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Commits on Apr 20, 2015
  1. @peff @gitster

    reachable: only mark local objects as recent

    peff authored gitster committed
    When pruning and repacking a repository that has an
    alternate object store configured, we may traverse a large
    number of objects in the alternate. This serves no purpose,
    and may be expensive to do. A longer explanation is below.
    
    Commits d3038d2 and abcb865 taught prune and pack-objects
    (respectively) to treat "recent" objects as tips for
    reachability, so that we keep whole chunks of history. They
    built on the object traversal in 660c889 (sha1_file: add
    for_each iterators for loose and packed objects,
    2014-10-15), which covers both local and alternate objects.
    
    In both cases, covering alternate objects is unnecessary, as
    both commands can only drop objects from the local
    repository. In the case of prune, we traverse only the local
    object directory. And in the case of repacking, while we may
    or may not include local objects in our pack, we will never
    reach into the alternate with "repack -d". The "-l" option
    is only a question of whether we are migrating objects from
    the alternate into our repository, or leaving them
    untouched.
    
    It is possible that we may drop an object that is depended
    upon by another object in the alternate. For example,
    imagine two repositories, A and B, with A pointing to B as
    an alternate. Now imagine a commit that is in B which
    references a tree that is only in A. Traversing from recent
    objects in B might prevent A from dropping that tree. But
    this case isn't worth covering. Repo B should take
    responsibility for its own objects. It would never have had
    the commit in the first place if it did not also have the
    tree, and assuming it is using the same "keep recent chunks
    of history" scheme, then it would itself keep the tree, as
    well.
    
    So checking the alternate objects is not worth doing, and
    come with a significant performance impact. In both cases,
    we skip any recent objects that have already been marked
    SEEN (i.e., that we know are already reachable for prune, or
    included in the pack for a repack). So there is a slight
    waste of time in opening the alternate packs at all, only to
    notice that we have already considered each object. But much
    worse, the alternate repository may have a large number of
    objects that are not reachable from the local repository at
    all, and we end up adding them to the traversal.
    
    We can fix this by considering only local unseen objects.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
Commits on Oct 19, 2014
  1. @peff @gitster

    reachable: use revision machinery's --indexed-objects code

    peff authored gitster committed
    This does the same thing as our custom code, so let's not
    repeat ourselves.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
Commits on Oct 16, 2014
  1. @peff @gitster

    pack-objects: match prune logic for discarding objects

    peff authored gitster committed
    A recent commit taught git-prune to keep non-recent objects
    that are reachable from recent ones. However, pack-objects,
    when loosening unreachable objects, tries to optimize out
    the write in the case that the object will be immediately
    pruned. It now gets this wrong, since its rule does not
    reflect the new prune code (and this can be seen by running
    t6501 with a strategically placed repack).
    
    Let's teach pack-objects similar logic.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
  2. @peff @gitster

    prune: keep objects reachable from recent objects

    peff authored gitster committed
    Our current strategy with prune is that an object falls into
    one of three categories:
    
      1. Reachable (from ref tips, reflogs, index, etc).
    
      2. Not reachable, but recent (based on the --expire time).
    
      3. Not reachable and not recent.
    
    We keep objects from (1) and (2), but prune objects in (3).
    The point of (2) is that these objects may be part of an
    in-progress operation that has not yet updated any refs.
    
    However, it is not always the case that objects for an
    in-progress operation will have a recent mtime. For example,
    the object database may have an old copy of a blob (from an
    abandoned operation, a branch that was deleted, etc). If we
    create a new tree that points to it, a simultaneous prune
    will leave our tree, but delete the blob. Referencing that
    tree with a commit will then work (we check that the tree is
    in the object database, but not that all of its referred
    objects are), as will mentioning the commit in a ref. But
    the resulting repo is corrupt; we are missing the blob
    reachable from a ref.
    
    One way to solve this is to be more thorough when
    referencing a sha1: make sure that not only do we have that
    sha1, but that we have objects it refers to, and so forth
    recursively. The problem is that this is very expensive.
    Creating a parent link would require traversing the entire
    object graph!
    
    Instead, this patch pushes the extra work onto prune, which
    runs less frequently (and has to look at the whole object
    graph anyway). It creates a new category of objects: objects
    which are not recent, but which are reachable from a recent
    object. We do not prune these objects, just like the
    reachable and recent ones.
    
    This lets us avoid the recursive check above, because if we
    have an object, even if it is unreachable, we should have
    its referent. We can make a simple inductive argument that
    with this patch, this property holds (that there are no
    objects with missing referents in the repository):
    
      0. When we have no objects, we have nothing to refer or be
         referred to, so the property holds.
    
      1. If we add objects to the repository, their direct
         referents must generally exist (e.g., if you create a
         tree, the blobs it references must exist; if you create
         a commit to point at the tree, the tree must exist).
         This is already the case before this patch. And it is
         not 100% foolproof (you can make bogus objects using
         `git hash-object`, for example), but it should be the
         case for normal usage.
    
         Therefore for any sequence of object additions, the
         property will continue to hold.
    
      2. If we remove objects from the repository, then we will
         not remove a child object (like a blob) if an object
         that refers to it is being kept. That is the part
         implemented by this patch.
    
         Note, however, that our reachability check and the
         actual pruning are not atomic. So it _is_ still
         possible to violate the property (e.g., an object
         becomes referenced just as we are deleting it). This
         patch is shooting for eliminating problems where the
         mtimes of dependent objects differ by hours or days,
         and one is dropped without the other. It does nothing
         to help with short races.
    
    Naively, the simplest way to implement this would be to add
    all recent objects as tips to the reachability traversal.
    However, this does not perform well. In a recently-packed
    repository, all reachable objects will also be recent, and
    therefore we have to look at each object twice. This patch
    instead performs the reachability traversal, then follows up
    with a second traversal for recent objects, skipping any
    that have already been marked.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
  3. @peff @gitster

    reachable: mark index blobs as SEEN

    peff authored gitster committed
    When we mark all reachable objects for pruning, that
    includes blobs mentioned by the index. However, we do not
    mark these with the SEEN flag, as we do for objects that we
    find by traversing (we also do not add them to the pending
    list, but that is because there is nothing further to
    traverse with them).
    
    This doesn't cause any problems with prune, because it
    checks only that the object exists in the global object
    hash, and not its flags. However, let's mark these objects
    to be consistent and avoid any later surprises.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
  4. @peff @gitster

    reachable: reuse revision.c "add all reflogs" code

    peff authored gitster committed
    We want to add all reflog entries as tips for finding
    reachable objects. The revision machinery can already do
    this (to support "rev-list --reflog"); we can reuse that
    code.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
  5. @peff @gitster

    reachable: use traverse_commit_list instead of custom walk

    peff authored gitster committed
    To find the set of reachable objects, we add a bunch of
    possible sources to our rev_info, call prepare_revision_walk,
    and then launch into a custom walker that handles each
    object top. This is a subset of what traverse_commit_list
    does, so we can just reuse that code (it can also handle
    more complex cases like UNINTERESTING commits and pathspecs,
    but we don't use those features).
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
Commits on Jun 6, 2013
  1. @peff @gitster

    clear parsed flag when we free tree buffers

    peff authored gitster committed
    Many code paths will free a tree object's buffer and set it
    to NULL after finishing with it in order to keep memory
    usage down during a traversal. However, out of 8 sites that
    do this, only one actually unsets the "parsed" flag back.
    Those sites that don't are setting a trap for later users of
    the tree object; even after calling parse_tree, the buffer
    will remain NULL, causing potential segfaults.
    
    It is not known whether this is triggerable in the current
    code. Most commands do not do an in-memory traversal
    followed by actually using the objects again. However, it
    does not hurt to be safe for future callers.
    
    In most cases, we can abstract this out to a
    "free_tree_buffer" helper. However, there are two
    exceptions:
    
      1. The fsck code relies on the parsed flag to know that we
         were able to parse the object at one point. We can
         switch this to using a flag in the "flags" field.
    
      2. The index-pack code sets the buffer to NULL but does
         not free it (it is freed by a caller). We should still
         unset the parsed flag here, but we cannot use our
         helper, as we do not want to free the buffer.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
Commits on Mar 17, 2013
  1. @peff @gitster

    use parse_object_or_die instead of die("bad object")

    peff authored gitster committed
    Some call-sites do:
    
      o = parse_object(sha1);
      if (!o)
    	  die("bad object %s", some_name);
    
    We can now handle that as a one-liner, and get more
    consistent output.
    
    In the third case of this patch, it looks like we are losing
    information, as the existing message also outputs the sha1
    hex; however, parse_object will already have written a more
    specific complaint about the sha1, so there is no point in
    repeating it here.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
Commits on Nov 8, 2011
  1. @peff @gitster

    reachable: per-object progress

    peff authored gitster committed
    The current progress code really just counts commits.
    This patch makes it count all objects, giving us a "total"
    count close to what a repack would show. This is nice when
    using "git gc", which will usually have just repacked the
    whole repo.
    
    Signed-off-by: Jeff King <peff@peff.net>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
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