When a reference that existed in the packed-refs file is deleted, the
packed-refs file must be rewritten. Previously, the file was
rewritten without any peeled refs, even if the file contained peeled
refs when it was read. This was not a bug, because the packed-refs
file header didn't claim that the file contained peeled values. But
it had a performance cost, because the repository would lose the
benefit of having precomputed peeled references until pack-refs was
run again.
Teach repack_without_ref() to write peeled refs to the packed-refs
file (regardless of whether they were present in the old version of
the file).
This means that if the old version of the packed-refs file was not
fully peeled, then repack_without_ref() will have to peel references.
To avoid the expense of reading lots of loose references, we take two
shortcuts relative to pack-refs:
* If the peeled value of a reference is already known (i.e., because
it was read from the old version of the packed-refs file), then
output that peeled value again without any checks. This is the
usual code path and should avoid any noticeable overhead. (This is
different than pack-refs, which always re-peels references.)
* We don't verify that the packed ref is still current. It could be
that a packed references is overridden by a loose reference, in
which case the packed ref is no longer needed and might even refer
to an object that has been garbage collected. But we don't check;
instead, we just try to peel all references. If peeling is
successful, the peeled value is written out (even though it might
not be needed any more); if not, then the reference is silently
omitted from the output.
The extra overhead of peeling references in repack_without_ref()
should only be incurred the first time the packed-refs file is written
by a version of Git that knows about the "fully-peeled" attribute.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add a test that demonstrates that the peeled values recorded in
packed-refs are lost if a packed ref is deleted. (The code in
repack_without_ref() doesn't even attempt to write peeled refs.) This
will be fixed in a moment.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Older versions of pack-refs did not write peel lines for
refs outside of refs/tags. This meant that on reading the
pack-refs file, we might set the REF_KNOWS_PEELED flag for
such a ref, even though we do not know anything about its
peeled value.
The previous commit updated the writer to always peel, no
matter what the ref is. That means that packed-refs files
written by newer versions of git are fine to be read by both
old and new versions of git. However, we still have the
problem of reading packed-refs files written by older
versions of git, or by other implementations which have not
yet learned the same trick.
The simplest fix would be to always unset the
REF_KNOWS_PEELED flag for refs outside of refs/tags that do
not have a peel line (if it has a peel line, we know it is
valid, but we cannot assume a missing peel line means
anything). But that loses an important optimization, as
upload-pack should not need to load the object pointed to by
refs/heads/foo to determine that it is not a tag.
Instead, we add a "fully-peeled" trait to the packed-refs
file. If it is set, we know that we can trust a missing peel
line to mean that a ref cannot be peeled. Otherwise, we fall
back to assuming nothing.
[commit message and tests by Jeff King <peff@peff.net>]
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When we pack an annotated tag ref, we write not only the
sha1 of the tag object along with the ref, but also the sha1
obtained by peeling the tag. This lets readers of the
pack-refs file know the peeled value without having to
actually load the object, speeding up upload-pack's ref
advertisement.
The writer marks a packed-refs file with peeled refs using
the "peeled" trait at the top of the file. When the reader
sees this trait, it knows that each ref is either followed
by its peeled value, or it is not an annotated tag.
However, there is a mismatch between the assumptions of the
reader and writer. The writer will only peel refs under
refs/tags, but the reader does not know this; it will assume
a ref without a peeled value must not be a tag object. Thus
an annotated tag object placed outside of the refs/tags
hierarchy will not have its peeled value printed by
upload-pack.
The simplest way to fix this is to start writing peel values
for all refs. This matches what the reader expects for both
new and old versions of git.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
