My prior change to git-describe attempts to print the distance
between the input commit and the best matching tag, but this distance
was usually only an estimate as we always aborted revision walking
as soon as we overflowed the configured limit on the number of
possible tags (as set by --candidates).
Displaying an estimated distance is not very useful and can just be
downright confusing. Most users (heck, most Git developers) don't
immediately understand why this distance differs from the output
of common tools such as `git rev-list | wc -l`. Even worse, the
estimated distance could change in the future (including decreasing
despite no rebase occuring) if we find more possible tags earlier
on during traversal. (This could happen if more tags are merged
into the branch between queries.) These factors basically make an
estimated distance useless.
Fortunately we are usually most of the way through an accurate
distance computation by the time we abort (due to reaching the
current --candidates limit). This means we can simply finish
counting out the revisions still in our commit queue to present
the accurate distance at the end. The number of commits remaining
in the commit queue is probably less than the number of commits
already traversed, so finishing out the count is not likely to take
very long. This final distance will then always match the output of
`git rev-list | wc -l`.
We can easily reduce the total number of commits that need to be
walked at the end by stopping as soon as all of the commits in the
commit queue are flagged as being merged into the already selected
best possible tag. If that's true then there are no remaining
unseen commits which can contribute to our best possible tag's
depth counter, so further traversal is useless.
Basic testing on my Mac OS X system shows there is no noticable
performance difference between this accurate distance counting
version of git-describe and the prior version of git-describe,
at least when run on git.git.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
If you get two different describes at different
times from a non-rewinding branch and they both come up with the same
tag name, you can tell which is the 'newer' one by distance. This is
rather common in practice, so its incredibly useful.
[jc: still needs documentation and fixups when traversal gives up
early.]
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
When on a non-tag commit, git-describe normally outputs descriptions of
the form
v1.0.0-g1234567890
Some scripts (for example the update hook script) might just want to
know the name of the nearest tag, so they then have to do
x=$(git-describe HEAD | sed 's/-g*//')
This is costly, but more importantly is fragile as it is relying on the
output format of git-describe, which we would then have to maintain
forever.
This patch adds support for setting the --abbrev option to zero. In
that case git-describe does as it always has, but outputs only the
nearest found tag instead of a completely unique name. This means that
scripts would not have to parse the output format and won't need
changing if the git-describe suffix is ever changed.
Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Junio added the found variable to enforce commit date order when two
tags have the same distance from the requested commit. Except it is
unnecessary as match_cnt is already used to record how many possible
tags have been identified thus far.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Currently we don't use the util field of struct commit but we want
fast access to the highest priority name that references any given
commit object during our matching loop. A really simple approach
is to just store the name directly in the util field.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
We really want to always favor an annotated tag over a lightweight
tag when describing a commit. Unfortunately git-describe wasn't
doing this as it was favoring the depth attribute of a possible_tag
over the priority. Now priority is the highest sort and we only
consider a lightweight tag if no annotated tags were identified.
Rather than searching for the minimum tag using a simple loop we
now sort them using a stable sort algorithm, this way the possible
tags display in order if --debug gets used. The stable sort helps
to preseve the inherit topology/date order that we obtain during
our search loop.
This fix allows the tests in t6120-describe.sh to pass.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
My prior version of git-describe ran very slowly on even reasonably
sized projects like git.git and linux.git as it tended to identify
a large number of possible tags and then needed to generate the
revision list for each of those tags to sort them and select the
best tag to describe the input commit.
All we really need is the number of commits in the input revision
which are not in the tag. We can generate these counts during
the revision walking and tag matching loop by assigning a color to
each tag and coloring the commits as we walk them. This limits us
to identifying no more than 26 possible tags, as there is limited
space available within the flags field of struct commit.
The limitation of 26 possible tags is hopefully not going to be a
problem in real usage, as most projects won't create 26 maintenance
releases and merge them back into a development trunk after the
development trunk was tagged with a release candidate tag. If that
does occur git-describe will start to revert to its old behavior of
using the newer maintenance release tag to describe the development
trunk, rather than the development trunk's own tag. The suggested
workaround would be to retag the development trunk's tip.
However since even 26 possible tags can take a while to generate a
description for on some projects I'm defaulting the limit to 10 but
offering the user --candidates to increase the number of possible
matches if they need a more accurate result. I specifically chose
10 for the default as it seems unlikely projects will have more
than 10 maintenance releases merged into a development trunk before
retagging the development trunk, and it seems to perform about the
same on linux.git as v1.4.4.4 git-describe.
A large amount of debugging information was also added during
the development of this change, so I've left it in to be toggled
on with --debug. It may be useful to the end user to help them
understand why git-describe took one particular tag over another.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
If a project has a really huge number of tags (such as several
thousand tags) then we are likely to have nearly a hundred tags in
some buckets. Scanning those buckets as linked lists could take
a large amount of time if done repeatedly during history traversal.
Since we are searching for a unique commit SHA1 we can sort all
tags by commit SHA1 and perform a binary search within the bucket.
Once we identify a particular tag as matching this commit we walk
backwards within the bucket matches to make sure we pick up the
highest priority tag for that commit, as the binary search may
have landed us in the middle of a set of tags which point at the
same commit.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
If a project has a very large number of tags then git-describe
will spend a good part of its time looping over the tags testing
them one at a time to determine if it matches a given commit.
For 10 tags this is not a big deal, but for hundreds of tags the
time could become considerable if we don't find an exact match for
the input commit and we need to walk back along the history chain.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Several people have suggested that its always better to describe
a commit using an annotated tag, and to only use a lightweight tag
if absolutely no annotated tag matches the input commit.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
Recently git.git itself encountered a situation on its master and
next branches where git-describe stopped reporting 'v1.5.0-rc0-gN'
and instead started reporting 'v1.4.4.4-gN'. This appeared to be
a backward jump in version numbering.
maint o-------------------4
\ \
master o-o-o-o-o-o-o-5-o-C-o-W
The issue is that commit C in the diagram claims it is version
1.5.0, as the tag v1.5.0 is placed on commit 5. Yet commit W
claims it is version 1.4.4.4 as the tag v1.5.0 has an older tag
date than the v1.4.4.4 tag.
As it turns out this situation is very common. A bug fix applied
to maint and later merged into master occurs frequently enough that
it should Just Work Right(tm).
Rather than taking the first tag that gets found git-describe will
now generate a list of all possible tags and select the one which
has the most number of commits in common with HEAD (or whatever
revision the user requested the description of).
This rule is based on the principle shown in the diagram above.
There are a large number of commits on the primary development branch
'master' which do not appear in the 'maint' branch, and many of
these are already tagged as part of v1.5.0-rc0. Additionally these
commits are not in v1.4.4.4, as they are part of the v1.5.0 release
still being developed. The v1.5.0-rc0 tag is more descriptive of
W than v1.4.4.4 is, and therefore should be used.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
