Bug 1744524: part 5) Add pref for enabling `clipboard.readText()` gated by a "Paste" button. r=edgar

The web platform test (WPT) framework doesn't support such user agent
specific buttons. The corresponding WPTs continue to use the pref
"dom.events.testing.asyncClipboard", which skips showing the button.

Differential Revision: https://phabricator.services.mozilla.com/D145058

dom/events/Clipboard.cpp

@@ -116,6 +116,31 @@ already_AddRefed<nsIRunnable> Clipboard::ReadTextRequest::Answer() {
       });
 }
 
+static bool IsReadTextExposedToContent() {
+  return StaticPrefs::dom_events_asyncClipboard_readText_DoNotUseDirectly();
+}
+
+already_AddRefed<nsIRunnable>
+Clipboard::CheckReadTextPermissionAndHandleRequest(
+    Promise& aPromise, nsIPrincipal& aSubjectPrincipal) {
+  if (IsTestingPrefEnabledOrHasReadPermission(aSubjectPrincipal)) {
+    MOZ_LOG(GetClipboardLog(), LogLevel::Debug,
+            ("%s: testing pref enabled or has read permission", __FUNCTION__));
+    return ReadTextRequest{aPromise, aSubjectPrincipal}.Answer();
+  }
+
+  if (aSubjectPrincipal.GetIsAddonOrExpandedAddonPrincipal()) {
+    // TODO: enable showing the "Paste" button in this case; see bug 1773681.
+    MOZ_LOG(GetClipboardLog(), LogLevel::Debug,
+            ("%s: Addon without read permssion.", __FUNCTION__));
+    aPromise.MaybeRejectWithUndefined();
+    return nullptr;
+  }
+
+  return HandleReadTextRequestWhichRequiresPasteButton(aPromise,
+                                                       aSubjectPrincipal);
+}
+
 already_AddRefed<nsIRunnable>
 Clipboard::HandleReadTextRequestWhichRequiresPasteButton(
     Promise& aPromise, nsIPrincipal& aSubjectPrincipal) {
@@ -188,18 +213,10 @@ already_AddRefed<Promise> Clipboard::ReadHelper(
     return nullptr;
   }
 
-  // We want to disable security check for automated tests that have the pref
-  //  dom.events.testing.asyncClipboard set to true
-  const bool isTestingPrefEnabledOrHasReadPermission =
-      IsTestingPrefEnabledOrHasReadPermission(aSubjectPrincipal);
-
   switch (aClipboardReadType) {
     case eReadText: {
       RefPtr<nsIRunnable> runnable =
-          isTestingPrefEnabledOrHasReadPermission
-              ? ReadTextRequest{*p, aSubjectPrincipal}.Answer()
-              : HandleReadTextRequestWhichRequiresPasteButton(
-                    *p, aSubjectPrincipal);
+          CheckReadTextPermissionAndHandleRequest(*p, aSubjectPrincipal);
 
       if (runnable) {
         GetParentObject()->Dispatch(TaskCategory::Other, runnable.forget());
@@ -208,6 +225,12 @@ already_AddRefed<Promise> Clipboard::ReadHelper(
       break;
     }
     case eRead: {
+      // We want to disable security check for automated tests that have the
+      // pref
+      //  dom.events.testing.asyncClipboard set to true
+      const bool isTestingPrefEnabledOrHasReadPermission =
+          IsTestingPrefEnabledOrHasReadPermission(aSubjectPrincipal);
+
       if (!isTestingPrefEnabledOrHasReadPermission) {
         MOZ_LOG(GetClipboardLog(), LogLevel::Debug,
                 ("Clipboard, ReadHelper, "
@@ -765,7 +788,8 @@ LogModule* Clipboard::GetClipboardLog() { return gClipboardLog; }
 /* static */
 bool Clipboard::ReadTextEnabled(JSContext* aCx, JSObject* aGlobal) {
   nsIPrincipal* prin = nsContentUtils::SubjectPrincipal(aCx);
-  return IsTestingPrefEnabled() || prin->GetIsAddonOrExpandedAddonPrincipal() ||
+  return IsReadTextExposedToContent() ||
+         prin->GetIsAddonOrExpandedAddonPrincipal() ||
          prin->IsSystemPrincipal();
 }
 

dom/events/Clipboard.h

@@ -71,6 +71,10 @@ class Clipboard : public DOMEventTargetHelper {
   static bool IsTestingPrefEnabledOrHasReadPermission(
       nsIPrincipal& aSubjectPrincipal);
 
+  // @return the remaining work to fill aPromise.
+  already_AddRefed<nsIRunnable> CheckReadTextPermissionAndHandleRequest(
+      Promise& aPromise, nsIPrincipal& aSubjectPrincipal);
+
   // @return the remaining work to fill aPromise.
   already_AddRefed<nsIRunnable> HandleReadTextRequestWhichRequiresPasteButton(
       Promise& aPromise, nsIPrincipal& aSubjectPrincipal);

modules/libpref/init/StaticPrefList.yaml

@@ -2247,6 +2247,7 @@
   value: false
   mirror: always
 
+# Skips checking permission and user activation when accessing the clipboard.
 # Should only be enabled in tests.
 # Access with Clipboard::IsTestingPrefEnabled().
 - name: dom.events.testing.asyncClipboard
@@ -2255,6 +2256,13 @@
   mirror: always
   do_not_use_directly: true
 
+# Control whether `navigator.clipboard.readText()` is exposed to content.
+- name: dom.events.asyncClipboard.readText
+  type: bool
+  value: false
+  mirror: always
+  do_not_use_directly: true
+
 # This pref controls whether or not the `protected` dataTransfer state is
 # enabled. If the `protected` dataTransfer stae is disabled, then the
 # DataTransfer will be read-only whenever it should be protected, and will not

testing/web-platform/meta/clipboard-apis/__dir__.ini

@@ -1 +1 @@
-prefs: [dom.events.asyncClipboard.clipboardItem: true, dom.events.testing.asyncClipboard:true, marionette.setpermission.enabled:true]
+prefs: [dom.events.asyncClipboard.clipboardItem: true, dom.events.asyncClipboard.readText: true, dom.events.testing.asyncClipboard:true, marionette.setpermission.enabled:true]