Re: [BUG?] fetch into shallow sends a large number of objects

Jeff King [off-list ref] writes:

I don't know how the client invoked git, but we can guess what happened
and simulate with:

  git tag shallow ecd7ea6033fe8a05d5c21f3a54355fded6942659
  git tag old 067f265bb512c95b22b83ccd121b9facbddcf6b1
  git tag new d7a6d9295d718c6015be496880f1a293bdd89185

  git clone --no-local --bare --branch=shallow --depth=1 . clone.git
  cd clone.git
  git fetch origin old:refs/tags/old
  git fetch origin new:refs/tags/new

Of the two follow-up fetches in the clone, the first is reasonably fast
(it just grabs a few new commits on top of the shallow base), but the
second is expensive (it grabs the merge which pulls in the whole
history). If we add "--depth=1" to each of those fetches, everything
remains fast.

Is this user error to call "git fetch" without "--depth" in the
subsequent cases? Or should git remember that we are in a shallow repo,
and presume that the user by default wants to keep things shallow?

Hmph, you shouldn't, and I somehow thought that you do not, have to
explicitly say things like "--deepen" to break the original
shallowness, but your example illustrates that the logic to do so is
not well thought out.  A new side branch will prevent you from
hitting an already-known shallow cut-off and traverse down to the
root.

Giving a random "depth" in subsequent fetch would however not work
very well, I suspect, as that is very prone to make the part of the
history the user originally obtained, and presumably used to build
her own history, into an island that is unconnected to the updated
tip of the history.  

I also do not offhand think of a good way to use the topology or
timestamp to figure out the best "depth" to truncate the side branch
at.  The server side may be able to figure out that things before 'F'
in your picture is not relevant for a client that has the shallow
cut-off at 067f265, but the side branch can be forked arbitrarily
long in the past, or it may not even share the ancient part of the
history and has its own root commit.