Re: [PATCH] git-daemon server

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"JCH" == Junio C Hamano [off-list ref] writes:

JCH> Looks very nice.  Some comments.

JCH> This being a dedicated GIT specific sync mechanism, you may want
JCH> to give more smarts to the server, so that the client can say "I
JCH> have these commits as HEADs in my forest, here are their SHA1s,
JCH> now sync me up to the head you said you have whose SHA1 is
JCH> this", implying he has all their HEADs dependents.  Of course
JCH> this can come later.

About the protocol, here is one change you may want to have even
in the initial version to futureproof yourself, and let you make
a "low hanging fruit" optimization without bumping the protocol
version up.

Make "request" capable to optionally have this format:

	request <sha1> <commit-sha1> <ce-path>

Note.  You have to make sure you have some way to quote embedded
newlines in ce-path since your protocol is mostly line based.

When this optional form is used, the requestor is telling the
responder the following:

    (1) it wants to retrieve <sha1>; this is the same as a
        request without the optional two fields.

    (2) it wants <sha1> because it is trying to complete a tree
        associated with <commit-sha1>; it already has the commit
        object itself and knows what the parents of the commit
        are.

    (3) it already has trees and blobs associated with all the
        parents of <commit-sha1>.

    (4) it knows that <sha1> resides at <ce-path> in the tree
        associated with <commit-sha1>.  As a special case, ""
        (an empty string) as <ce-path> means "the root level
        tree object associated with <commit-sha1>".

The initial implementation of a requestor does not even send
this extended form.  The initial implementation of a responder
must be able to parse this extended form, but it does not have
to do anything special about it; just do what your cmd_request()
currently does.  However, this extended request lets your later
implementation of the responder create and send delta on the
fly, by:

    (0) Look at <sha1> in the local storage.  If it is already
        deltified, do not do anything special but just send it
        out.

    (1) Look at <commit-sha1> and its parents.  Compare the
        object (either a blob or a tree) that corresponds to
        <ce-path> in the trees associated with these commits.
        Verify <sha1> is indeed what the requestor thinks it is
        while you are at it.

    (2) Try to synthesize a reasonable delta to create <sha1>
        based on the objects you find in step (1).  Upon finding
        a reasonable delta, send that as a delta object to the
        requestor.  Optionally you may want to replace the
        <sha1> found at the local store in step (0) with this
        delta.  If you have many parents, this "reasonable"
        delta does not necessarily have to be the minimal delta.

Unlike a full-blown "ihave/sendme" protocol extension, this does
not require responder side to keep much client state, and should
give you the ability to create and send a reasonable if not
minimum delta lazily.

Hmm.