Re: [PATCH v2] send-pack: never fetch when checking exclusions

From: Jonathan Tan <jonathantanmy@google.com>
Date: Oct 11, 2019, 22:08 UTC
Message-ID: <20191011220822.154063-1-jonathantanmy@google.com>
In-Reply-To: <20191011161504.GB19741@sigill.intra.peff.net>

Show 11 quoted lines

> As a general rule (and why I'm raising this issue in reply to Jonathan's
> patch), I think most or all sites that want OBJECT_INFO_QUICK will want
> SKIP_FETCH_OBJECT as well, and vice versa. The reasoning is generally
> the same:
> 
>   - it's OK to racily have a false negative (we'll still be correct, but
>     possibly a little less optimal)
> 
>   - it's expected and normal to be missing the object, so spending time
>     double-checking the pack store wastes measurable time in real-world
>     cases

I took a look on "next" and it's true for these reasons in most cases but not all.

QUICK implies SKIP_FETCH_OBJECT:

  fetch-pack.c: Run with fetch_if_missing=0 (from builtin/fetch.c,
  builtin/fetch-pack.c, or through a lazy fetch) so OK.
  
  builtin/index-pack.c: Run with fetch_if_missing=0, so OK.
  
  builtin/fetch.c: Run with fetch_if_missing=0, so OK.
  
  object-store.h, sha1-file.c: Definition and implementation of this
  flag.

Everything is OK here. Now, SKIP_FETCH_OBJECT implies QUICK:

  cache-tree.c: I added this recently in f981ec18cf ("cache-tree: do not
  lazy-fetch tentative tree", 2019-09-09). No problem with a false
  negative, since we know how to reconstruct the tree. OK.
  
  object-store.h, sha1-file.c: Definition and implementation of this
  flag.
  
  send-pack.c: This patch (which is already in "next"). If we have a
  false negative, we might accidentally send more than we need. But that
  is not too bad.
  
  promisor-remote.c: This is the slightly tricky one. We use this
  information to determine if we got our lazily-fetched object from the
  most recent lazy fetch, or if we should continue attempting to fetch the
  given object from other promisor remotes; so this information is
  important. However, adding QUICK doesn't lose us anything because the
  lack of QUICK only helps us when there is another process packing
  loose objects: if we got our object, our object will be in a pack
  (because of the way the fetch is implemented - in particular, we need
  a pack because we need the ".promisor" file).

So everything is OK except for promisor-remote.c, but even that is OK for another reason.

Having said that, perhaps we should consider promisor-remote.c as low-level code and expect it to know that objects are fetched into a packfile (as opposed to loose objects), so it can safely use QUICK (which is documented as checking packed after packed and loose). If no one disagrees, I can make such a patch after jt/push-avoid-lazy-fetch is merged to master (as is the plan, according to What's Cooking [1]).

[1] https://public-inbox.org/git/xmqq8sprhgzc.fsf@gitster-ct.c.googlers.com/