[webkit-dev] Limiting slow unload handlers (Re: Back/forward cache for pages with unload handlers)

Maciej Stachowiak mjs at apple.com
Thu Sep 17 16:17:04 PDT 2009

On Sep 17, 2009, at 3:35 PM, Brady Eidson wrote:

> On Sep 17, 2009, at 2:17 PM, Ojan Vafai wrote:
>> On Thu, Sep 17, 2009 at 2:11 PM, John Abd-El-Malek <jam at google.com>  
>> wrote:
>> On Thu, Sep 17, 2009 at 2:09 PM, Darin Fisher <darin at chromium.org>  
>> wrote:
>> On Thu, Sep 17, 2009 at 12:52 PM, Maciej Stachowiak <mjs at apple.com>  
>> wrote:
>> Based on your comments below, I think the expedient thing to do is  
>> to let Image loads (only) complete their I/O when initiated from  
>> unload or pagehide.
>> Why exclude beforeunload?  Some of the sites we found use the busy  
>> loop hack in beforeunload.
>> These sites presumably did it to split the sleep calls across as  
>> many handlers as possible to avoid hung script detectors.  If they  
>> rewrite their pages to use one clean method, it seems they only  
>> need to do it in one place.
>> There a plenty of sites that only have a beforeunload handler and  
>> have no reason to register an unload handler. If you're not  
>> prompting the user to stay on the page, the two events are  
>> essentially equivalent.
> This is not true.
> I couldn't test IE right now but for Safari, Opera, and Firefox, I  
> just independently verified that:
> -unload handlers prevent a page from going into the page cache.
> -beforeunload handlers do not
> -beforeunload handlers are called before a page is navigated away  
> from, even when the page is going into the page cache.
> So the key difference is that while an unload handler will *only*  
> ever run once, when a page is destroyed, a beforeunload handler  
> might run multiple times, each time the user navigates away from the  
> page after returning to it.
> This might not be immediately relevant to the conversation, but they  
> are not the same and I'd rather the conversation not assume they are.
> In practice, if an author uses beforeunload as their unload handler,  
> they are doing The Wrong Thing(tm)

Another obvious difference is that the beforeunload event is  
dispatched earlier in the navigation process and its handlers may  
cancel the navigation.

  - Maciej

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