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 +769,23 @@ 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". Not
+		# all product repositories are required to have 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 for the new Ubuntu version, this block of code will check if the "os-upgrade" branch
+		# exists in the repository and if it does not, it will build off of "develop" instead.
+		#
+		if [[ "$PACKAGE_GIT_BRANCH" == "os-upgrade" ]]; then
+			if [[ -n $(git_fetch_helper "$PACKAGE_GIT_URL" --no-tags "$PACKAGE_GIT_BRANCH" --depth=1) ]]; then
+				echo "NOTE: The branch 'os-upgrade' does not exist for this repo. Using 'develop' instead."
+				PACKAGE_GIT_BRANCH="develop"
+			fi
+		fi
+
 		logmust git_fetch_helper "$PACKAGE_GIT_URL" --no-tags \
 			"+$PACKAGE_GIT_BRANCH:repo-HEAD" --depth=1
 		logmust git show-ref repo-HEAD