Hi,
I recently ran into a problem with the displayed version number for a
build of MSysgit. After much head scratching on my part, and divine
intervention from Dscho given his intimate knowledge of the codebase,
was it resolved.
My problem was that although I had checked out the tip of devel branch,
every time I ran make, my build version was incorrect, although the hash
suffix corresponded to the tip of the devel branch.
$ make
GIT_VERSION = 1.6.4.msysgit.0.2049.g91809
...
I was expecting to see 1.7.0 as the version which had been merged a few
days ago, and simply inspecting the files did not reveal where the
discrepancy originated.
I had been concentrating on GIT-VERSION-GEN to see how the version was
generated, and then comparing between two machines - one with a
successful build showing correct version and the other not, but both
showing 1.7.0 as DEF_VER and apparently identical repositories -
Johannes then replied to my query with a request to fetch some tags
which had been overlooked. Viewing the two repositories graphically side
by side immediately revealed that the build with the incorrect version
label did not have recent tags, and it was not the files I should have
been diffing, but comparing the output of "git describe". Problem solved
after fetching the new tags.
This has been a most beneficial learning exercise for me, and I am most
grateful, and heartened I was on the right track, but I think I also see
a potential problem.
GIT-VERSION-GEN sets a default value DEF_VER according to the version at
the time.
The two most recent being 1.6.6.2 at revision 82221 and 1.7.0 at e923e
However, in the absence of a version, the script uses "git describe" to
retrieve the latest tag, and goes on to use this to create the version
file along with the hash suffix at the current HEAD. In my case, the
latest tag was 1.6.4 but I was building from the latest source at
revision 91809.
Reading the manual entry for "git describe"[1] there is a note saying
that the hash suffix does not guarantee disambiguity, and given that a
tag may be incorrect or missing, there is a chance - albeit with
diminishing odds - that the 5 digit hash/tag combination might lead to
some obscure problems at some point along the line.
The chance of this happening really is low, but there is a chance all
the same. We cannot foresee all errors, but identifying, and further
reducing the odds of some must be good. Without doing the math, a guess
would be that the probability of a repeat 5 digit abbreviated hash
suffix increases the longer a tagged version is in place, so never will
be 100% safe. Relying on the build version alone is not a good test
under most circumstances, but in my case I could see that the hash was
correct and the displayed version was unexpected. The other way around
or one of those rare occasions of a repeat would have gone completely
unnoticed.
I may be wrong, but the only scenario where I see DEF_VER being used by
GIT-VERSION-GEN, would be when there are no tags for git describe to
retrieve. ie "git pull --no-tags"
If my understanding is correct, DEF_VER is unique and set at the same
time as the tagged version, so wouldn't it be desirable to cross check,
or include this value instead of relying solely on the tag when present
during the generation of GIT-VERSION-FILE at build time?
Steve
1. http://www.kernel.org/pub/software/scm/git/docs/git-describe.html
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