travis-ci: skip a branch build if equal tag is present

If we push a branch and a tag pointing to the HEAD of this branch,
then Travis CI would run the build twice. This wastes resources and
slows the testing.

Add a function to detect this situation and skip the build the branch
if appropriate. Invoke this function on every build.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/ci/lib-travisci.sh b/ci/lib-travisci.sh
index 44d6ba2..9c4ae9b 100755 (executable)
--- a/ci/lib-travisci.sh
+++ b/ci/lib-travisci.sh
@@ -1,5 +1,28 @@
 # Library of functions shared by all CI scripts
 
+skip_branch_tip_with_tag () {
+       # Sometimes, a branch is pushed at the same time the tag that points
+       # at the same commit as the tip of the branch is pushed, and building
+       # both at the same time is a waste.
+       #
+       # Travis gives a tagname e.g. v2.14.0 in $TRAVIS_BRANCH when
+       # the build is triggered by a push to a tag.  Let's see if
+       # $TRAVIS_BRANCH is exactly at a tag, and if so, if it is
+       # different from $TRAVIS_BRANCH.  That way, we can tell if
+       # we are building the tip of a branch that is tagged and
+       # we can skip the build because we won't be skipping a build
+       # of a tag.
+
+       if TAG=$(git describe --exact-match "$TRAVIS_BRANCH" 2>/dev/null) &&
+               $TAG != $TRAVIS_BRANCH
+       then
+               echo "Tip of $TRAVIS_BRANCH is exactly at $TAG"
+               exit 0
+       fi
+}
+
 # Set 'exit on error' for all CI scripts to let the caller know that
 # something went wrong
 set -e
+
+skip_branch_tip_with_tag