[RFC] Repository.get_file_texts API and planning for it

[Robert Collins]

Aaron and I chatted on IRC earlier today. We'd like to add an interface
which will extract a number of file texts from a repository in whatever
manner is best for that repository, calling back to a tree transform (or
possibly a callback ?) for each text.

This lets the repository optimise access. For instance, an RPC based
repository like RemoteRepository might ask for a stream of texts. A knit
based one might group all the texts with the same disk hash prefix to
maximise locality of reference. And a pack based one will group readvs
to do single passes over each pack (as much as possible).

This needs changes to Repository - adding the new method (we need a good
name - its not a regular getter, because it calls back with the text, or
the text lines, or a file object - we should decide what is least
friction here too).

And code like checkout, merge, revert will need to learn about it.

the proposed signature is:
def FOO(self, requestor, file_details):
    """Extract a number of historyical file texts.

    :param requestor: An object which offers a create_file method which
        will be called for each element of file_details, though not
        necessarily in the order supplied. The create_file method has 
        the signature (callback_data, bytes).
    :param file_details: An iterable containing tuples 
        (text_key, callback_data). text_key is a (file_id, revision_id) 
        tuple, and callback_data is opaque to this function and simply
        serves to identify the file to the requestor.
    """

I propose that I will write up an implementation for our generic
repository asap, and Aaron do the tree-transform related changes,
because I'll be writing the pack based version anyhow, I may as well do
both the generic and specialised versions.

-Rob

-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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