The peel_ref() interface is confusing and error-prone:
- it's typically used by ref iteration callbacks that have both a
refname and oid. But since they pass only the refname, we may load
the ref value from the filesystem again. This is inefficient, but
also means we are open to a race if somebody simultaneously updates
the ref. E.g., this:
int some_ref_cb(const char *refname, const struct object_id *oid, ...)
{
if (!peel_ref(refname, &peeled))
printf("%s peels to %s",
oid_to_hex(oid), oid_to_hex(&peeled);
}
could print nonsense. It is correct to say "refname peels to..."
(you may see the "before" value or the "after" value, either of
which is consistent), but mentioning both oids may be mixing
before/after values.
Worse, whether this is possible depends on whether the optimization
to read from the current iterator value kicks in. So it is actually
not possible with:
for_each_ref(some_ref_cb);
but it _is_ possible with:
head_ref(some_ref_cb);
which does not use the iterator mechanism (though in practice, HEAD
should never peel to anything, so this may not be triggerable).
- it must take a fully-qualified refname for the read_ref_full() code
path to work. Yet we routinely pass it partial refnames from
callbacks to for_each_tag_ref(), etc. This happens to work when
iterating because there we do not call read_ref_full() at all, and
only use the passed refname to check if it is the same as the
iterator. But the requirements for the function parameters are quite
unclear.
Instead of taking a refname, let's instead take an oid. That fixes both
problems. It's a little funny for a "ref" function not to involve refs
at all. The key thing is that it's optimizing under the hood based on
having access to the ref iterator. So let's change the name to make it
clear why you'd want this function versus just peel_object().
There are two other directions I considered but rejected:
- we could pass the peel information into the each_ref_fn callback.
However, we don't know if the caller actually wants it or not. For
packed-refs, providing it is essentially free. But for loose refs,
we actually have to peel the object, which would be wasteful in most
cases. We could likewise pass in a flag to the callback indicating
whether the peeled information is known, but that complicates those
callbacks, as they then have to decide whether to manually peel
themselves. Plus it requires changing the interface of every
callback, whether they care about peeling or not, and there are many
of them.
- we could make a function to return the peeled value of the current
iterated ref (computing it if necessary), and BUG() otherwise. I.e.:
int peel_current_iterated_ref(struct object_id *out);
Each of the current callers is an each_ref_fn callback, so they'd
mostly be happy. But:
- we use those callbacks with functions like head_ref(), which do
not use the iteration code. So we'd need to handle the fallback
case there, anyway.
- it's possible that a caller would want to call into generic code
that sometimes is used during iteration and sometimes not. This
encapsulates the logic to do the fast thing when possible, and
fallback when necessary.
The implementation is mostly obvious, but I want to call out a few
things in the patch:
- the test-tool coverage for peel_ref() is now meaningless, as it all
collapses to a single peel_object() call (arguably they were pretty
uninteresting before; the tricky part of that function is the
fast-path we see during iteration, but these calls didn't trigger
that). I've just dropped it entirely, though note that some other
tests relied on the tags we created; I've moved that creation to the
tests where it matters.
- we no longer need to take a ref_store parameter, since we'd never
look up a ref now. We do still rely on a global "current iterator"
variable which _could_ be kept per-ref-store. But in practice this
is only useful if there are multiple recursive iterations, at which
point the more appropriate solution is probably a stack of
iterators. No caller used the actual ref-store parameter anyway
(they all call the wrapper that passes the_repository).
- the original only kicked in the optimization when the "refname"
pointer matched (i.e., not string comparison). We do likewise with
the "oid" parameter here, but fall back to doing an actual oideq()
call. This in theory lets us kick in the optimization more often,
though in practice no current caller cares. It should never be
wrong, though (peeling is a property of an object, so two refs
pointing to the same object would peel identically).
- the original took care not to touch the peeled out-parameter unless
we found something to put in it. But no caller cares about this, and
anyway, it is enforced by peel_object() itself (and even in the
optimized iterator case, that's where we eventually end up). We can
shorten the code and avoid an extra copy by just passing the
out-parameter through the stack.
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an
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and full access to internals.
Git is an Open Source project covered by the GNU General Public
License version 2 (some parts of it are under different licenses,
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Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/
including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
Issues which are security relevant should be disclosed privately to
the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that
list the current status of various development topics to the mailing
list. The discussion following them give a good reference for
project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very
first version. He described the tool as "the stupid content tracker"
and the name as (depending on your mood):
random three-letter combination that is pronounceable, and not
actually used by any common UNIX command. The fact that it is a
mispronunciation of "get" may or may not be relevant.
stupid. contemptible and despicable. simple. Take your pick from the
dictionary of slang.
"global information tracker": you're in a good mood, and it actually
works for you. Angels sing, and a light suddenly fills the room.
"goddamn idiotic truckload of sh*t": when it breaks
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