[GStreamer] BubblewrapLauncher sandbox lacks directory for the gstrea…

…mer user registry cache directory

https://bugs.webkit.org/show_bug.cgi?id=268759

Reviewed by Michael Catanzaro.

Grant read-write access in the default GStreamer registry path within the WebProcess sandbox. This
is safe because the GStreamer registry file format is binary and the file is not executable. The
internal GStreamer code responsible for loading this file is able to handle parsing errors.

For additional context, the registry file stores informations about the plugins available on the
system, so that the plugin scanner doesn't need to rescan plugins every time GStreamer is
initialized.

* Source/WebKit/UIProcess/Launcher/glib/BubblewrapLauncher.cpp:
(WebKit::bindGStreamerData):

Canonical link: https://commits.webkit.org/274144@main

Source/WebKit/UIProcess/Launcher/glib/BubblewrapLauncher.cpp

@@ -423,10 +423,10 @@ static void bindGStreamerData(Vector<CString>& args)
 
     // The plugin scanner needs write permissions in the parent directory of GST_REGISTRY in order to
     // write the registry file.
-    if (const char* registryPath = g_getenv("GST_REGISTRY")) {
-        auto registryDir = FileSystem::parentPath(FileSystem::stringFromFileSystemRepresentation(registryPath));
-        bindIfExists(args, registryDir.utf8().data(), BindFlags::ReadWrite);
-    }
+    GUniquePtr<char> defaultRegistryPath(g_build_filename(g_get_user_cache_dir(), "gstreamer-1.0", nullptr));
+    const char* registryPath = environmentVariableValue("GST_REGISTRY", defaultRegistryPath.get());
+    auto registryDir = FileSystem::parentPath(FileSystem::stringFromFileSystemRepresentation(registryPath));
+    bindIfExists(args, registryDir.utf8().data(), BindFlags::ReadWrite);
 
     bindPathVar(args, "GST_PRESET_PATH");