Jim's Depository

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If you find yourself making ajax requests for HTML pages, then using jquery to massage them or extract data before displaying it, you should be aware that you may be attempting to load the <img> resources from the page. This can be a performance killer or information leak.

Consider…

function parseSomeDudesPage( pageText) {
    var page = $(pageText);   // <<<<------   This is the problem
    ... do a bunch of stuff with page...
}

...
   $.get('http://somedudeswebsite.com/blah/blah.html', parseSomeDudesPage);
...

That line marked as the problem will parse the text into DOM nodes which will be in your main document. The browser may decide to start fetching the images contained in pageText, (Safari 8.0.2 does). Depending how you fetched the document you could be getting images, broken links, or security domain violations for pulling http: resources into your https: page.

I think the solution is to keep the nodes out of your main document by creating a new one. Reading this stackoverflow comment and not caring about old browsers in my application I went with…

function parseSomeDudesPage( pageText) {
    var page = (new DOMParser).parseFromString( pageText, 'text/html');
    ... do a bunch of stuff with page which is a new document and doesn't trigger <img> loading.
}

Since you have a new document, you won’t be able to move nodes from page to your main document, but I tend to create nodes anyway, so I don’t miss the ability.

A quick warning: When you upgrade your Debian installation to Jessie, you may find the grub2 can no longer install itself and you are left with an unbootable system. (Been there, done that, managed to save myself.)

Preconditions:

  • Your disks have been MBR partitioned so far in the past that there isn’t a megabyte hiding between the MBR and your first partition.

  • Your /boot is sitting on something which requires more than a tiny amount of code to access. In my case ext3 on an LVM partition.

When grub2 tries to update you will get messages about core.img being too big. At this point you may be screwed.

Am I safe?

If you are MBR partitioned, then check where your first partition is. If it starts at block 2048, you have plenty of space. If it is 63, you are in trouble.

If you are GUID partitioned, make sure you have a 1MB partition for boot loader.

How I recovered:

Since I was using LVM I plowed all the data off of my /dev/sda spindle using pvmove (many times), that involved de-mirroring things since I only had three spindles in the machine. Then I could repartition the drive. I went to GUID partitioning, where you explicitly make a boot loader partition for the use of grub2.

Then I could create an LVM physical volume on the remaining space of /dev/sda and shuffle all my data back into mirrors. The process was more complicated since I went through and reformatted each of the drives in case one blows and the BIOS chooses another, but all told it is possible to perform without a reboot.

I have a web site which displays a remote camera view. I like to update the <img> once a minute or so without forcing a page reload and all of its flicker. I’ve done many horrible things over the years, mostly related to adding a query parameter to the end of the URL and accepting that I will trash the cache and perform redundant downloads when the image is not changing.

The <img> tag is 21 years old, surely if it can drink in the USA it has had a .refresh() method added. brief check of specifications: and no.

stackoverflow is littered with questions and solutions about how to reload an <img>, most of them involve cache busting serial numbers in the query parameters.

Notable exceptions:

  • Some answers recommend using a hash tag serial, since this should leave the URL to the server unchanged. This works in some browsers, but doesn’t cause a reload in others. Sometimes there is a combination of HTTP Cache-control headers that will cause a reload, sometimes not. Too touchy to use.

  • You can use an <iframe> containing just the URI of your image to force a reload using an actual, supported mechanism! When that completes you can bludgeon your <img> tag enough to use the new version. See Method #4. I have dispensed with replacing the <img> element and just set the src to null and back. This keeps Safari and Chrome from flickering in January of 2015.

The <iframe> method is my new favorite. It requires Javascript, but I’m not sure how you would be deciding to refresh without Javascript, so that isn’t a loss. From Safari, it does cause two requests for the resource, but the second will be a 304 if you are using any sane caching mechanisms, and the first will only be a 200 if it really changed. (We are in 2015 and I don’t worry so much about the extra request since the servers are all SPDY and its not like its a new TCP connection or blocking anything else.)

Note to W3C: Add .reload() method which tells an <img> to reconsider the caching information and make a new request if required. I suppose a force boolean argument wouldn’t be too much. But nothing more complicated.