullptr;
     }
@@ -627,7 +632,7 @@ Cache::AddAll(const GlobalObject& aGlobal,
 }
 
 already_AddRefed<Promise>
-Cache::PutAll(const nsTArray<RefPtr<Request>>& aRequestList,
+Cache::PutAll(JSContext* aCx, const nsTArray<RefPtr<Request>>& aRequestList,
               const nsTArray<RefPtr<Response>>& aResponseList,
               ErrorResult& aRv)
 {
@@ -644,7 +649,8 @@ Cache::PutAll(const nsTArray<RefPtr<Request>>& aRequestList,
 
   for (uint32_t i = 0; i < aRequestList.Length(); ++i) {
     RefPtr<InternalRequest> ir = aRequestList[i]->GetInternalRequest();
-    args.Add(ir, ReadBody, TypeErrorOnInvalidScheme, *aResponseList[i], aRv);
+    args.Add(aCx, ir, ReadBody, TypeErrorOnInvalidScheme, *aResponseList[i],
+             aRv);
     if (NS_WARN_IF(aRv.Failed())) {
       return nullptr;
     }
diff --git a/dom/cache/Cache.h b/dom/cache/Cache.h
index 04f891dca5..6fea3d36a0 100644
--- a/dom/cache/Cache.h
+++ b/dom/cache/Cache.h
@@ -43,10 +43,10 @@ public:
 
   // webidl interface methods
   already_AddRefed<Promise>
-  Match(const RequestOrUSVString& aRequest, const CacheQueryOptions& aOptions,
-        ErrorResult& aRv);
+  Match(JSContext* aCx, const RequestOrUSVString& aRequest,
+        const CacheQueryOptions& aOptions, ErrorResult& aRv);
   already_AddRefed<Promise>
-  MatchAll(const Optional<RequestOrUSVString>& aRequest,
+  MatchAll(JSContext* aCx, const Optional<RequestOrUSVString>& aRequest,
            const CacheQueryOptions& aOptions, ErrorResult& aRv);
   already_AddRefed<Promise>
   Add(JSContext* aContext, const RequestOrUSVString& aRequest,
@@ -55,13 +55,13 @@ public:
   AddAll(JSContext* aContext,
          const Sequence<OwningRequestOrUSVString>& aRequests, ErrorResult& aRv);
   already_AddRefed<Promise>
-  Put(const RequestOrUSVString& aRequest, Response& aResponse,
+  Put(JSContext* aCx, const RequestOrUSVString& aRequest, Response& aResponse,
       ErrorResult& aRv);
   already_AddRefed<Promise>
-  Delete(const RequestOrUSVString& aRequest, const CacheQueryOptions& aOptions,
-         ErrorResult& aRv);
+  Delete(JSContext* aCx, const RequestOrUSVString& aRequest,
+         const CacheQueryOptions& aOptions, ErrorResult& aRv);
   already_AddRefed<Promise>
-  Keys(const Optional<RequestOrUSVString>& aRequest,
+  Keys(JSContext* aCx, const Optional<RequestOrUSVString>& aRequest,
        const CacheQueryOptions& aParams, ErrorResult& aRv);
 
   // binding methods
@@ -100,7 +100,7 @@ private:
          ErrorResult& aRv);
 
   already_AddRefed<Promise>
-  PutAll(const nsTArray<RefPtr<Request>>& aRequestList,
+  PutAll(JSContext* aCx, const nsTArray<RefPtr<Request>>& aRequestList,
          const nsTArray<RefPtr<Response>>& aResponseList,
          ErrorResult& aRv);
 
diff --git a/dom/cache/CacheStorage.cpp b/dom/cache/CacheStorage.cpp
index 3ace28c156..05df8ddc69 100644
--- a/dom/cache/CacheStorage.cpp
+++ b/dom/cache/CacheStorage.cpp
@@ -316,7 +316,7 @@ CacheStorage::CacheStorage(nsresult aFailureResult)
 }
 
 already_AddRefed<Promise>
-CacheStorage::Match(const RequestOrUSVString& aRequest,
+CacheStorage::Match(JSContext* aCx, const RequestOrUSVString& aRequest,
                     const CacheQueryOptions& aOptions, ErrorResult& aRv)
 {
   NS_ASSERT_OWNINGTHREAD(CacheStorage);
@@ -326,8 +326,8 @@ CacheStorage::Match(const RequestOrUSVString& aRequest,
     return nullptr;
   }
 
-  RefPtr<InternalRequest> request = ToInternalRequest(aRequest, IgnoreBody,
-                                                        aRv);
+  RefPtr<InternalRequest> request =
+    ToInternalRequest(aCx, aRequest, IgnoreBody, aRv);
   if (NS_WARN_IF(aRv.Failed())) {
     return nullptr;
   }
diff --git a/dom/cache/CacheStorage.h b/dom/cache/CacheStorage.h
index 04a2fa0dd9..c1cef4b428 100644
--- a/dom/cache/CacheStorage.h
+++ b/dom/cache/CacheStorage.h
@@ -59,9 +59,9 @@ public:
   DefineCaches(JSContext* aCx, JS::Handle<JSObject*> aGlobal);
 
   // webidl interface methods
-  already_AddRefed<Promise> Match(const RequestOrUSVString& aRequest,
-                                  const CacheQueryOptions& aOptions,
-                                  ErrorResult& aRv);
+  already_AddRefed<Promise>
+  Match(JSContext* aCx, const RequestOrUSVString& aRequest,
+        const CacheQueryOptions& aOptions, ErrorResult& aRv);
   already_AddRefed<Promise> Has(const nsAString& aKey, ErrorResult& aRv);
   already_AddRefed<Promise> Open(const nsAString& aKey, ErrorResult& aRv);
   already_AddRefed<Promise> Delete(const nsAString& aKey, ErrorResult& aRv);
diff --git a/dom/cache/TypeUtils.cpp b/dom/cache/TypeUtils.cpp
index f849f18874..1af5ee9458 100644
--- a/dom/cache/TypeUtils.cpp
+++ b/dom/cache/TypeUtils.cpp
@@ -79,7 +79,7 @@ ToHeadersEntryList(nsTArray<HeadersEntry>& aOut, InternalHeaders* aHeaders)
 } // namespace
 
 already_AddRefed<InternalRequest>
-TypeUtils::ToInternalRequest(const RequestOrUSVString& aIn,
+TypeUtils::ToInternalRequest(JSContext* aCx, const RequestOrUSVString& aIn,
                              BodyAction aBodyAction, ErrorResult& aRv)
 {
   if (aIn.IsRequest()) {
@@ -87,7 +87,7 @@ TypeUtils::ToInternalRequest(const RequestOrUSVString& aIn,
 
     // Check and set bodyUsed flag immediately because its on Request
     // instead of InternalRequest.
-    CheckAndSetBodyUsed(&request, aBodyAction, aRv);
+    CheckAndSetBodyUsed(aCx, &request, aBodyAction, aRv);
     if (aRv.Failed()) { return nullptr; }
 
     return request.GetInternalRequest();
@@ -97,7 +97,8 @@ TypeUtils::ToInternalRequest(const RequestOrUSVString& aIn,
 }
 
 already_AddRefed<InternalRequest>
-TypeUtils::ToInternalRequest(const OwningRequestOrUSVString& aIn,
+TypeUtils::ToInternalRequest(JSContext* aCx,
+                             const OwningRequestOrUSVString& aIn,
                              BodyAction aBodyAction, ErrorResult& aRv)
 {
 
@@ -106,7 +107,7 @@ TypeUtils::ToInternalRequest(const OwningRequestOrUSVString& aIn,
 
     // Check and set bodyUsed flag immediately because its on Request
     // instead of InternalRequest.
-    CheckAndSetBodyUsed(request, aBodyAction, aRv);
+    CheckAndSetBodyUsed(aCx, request, aBodyAction, aRv);
     if (aRv.Failed()) { return nullptr; }
 
     return request->GetInternalRequest();
@@ -204,7 +205,7 @@ TypeUtils::ToCacheResponseWithoutBody(CacheResponse& aOut,
 }
 
 void
-TypeUtils::ToCacheResponse(CacheResponse& aOut, Response& aIn,
+TypeUtils::ToCacheResponse(JSContext* aCx, CacheResponse& aOut, Response& aIn,
                            nsTArray<UniquePtr<AutoIPCStream>>& aStreamCleanupList,
                            ErrorResult& aRv)
 {
@@ -222,7 +223,10 @@ TypeUtils::ToCacheResponse(CacheResponse& aOut, Response& aIn,
   nsCOMPtr<nsIInputStream> stream;
   ir->GetUnfilteredBody(getter_AddRefs(stream));
   if (stream) {
-    aIn.SetBodyUsed();
+    aIn.SetBodyUsed(aCx, aRv);
+    if (NS_WARN_IF(aRv.Failed())) {
+      return;
+    }
   }
 
   SerializeCacheStream(stream, &aOut.body(), aStreamCleanupList, aRv);
@@ -426,8 +430,8 @@ TypeUtils::ProcessURL(nsACString& aUrl, bool* aSchemeValidOut,
 }
 
 void
-TypeUtils::CheckAndSetBodyUsed(Request* aRequest, BodyAction aBodyAction,
-                               ErrorResult& aRv)
+TypeUtils::CheckAndSetBodyUsed(JSContext* aCx, Request* aRequest,
+                               BodyAction aBodyAction, ErrorResult& aRv)
 {
   MOZ_DIAGNOSTIC_ASSERT(aRequest);
 
@@ -443,7 +447,10 @@ TypeUtils::CheckAndSetBodyUsed(Request* aRequest, BodyAction aBodyAction,
   nsCOMPtr<nsIInputStream> stream;
   aRequest->GetBody(getter_AddRefs(stream));
   if (stream) {
-    aRequest->SetBodyUsed();
+    aRequest->SetBodyUsed(aCx, aRv);
+    if (NS_WARN_IF(aRv.Failed())) {
+      return;
+    }
   }
 }
 
diff --git a/dom/cache/TypeUtils.h b/dom/cache/TypeUtils.h
index 274586e3f7..ff5816db3e 100644
--- a/dom/cache/TypeUtils.h
+++ b/dom/cache/TypeUtils.h
@@ -73,12 +73,12 @@ public:
   GetIPCManager() = 0;
 
   already_AddRefed<InternalRequest>
-  ToInternalRequest(const RequestOrUSVString& aIn, BodyAction aBodyAction,
-                    ErrorResult& aRv);
+  ToInternalRequest(JSContext* aCx, const RequestOrUSVString& aIn,
+                    BodyAction aBodyAction, ErrorResult& aRv);
 
   already_AddRefed<InternalRequest>
-  ToInternalRequest(const OwningRequestOrUSVString& aIn, BodyAction aBodyAction,
-                    ErrorResult& aRv);
+  ToInternalRequest(JSContext* aCx, const OwningRequestOrUSVString& aIn,
+                    BodyAction aBodyAction, ErrorResult& aRv);
 
   void
   ToCacheRequest(CacheRequest& aOut, InternalRequest* aIn,
@@ -91,7 +91,7 @@ public:
                              ErrorResult& aRv);
 
   void
-  ToCacheResponse(CacheResponse& aOut, Response& aIn,
+  ToCacheResponse(JSContext* aCx, CacheResponse& aOut, Response& aIn,
                   nsTArray<UniquePtr<mozilla::ipc::AutoIPCStream>>& aStreamCleanupList,
                   ErrorResult& aRv);
 
@@ -133,7 +133,7 @@ public:
 
 private:
   void
-  CheckAndSetBodyUsed(Request* aRequest, BodyAction aBodyAction,
+  CheckAndSetBodyUsed(JSContext* aCx, Request* aRequest, BodyAction aBodyAction,
                       ErrorResult& aRv);
 
   already_AddRefed<InternalRequest>
diff --git a/dom/fetch/Fetch.cpp b/dom/fetch/Fetch.cpp
index ce21cd391a..53ada9378b 100644
--- a/dom/fetch/Fetch.cpp
+++ b/dom/fetch/Fetch.cpp
@@ -948,6 +948,52 @@ template
 bool
 FetchBody<Response>::BodyUsed() const;
 
+template <class Derived>
+void
+FetchBody<Derived>::SetBodyUsed(JSContext* aCx, ErrorResult& aRv)
+{
+  MOZ_ASSERT(aCx);
+  MOZ_ASSERT(mOwner->EventTargetFor(TaskCategory::Other)->IsOnCurrentThread());
+
+  if (mBodyUsed) {
+    return;
+  }
+
+  mBodyUsed = true;
+
+  // If we already have a ReadableStreamBody and it has been created by DOM, we
+  // have to lock it now because it can have been shared with other objects.
+  if (mReadableStreamBody) {
+    JS::Rooted<JSObject*> readableStreamObj(aCx, mReadableStreamBody);
+    if (JS::ReadableStreamGetMode(readableStreamObj) ==
+          JS::ReadableStreamMode::ExternalSource) {
+      LockStream(aCx, readableStreamObj, aRv);
+      if (NS_WARN_IF(aRv.Failed())) {
+        return;
+      }
+    } else {
+      // If this is not a native ReadableStream, let's activate the
+      // FetchStreamReader.
+      MOZ_ASSERT(mFetchStreamReader);
+      JS::Rooted<JSObject*> reader(aCx);
+      mFetchStreamReader->StartConsuming(aCx, readableStreamObj, &reader, aRv);
+      if (NS_WARN_IF(aRv.Failed())) {
+        return;
+      }
+
+      mReadableStreamReader = reader;
+    }
+  }
+}
+
+template
+void
+FetchBody<Request>::SetBodyUsed(JSContext* aCx, ErrorResult& aRv);
+
+template
+void
+FetchBody<Response>::SetBodyUsed(JSContext* aCx, ErrorResult& aRv);
+
 template <class Derived>
 already_AddRefed<Promise>
 FetchBody<Derived>::ConsumeBody(JSContext* aCx, FetchConsumeType aType, ErrorResult& aRv)
@@ -963,30 +1009,9 @@ FetchBody<Derived>::ConsumeBody(JSContext* aCx, FetchConsumeType aType, ErrorRes
     return nullptr;
   }
 
-  SetBodyUsed();
-
-  // If we already have a ReadableStreamBody and it has been created by DOM, we
-  // have to lock it now because it can have been shared with other objects.
-  if (mReadableStreamBody) {
-    JS::Rooted<JSObject*> readableStreamObj(aCx, mReadableStreamBody);
-    if (JS::ReadableStreamGetMode(readableStreamObj) ==
-          JS::ReadableStreamMode::ExternalSource) {
-      LockStream(aCx, readableStreamObj, aRv);
-      if (NS_WARN_IF(aRv.Failed())) {
-        return nullptr;
-      }
-    } else {
-      // If this is not a native ReadableStream, let's activate the
-      // FetchStreamReader.
-      MOZ_ASSERT(mFetchStreamReader);
-      JS::Rooted<JSObject*> reader(aCx);
-      mFetchStreamReader->StartConsuming(aCx, readableStreamObj, &reader, aRv);
-      if (NS_WARN_IF(aRv.Failed())) {
-        return nullptr;
-      }
-
-      mReadableStreamReader = reader;
-    }
+  SetBodyUsed(aCx, aRv);
+  if (NS_WARN_IF(aRv.Failed())) {
+    return nullptr;
   }
 
   nsCOMPtr<nsIGlobalObject> global = DerivedClass()->GetParentObject();
diff --git a/dom/fetch/Fetch.h b/dom/fetch/Fetch.h
index d65b66b883..01f0ab4ace 100644
--- a/dom/fetch/Fetch.h
+++ b/dom/fetch/Fetch.h
@@ -190,11 +190,28 @@ public:
 
   // Utility public methods accessed by various runnables.
 
+  // This method _must_ be called in order to set the body as used. If the body
+  // is a ReadableStream, this method will start reading the stream.
+  // More in details, this method does:
+  // 1) It uses an internal flag to track if the body is used.  This is tracked
+  // separately from the ReadableStream disturbed state due to purely native
+  // streams.
+  // 2) If there is a ReadableStream reflector for the native stream it is
+  // Locked.
+  // 3) If there is a JS ReadableStream then we begin pumping it into the native
+  // body stream.  This effectively locks and disturbs the stream.
+  //
+  // Note that JSContext is used only if there is a ReadableStream (this can
+  // happen because the body is a ReadableStream or because attribute body has
+  // already been used by content). If something goes wrong using
+  // ReadableStream, errors will be reported via ErrorResult and not as JS
+  // exceptions in JSContext. This is done in order to have a centralized error
+  // reporting way.
+  //
+  // Exceptions generated when reading from the ReadableStream are directly sent
+ // to the Console (NOTE FOR THE REVIEWER: this is part of patch 16)
   void
-  SetBodyUsed()
-  {
-    mBodyUsed = true;
-  }
+  SetBodyUsed(JSContext* aCx, ErrorResult& aRv);
 
   const nsCString&
   MimeType() const
diff --git a/dom/fetch/Request.cpp b/dom/fetch/Request.cpp
index ec140a22d4..b79b539772 100644
--- a/dom/fetch/Request.cpp
+++ b/dom/fetch/Request.cpp
@@ -614,7 +614,10 @@ Request::Constructor(const GlobalObject& aGlobal,
     inputReq->GetBody(getter_AddRefs(body));
     if (body) {
       inputReq->SetBody(nullptr);
-      inputReq->SetBodyUsed();
+      inputReq->SetBodyUsed(aGlobal.Context(), aRv);
+      if (NS_WARN_IF(aRv.Failed())) {
+        return nullptr;
+      }
     }
   }
   return domRequest.forget();
diff --git a/dom/workers/ScriptLoader.cpp b/dom/workers/ScriptLoader.cpp
index 308ef94e38..123a46526a 100644
--- a/dom/workers/ScriptLoader.cpp
+++ b/dom/workers/ScriptLoader.cpp
@@ -701,9 +701,14 @@ private:
     request.SetAsUSVString().Rebind(loadInfo.mFullURL.Data(),
                                     loadInfo.mFullURL.Length());
 
+    // This JSContext will not end up executing JS code because here there are
+    // no ReadableStreams involved.
+    AutoJSAPI jsapi;
+    jsapi.Init();
+
     ErrorResult error;
     RefPtr<Promise> cachePromise =
-      mCacheCreator->Cache_()->Put(request, *response, error);
+      mCacheCreator->Cache_()->Put(jsapi.cx(), request, *response, error);
     if (NS_WARN_IF(error.Failed())) {
       nsresult rv = error.StealNSResult();
       channel->Cancel(rv);
@@ -1668,8 +1673,13 @@ CacheScriptLoader::Load(Cache* aCache)
 
   mozilla::dom::CacheQueryOptions params;
 
+  // This JSContext will not end up executing JS code because here there are
+  // no ReadableStreams involved.
+  AutoJSAPI jsapi;
+  jsapi.Init();
+
   ErrorResult error;
-  RefPtr<Promise> promise = aCache->Match(request, params, error);
+  RefPtr<Promise> promise = aCache->Match(jsapi.cx(), request, params, error);
   if (NS_WARN_IF(error.Failed())) {
     Fail(error.StealNSResult());
     return;
diff --git a/dom/workers/ServiceWorkerEvents.cpp b/dom/workers/ServiceWorkerEvents.cpp
index ce2e5e7aae..7c9add4060 100644
--- a/dom/workers/ServiceWorkerEvents.cpp
+++ b/dom/workers/ServiceWorkerEvents.cpp
@@ -505,6 +505,44 @@ public:
     }
   }
 
+  // This function steals the error message from a ErrorResult.
+  void
+  SetCancelErrorResult(JSContext* aCx, ErrorResult& aRv)
+  {
+    MOZ_DIAGNOSTIC_ASSERT(aRv.Failed());
+    MOZ_DIAGNOSTIC_ASSERT(!JS_IsExceptionPending(aCx));
+
+    // Storing the error as exception in the JSContext.
+    if (!aRv.MaybeSetPendingException(aCx)) {
+      return;
+    }
+
+    MOZ_ASSERT(!aRv.Failed());
+
+    // Let's take the pending exception.
+    JS::Rooted<JS::Value> exn(aCx);
+    if (!JS_GetPendingException(aCx, &exn)) {
+      return;
+    }
+
+    JS_ClearPendingException(aCx);
+
+    // Converting the exception in a js::ErrorReport.
+    js::ErrorReport report(aCx);
+    if (!report.init(aCx, exn, js::ErrorReport::WithSideEffects)) {
+      JS_ClearPendingException(aCx);
+      return;
+    }
+
+    MOZ_ASSERT(mOwner);
+    MOZ_ASSERT(mMessageName.EqualsLiteral("InterceptionFailedWithURL"));
+    MOZ_ASSERT(mParams.Length() == 1);
+
+    // Let's store the error message here.
+    mMessageName.Assign(report.toStringResult().c_str());
+    mParams.Clear();
+  }
+
   template<typename... Params>
   void SetCancelMessage(const nsACString& aMessageName, Params&&... aParams)
   {
@@ -659,7 +697,12 @@ RespondWithHandler::ResolvedCallback(JSContext* aCx, JS::Handle<JS::Value> aValu
   ir->GetUnfilteredBody(getter_AddRefs(body));
   // Errors and redirects may not have a body.
   if (body) {
-    response->SetBodyUsed();
+    IgnoredErrorResult error;
+    response->SetBodyUsed(aCx, error);
+    if (NS_WARN_IF(error.Failed())) {
+      autoCancel.SetCancelErrorResult(aCx, error);
+      return;
+    }
 
     nsCOMPtr<nsIOutputStream> responseBody;
     rv = mInterceptedChannel->GetResponseBody(getter_AddRefs(responseBody));
diff --git a/dom/workers/ServiceWorkerScriptCache.cpp b/dom/workers/ServiceWorkerScriptCache.cpp
index 05be250da8..f343c35586 100644
--- a/dom/workers/ServiceWorkerScriptCache.cpp
+++ b/dom/workers/ServiceWorkerScriptCache.cpp
@@ -395,7 +395,7 @@ public:
         return;
       }
 
-      WriteToCache(cache);
+      WriteToCache(aCx, cache);
       return;
     }
 
@@ -528,7 +528,7 @@ private:
   }
 
   void
-  WriteToCache(Cache* aCache)
+  WriteToCache(JSContext* aCx, Cache* aCache)
   {
     AssertIsOnMainThread();
     MOZ_ASSERT(aCache);
@@ -561,8 +561,10 @@ private:
     // For now we have to wait until the Put Promise is fulfilled before we can
     // continue since Cache does not yet support starting a read that is being
     // written to.
-    RefPtr<Promise> cachePromise = aCache->Put(request, *response, result);
+    RefPtr<Promise> cachePromise = aCache->Put(aCx, request, *response, result);
     if (NS_WARN_IF(result.Failed())) {
+      // No exception here because there are no ReadableStreams involved here.
+      MOZ_ASSERT(!result.IsJSException());
       MOZ_ASSERT(!result.IsErrorWithMessage());
       Fail(result.StealNSResult());
       return;
@@ -903,7 +905,7 @@ CompareCache::ManageCacheResult(JSContext* aCx, JS::Handle<JS::Value> aValue)
   request.SetAsUSVString().Rebind(mURL.Data(), mURL.Length());
   ErrorResult error;
   CacheQueryOptions params;
-  RefPtr<Promise> promise = cache->Match(request, params, error);
+  RefPtr<Promise> promise = cache->Match(aCx, request, params, error);
   if (NS_WARN_IF(error.Failed())) {
     mManager->CacheFinished(error.StealNSResult(), false);
     return;