In r172919 I moved the GTK+ symbols mix check earlier, before the plugin is loaded and initialized. That made impossible to use the gtk3 plugin process to scan gtk2 plugins, because we need to load the plugin to get its metadata.
$ bin/WebKitPluginProcess -scanPlugin /usr/lib/mozilla/plugins/flash-mozilla.so
$ echo $?
$ bin/WebKitPluginProcess2 -scanPlugin /usr/lib/mozilla/plugins/flash-mozilla.so
Shockwave Flash 11.2 r202
application/x-shockwave-flash:swf:Shockwave Flash;application/futuresplash:spl:FutureSplash Player
We need to load and initialize the plugin to get its metadata, but not to check if it requires GTK2, so we can move that check to the UI process to decide also which plugin process to use when scanning a plugin. This problem hasn't been noticed earlier thanks to the plugins cache, but after a flash plugin upgrade of my distro, the cache forces a new scan of the plugin that always fails.
Created attachment 238810 [details]
Committed r174054: <http://trac.webkit.org/changeset/174054>