How can I undo a change from a modification-hook?

[Clément Pit--Claudel]

Hey emacs-devel,

I'm using overlays to keep track of buffer regions that have been sent to a 
subprocess.  Edits in a previously sent ("processed") region are generally 
forbidden — the only two permissible kinds of changes in processed regions are 
editing a comment and adding or removing of space at the end of a line.

Adding a modification-hook to my overlays makes it easy to disallow changes (by 
raising an error), and to allow changes when in a comment.  I can't find a good 
way to allow whitespace edits at the end of a line, though.  I can detect such 
changes from the second call to the modification hook (during which it's easy 
to know whether the change is only adding or removing space at the end of a 
line), but I can't easily undo the changes and error out if they involve more 
than whitespace, can I?

What's the right way do disallow certain modifications in text covered by an 
overlay, in particular when the determination of whether to allow a given 
modification cannot be made during the first call to the overlay's modification 
hook?  For example, how can I make an overlay that allows insertion or deletion 
of space at the end of the lines that it covers, but no other modifications?

Thanks!
Clément.

smime.p7s
Description: S/MIME Cryptographic Signature