what can a sequence of handler::index_XXX be interrupted by?

Hello all,

As best as I understand, a handler mostly does reads by performing the
following sequence of operations (not including rnd_XXX functions):
 - call handler::index_init
 - perform a bunch of handler::index_XXX calls
 - call handler::index_end

I am wondering what other handler operations may occur in between
index_init and index_end that may block a handler::index_XXX call.

For instance, in the case of deletes and updates, I know that the
handler that performs an index_XXX call may then perform
handler::delete_row or handler::update_row. So, a subsequent call to
handler::index_XXX may not occur until the delete_row or update_row is
complete.

Are there any other cases where the sequence of index_XXX calls may be
blocked? For instance, is there a chance that some other handler's
work may block execution of and index_XXX call? Can joins or index
merges do such a thing?

Thanks
-Zardosht

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