Re: [RFC PATCH] During a shallow fetch, prevent sending over unneeded objects

Matthijs Kooijman [off-list ref] writes:

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In your discussion (including the comment), you talk about "shallow
root" (I think that is the same as what we call "shallow boundary"),

I think so, yes. I mean to refer to the commits referenced in
.git/shallow, that have their parents "hidden".

Could you confirm that I got the terms right here (or is the shallow
boundary the first hidden commit?)

As long as you are consistent it is fine. I _think_ boundary refers
to what is recorded in the .git/shallow file, so they are commits
that are missing from our repository, and their immediate children
are available.

My proposal was to only apply the fix for all have revisions when the
previous history traversal came across some shallow boundary commits. If
this happens, then that shallow boundary commit will be a "new" one and
it will have prevented the history traversal from finding the full list
of relevant "have" commits. In this case, we should just use all "have"
commits instead.

Now, looking at the code, I see a few options for detecting this case:

 1 Modify mark_edges_uninteresting to return a boolean (or have an
   output argument) if any of the commits in the list of commits to find
   (not the edges) is a shallow boundary.
 2 Modify mark_edges_uninteresting to have a "show_shallow" argument
   that gets called for every shallow boundary. The show_shallow
   function passed would then simply keep a boolean if it is passed at
   least once.
 3 Add another loop over the commits _after_ the call to
   mark_edges_uninteresting, that simply looks for any shallow boundary
   commit.

The last option seems sensible to me, since it prevents modifying the
somewhat generic mark_edges_uninteresting function for this specific
usecase. On the other hand, it does mean that the list of commits is
looped twice, not sure what that means for performance.

Before I go and implement one of these, which option seems best to you?

My gut feeling without looking at any patch is that the simplest
(i.e. 3.) would be the best among these three.

But I suspect, with any of these approaches, you would need to be
very careful futzing with the edge ones.  It may have an interesting
interactions with --thin transfer.