bloat check: make more robust

The use of 'C^' (parent) in 'git rebase' is counterintuitive,
at least to me: when C is a merge of multiple commits, rebase
picks each of those commits. That probably makes good sense
to a git expert, which I'm not.

Solution: forget using '^', just calculate the baseline sizes
by doing a checkout of the PR's parent.

Also: compute PR parent using git-merge-base instead of
blindly trusting $DEST_BRANCH (which may be volatile as
other PRs are merged).

Also: run git-rebase directly, not via make rule. That
indirection is too confusing here.

Signed-off-by: Ed Santiago <santiago@redhat.com>

1 files changed, 13 insertions, 6 deletions

diff --git a/contrib/cirrus/runner.sh b/contrib/cirrus/runner.sh
index c4a714691..1ec18c861 100755
--- a/contrib/cirrus/runner.sh
+++ b/contrib/cirrus/runner.sh
@@ -250,16 +250,23 @@ function _run_altbuild() {
     case "$ALT_NAME" in
         *Each*)
             git fetch origin
-            # The check-size script, introduced 2022-03-22 in #13518,
+            # The make-and-check-size script, introduced 2022-03-22 in #13518,
             # runs 'make' (the original purpose of this check) against
             # each commit, then checks image sizes to make sure that
             # none have grown beyond a given limit. That of course
-            # requires a baseline, which is why we use '^' to start
-            # with the *parent* commit of this PR, not the first commit.
+            # requires a baseline, so our first step is to build the
+            # branch point of the PR.
+            local context_dir savedhead pr_base
             context_dir=$(mktemp -d --tmpdir make-size-check.XXXXXXX)
-            make build-all-new-commits \
-                 GIT_BASE_BRANCH=origin/"${DEST_BRANCH}^" \
-                 MAKE="hack/make-and-check-size $context_dir"
+            savedhead=$(git rev-parse HEAD)
+            # Push to PR base. First run of the script will write size files
+            pr_base=$(git merge-base --fork-point origin/$DEST_BRANCH)
+            git checkout $pr_base
+            hack/make-and-check-size $context_dir
+            # pop back to PR, and run incremental makes. Subsequent script
+            # invocations will compare against original size.
+            git checkout $savedhead
+            git rebase $pr_base -x "hack/make-and-check-size $context_dir"
             rm -rf $context_dir
             ;;
         *Windows*)