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[DOM4] atomicity of DocumentFragment insertion

From: David Flanagan <dflanagan@mozilla.com>
Date: Fri, 28 Oct 2011 14:47:24 -0700
Message-ID: <4EAB22EC.5090002@mozilla.com>
To: www-dom@w3.org
I think the DOM4 spec should be clearer about the atomicity (or 
non-atomicity) of DocumentFragment insertion.  I assume that when a 
DocumentFragment is inserted, scripts should never be able to observe 
the document in state where some of the children of the fragment have 
been inserted and others have not.  But I don't think the spec currently 
states that anywhere, and I think it should.

This may be an issue for mutation events, and it also turns out to be an 
issue for HTML script tags, which execute when children are inserted.  
For example, what is the proper behavior of this code?:

     // s and f are global variables
     s = document.createElement("script");
     document.head.appendChild(s);
     f = document.createDocumentFragment();
     f.appendChild(document.createTextNode("alert(s.text); 
alert(f.childNodes.length);"));
     f.appendChild(document.createTextNode("alert(2);"));
     f.appendChild(document.createTextNode("alert(3);"));
     s.appendChild(f);
     alert(s.text);

Firefox runs the code in all 3 text nodes and displays 5 alerts total. 
The first and last alerts both contain the concatenated text of all 
three text nodes. The second alert display 0 because there are no 
children left in the document fragment.

But Chrome, Safari and Opera run only the code in the first text node 
and display 3 alerts. The first alert displays only the first text node, 
the second alert displays 2 because one child has been moved from the 
fragment into the script, and the third alert displays all 3 text nodes.

So, Firefox inserts the DocumentFragment atomically and the other 
browsers (I don't have a Windows installation to test IE) insert it 
non-atomically and allow the document to be observed in a partially 
inserted state.

I've raised this issue (with a slightly simpler example) on the 
whatwg.org list: 
http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2011-October/033705.html 
but I suspect that DOM4 might actually be the right place to fix it.

The DOM4 spec should either adopt language like "the document is never 
observable in a partially-inserted state" or it should simply define 
operations on document fragments using recursion so that it is clear 
that calling appendChild on a document fragment is exactly the same as 
sequentially calling appendChild on its children.

     David
Received on Friday, 28 October 2011 21:48:03 GMT

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