DLPX-91971 linux-pkg: Add support for os-upgrade branches

PR URL: https://www.github.com/delphix/linux-pkg/pull/320

Author: palash-gandhi

lib/common.sh

@@ -769,6 +769,24 @@ function fetch_repo_from_git() {
 		logmust git show-ref repo-HEAD
 		logmust git show-ref upstream-HEAD
 	else
+
+		#
+		# The "os-upgrade" branch is used for Ubuntu version upgrade projects. As such, the following
+		# block of code will only be executed if the PACKAGE_GIT_BRANCH is set to "os-upgrade" i.e. when
+		# the "os-upgrade" branch is marked active in dlpx-version-mapping. Not all product repositories
+		# require an "os-upgrade" branch, because some packages will successfully build on the new
+		# Ubuntu version as well as the old without any changes.
+		# To allow engineering to create the "os-upgrade" branch only in repositories that require
+		# changes, this block of code will check if the "os-upgrade" branch exists in the repository
+		# and if it does not, it will use the "develop" branch instead.
+		#
+		if [[ "$PACKAGE_GIT_BRANCH" == "os-upgrade" ]]; then
+			if [[ -n $(git ls-remote --exit-code --heads origin refs/heads/os-upgrade) ]]; then
+				echo "NOTE: The branch 'os-upgrade' does not exist for this repo. Using 'develop' instead."
+				PACKAGE_GIT_BRANCH="develop"
+			fi
+		fi
+
 		logmust git_fetch_helper "$PACKAGE_GIT_URL" --no-tags \
 			"+$PACKAGE_GIT_BRANCH:repo-HEAD" --depth=1
 		logmust git show-ref repo-HEAD