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Out, You Wretched, Corrupted Cache Entry... OUT!

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Out, You Wretched, Corrupted Cache Entry... OUT!

A developer talks about a performance issue he was having in is Firefox browser, and how he got the root of the problem.

· Performance Zone ·
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While I'm a Firefox fan, I often run into tiny issues of the browser, many of which cannot be reproduced in clean environments (and hence are somehow related to the dozens of customizations and the horde of add-ons that I take for granted).

I recently nailed one that had been bugging me for well over three years—practically ever since I discovered FF's offline mode.

While the offline mode does an excellent job almost all the time, sometimes it can screw up your cache entries so bad that the only way out is a full cache clear. This often happens if you place the browser in offline mode while a resource is still loading (CSS, JS, font,... and sometimes even the main HTML page, especially in the case of Wikipedia).

If you are unfortunate enough to run into such a mess, from then onwards, whenever you load the page from cache, the cache responds with the partially fetched (hence partially cached) broken resource—apparently a known bug. No matter how many times you refresh—even in online mode—the full version of the resource will not get cached (the browser would fetch the full resource and just discard it secretly, coughing up the corrupted entry right away during the next offline fetch).

Although FF has a "Forget about this site" option that could have shed some light (as you could simply ask the browser to clear just that page from the cache), the feature is bugged as well, and ends up clearing your whole cache anyway; so you have no easy way of discarding the corrupted entry in isolation.

And the ultimate and unfortunate solution, for getting the site to work again, would be to drop several hundred megabytes of cache, so that the browser could start from zero; or to stop using the site until the expiry time of the resource is hit, which could potentially be months ahead in the future.

The good news is, FF's Cache2 API allows you to access the offending resource by URL, and kick it out of the cache. The bad news, on the other hand, is that although there are a few plugins that allow you to do this by hand, all of them are generic cache-browsing solutions, so they take forever to iterate through the browser cache and build the entry index, during which you cannot practically do anything useful. I don't know how things would be on a fast disk like an SSD, but on my 5400-RPM magnetic disk, it takes well over 5 minutes to populate the list.

But since you already know the URL of the resource, why not invoke the Cache2 API directly with a few lines of code, and kick the bugger out yourself?

// load the disk cache
var cacheservice = Components.classes["@mozilla.org/netwerk/cache-storage-service;1"]
    .getService(Components.interfaces.nsICacheStorageService);
var {LoadContextInfo} = Components.utils.import("resource://gre/modules/LoadContextInfo.jsm",{})
var hdcache = cacheservice.diskCacheStorage(LoadContextInfo.default, true);

// compose the URL and submit it for dooming
var uri = Components.classes["@mozilla.org/network/io-service;1"]
    .getService(Components.interfaces.nsIIOService).newURI(prompt("Enter the URL to kick out:"), null, null);
hdcache.asyncDoomURI(uri, null, null);

Yes, that's all. Once the script is run on the browser console, with uri populated with the URL of the offending resource (which in this case is read in using a JS prompt()), poof! You just have to reload the resource (usually by loading the parent HTML page), taking care not to hit the offline mode prematurely, to get the site working fine again.

And that's the absolute beauty of Firefox.

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Topics:
browser cache ,javascript ,browser bugs ,cache eviction ,performance

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