Bug 1494436 - Unset MOZ_APP_LAUNCHER for external MIME handlers. r=emilio

If Thunderbird sets this in its startup script (as it does in Debian
and Fedora), Firefox does not set this in its startup script (it doesn't
in Debian), and Firefox is the handler for clicking a link in
Thunderbird, then Firefox will think it is part of Thunderbird and offer
to make Thunderbird (not Firefox!) the default browser. If the user
accepts this, it will break the ability to open normal HTTP links and
HTML files from other applications.

The same would be true for any other pair of Mozilla-based applications.
To avoid this, unset MOZ_APP_LAUNCHER for the Firefox child process.

MANUAL PUSH: Patch submitted via Splinter.

Co-Authored-By: Emilio Cobos Álvarez <emilio@crisal.io>
Bug-Debian: https://bugs.debian.org/948691

toolkit/system/gnome/nsGIOService.cpp

@@ -213,6 +213,17 @@ nsGIOMimeApp::Equals(nsIHandlerApp* aHandlerApp, bool* _retval) {
   return NS_OK;
 }
 
+static RefPtr<GAppLaunchContext> GetLaunchContext() {
+  RefPtr<GAppLaunchContext> context = dont_AddRef(g_app_launch_context_new());
+  // Unset this before launching third-party MIME handlers. Otherwise, if
+  // Thunderbird sets this in its startup script (as it does in Debian and
+  // Fedora), and Firefox does not set this in its startup script (it doesn't in
+  // Debian), then Firefox will think it is part of Thunderbird and try to make
+  // Thunderbird the default browser. See bug 1494436.
+  g_app_launch_context_unsetenv(context, "MOZ_APP_LAUNCHER");
+  return context;
+}
+
 NS_IMETHODIMP
 nsGIOMimeApp::LaunchWithURI(nsIURI* aUri,
                             mozilla::dom::BrowsingContext* aBrowsingContext) {
@@ -223,9 +234,8 @@ nsGIOMimeApp::LaunchWithURI(nsIURI* aUri,
   uris.data = const_cast<char*>(spec.get());
 
   GUniquePtr<GError> error;
-  gboolean result =
-      g_app_info_launch_uris(mApp, &uris, nullptr, getter_Transfers(error));
-
+  gboolean result = g_app_info_launch_uris(
+      mApp, &uris, GetLaunchContext().get(), getter_Transfers(error));
   if (!result) {
     g_warning("Cannot launch application: %s", error->message);
     return NS_ERROR_FAILURE;
@@ -525,7 +535,7 @@ nsresult nsGIOService::ShowURI(nsIURI* aURI) {
   nsAutoCString spec;
   MOZ_TRY(aURI->GetSpec(spec));
   GUniquePtr<GError> error;
-  if (!g_app_info_launch_default_for_uri(spec.get(), nullptr,
+  if (!g_app_info_launch_default_for_uri(spec.get(), GetLaunchContext().get(),
                                          getter_Transfers(error))) {
     g_warning("Could not launch default application for URI: %s",
               error->message);
@@ -539,7 +549,7 @@ static nsresult LaunchPath(const nsACString& aPath) {
       g_file_new_for_commandline_arg(PromiseFlatCString(aPath).get()));
   GUniquePtr<char> spec(g_file_get_uri(file));
   GUniquePtr<GError> error;
-  g_app_info_launch_default_for_uri(spec.get(), nullptr,
+  g_app_info_launch_default_for_uri(spec.get(), GetLaunchContext().get(),
                                     getter_Transfers(error));
   if (error) {
     g_warning("Cannot launch default application: %s", error->message);

widget/gtk/GRefPtr.h

@@ -37,6 +37,7 @@ GOBJECT_TRAITS(GSimpleAction)
 GOBJECT_TRAITS(GSimpleActionGroup)
 GOBJECT_TRAITS(GDBusProxy)
 GOBJECT_TRAITS(GAppInfo)
+GOBJECT_TRAITS(GAppLaunchContext)
 GOBJECT_TRAITS(GdkDragContext)
 GOBJECT_TRAITS(GdkPixbuf)