Fix checkouts on Travis

The way Travis performs git checkouts puts the working directory in
detached HEAD state. This breaks the logic for deriving version numbers
from branch names in the build script.

This commit updates the Travis build to replace the detached head
checkout with a branch checkout if necessary.

Author: cboehme

.travis.yml

@@ -4,6 +4,9 @@ language: java
 jdk:
   - oraclejdk8
 
+install:
+  - ./travis/switch-to-branch.sh
+
 after_success:
   - ./travis/publish.sh
 

travis/switch-to-branch.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+
+#
+# When building branches Travis does not check out the branch but a specific
+# commit. This results in the working copy being in "detached HEAD" state.  The
+# breaks the logic for deriving version numbers from branch names in the build
+# script.
+# 
+# This script checks if the current build corresponds to the tip of a branch.
+# If it is then the branch is checked out.
+#
+
+function main {
+    require_not_building_pull_request
+    require_not_triggered_by_tag
+    require_on_tip_of_branch
+    checkout_branch
+}
+
+function require_not_building_pull_request {
+    if [ -v TRAVIS_PULL_REQUEST -a "$TRAVIS_PULL_REQUEST" != "false" ]; then
+        echo "Building pull request. Will not replace detached head with branch"
+        exit 0
+    fi
+}
+
+#
+# In builds triggered by a tag Travis sets the variable TRAVIS_BRANCH to the
+# tag name instead of the branch name. It is not possible (and not necessary)
+# to check out the branch in this case.
+#
+function require_not_triggered_by_tag {
+    if [ -v TRAVIS_TAG -a "$TRAVIS_TAG" != "" ] ; then
+        echo "Build was triggered by a tag. Will not replace detached head with branch"
+        exit 0
+    fi
+}
+
+function require_on_tip_of_branch {
+    if [ $TRAVIS_COMMIT != $( git rev-parse --verify $TRAVIS_BRANCH ) ] ; then 
+        echo "Detached head does not match tip of current branch. Staying on detached head."
+        exit 0
+    fi
+}
+
+function checkout_branch {
+    echo "Detached head matches tip of current branch. Replacing detached head with branch"
+    git checkout $TRAVIS_BRANCH 
+}
+
+main
+