[wp-trac] [WordPress Trac] #7691: autosave newer version check can be slow enough to cause edit tab to time out

[WordPress Trac]

#7691: autosave newer version check can be slow enough to cause edit tab to time
out
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 Reporter:  tellyworth  |       Owner:  anonymous
     Type:  defect      |      Status:  new      
 Priority:  normal      |   Milestone:  2.6.2    
Component:  General     |     Version:           
 Severity:  normal      |    Keywords:           
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 in edit-form-advanced.php there's a call to wp_text_diff() that checks for
 a newer autosave revision that is different (around line 48).  Under
 certain circumstances this can cause a loop that will eat CPU for several
 minutes and prevent the edit post tab from opening.

 Specifically it happens if two revisions are very long, have many lines,
 and every line is different between the two revs.  I can reproduce this in
 unit testing with two different revisions, each with 10,000 lines of 50
 characters.  The real-world circumstances that triggered this was a very
 long post (approx 750kb) where one revision contained HTML tags, and the
 other revision had html special chars converted to entities.  That caused
 virtually every line to be different.

 There are three separate issues here:

 1. edit-form-advanced.php calls wp_text_diff() (a heavyweight function for
 displaying the differences between posts) when all it really needs is a
 fuzzy text comparison function.

 2. wp_text_diff() calls WP_Text_Diff_Renderer_Table, which is very
 inefficient under these circumstances.  I don't know enough about that
 code to know the cause or appropriate fix, but it seems to get stuck in
 the compute_string_distance() loop at the top of
 interleave_changed_lines().  It's not an infinite loop as far as I can
 tell, just very inefficient in this particular case.

 3. Under some unknown circumstances, revisions are saved with special
 chars entity-encoded.  That will screw with versioning diffs even if the
 above two issues are fixed.

 Working on a fix for 1.  I'll leave 2 and 3 for folks with specific
 knowledge about those things.

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Ticket URL: <http://trac.wordpress.org/ticket/7691>
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