cmd-build: Clarify error message and add explanatory comment
This is a pretty rare occurrence for the user, though it gives more
context to readers.
@@ -172,7 +172,10 @@ else
cp -a --reflink=auto "${cached_previous_composejson}" "${composejson}"
else
if [ -z "${previous_build}" ]; then
- fatal "compose tree had no changes, but no previous build or cached data"
+ # This can happen if building the OSTree worked on the first time,
+ # but image creation failed, and then tmp/ was nuked before trying a
+ # second time. Just recommend re-running with --force.
+ fatal "compose tree had no changes, but no previous build or cached data; try rerunning with --force"
fi
echo "Commit ${commit} unchanged; reusing previous build's rpm-ostree metadata"
# This will have all of the data from the previous build, but we'll
