Add a check to see if Jenkins is clobbering tags

[agocs]

If the git tag begins with "jenkins", there's a pretty strong chance
that Jenkins is clobbering our git tag. I've added logic that checks for
a new env var, semverIfJenkins. If that is set to 1 when ldflags.sh is
called, ldflags.sh will check to see if the git tag begins with
"jenkins", and, if so, it grabs the latest git tag beginning with the
character "v" assuming that is a semver tag.

If semverIfJenkins is set but there is no semver tag, gittag will be
empty string.

Usage:

semverIfJenkins=1 source ldflags.sh

[paddycarver]

So, thoughts on this:

If we're going to use environment variables, let's use uppercase with underscores, just to be consistent.

I'd much rather modify the external-facing $TAG variable than the internal $gittag variable, just for sanity's sake.

I'm tempted, so this doesn't become a shell monstrosity as we add exceptions and special logic, to say we should pull this into its own file. The idea kicking around in my head at the moment is to put this in a new jenkins-semver.sh file that sets $TAG if git name-rev --tags --name-only $(git rev-parse --short HEAD) has jenkins prefix. Then programs could do something like

source vendor/darlinggo.co/version/jenkins-semver.sh
source vendor/darlinggo.co/version/ldflags.sh
go build -ldflags "${LDFLAGS}" .

But that's a bit more composable and sustainable if we start needing to add more exceptions in. It also patterns how people could modify the logic that gets built into the LDFLAGS environment variable without getting code merged to master or forking.

What do you think? Any thoughts on any of this?

[paddycarver]

(Also, thanks for the PR! Sorry for the delay in reviewing it.)

[agocs]

Sorry if I've let this sit too long. I think that's wise. I'll see if I can find time to hack on it some more.

[paddycarver]

Not at all! If you want help, let me know, and I can probably implement some of this, too. I don't have a jenkins server handy, but I can do some of the bash stuff if necessary.

Also, as a note, just make sure your PR adds you (or DramaFever, if that's who owns the work you're contributing) to the AUTHORS file, and you (regardless of who owns the work) to the CONTRIBUTORS file.

... neither of which exists. I'll remedy that right now. I'm the worst.

(Basically, this just makes sure that the copyright owner retains their copyright claim on the code, and the people that actually wrote the code get the credit they deserve for writing it.)