Optimization: skip tests in some circumstances

A common pattern is to submit PRs that update only tests or docs.

When the only changes are to test/e2e, there is no point in running
test/system or test/upgrade or test/buildah-bud. Likewise, reciprocally,
and similarly for a bunch of other tests (alt, cross, apiv2, ...)

And when the only changes are under docs/ , there is no point in
running any of the above.

Exception: if $CIRRUS_<mumble> are undefined (e.g., cron), never skip

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

contrib/cirrus/pr-should-include-tests

@@ -30,8 +30,7 @@ fi
 # Nothing changed under test subdirectory.
 #
 # This is OK if the only files being touched are "safe" ones.
-filtered_changes=$(git diff --name-status $base $head |
-                       awk '{print $2}'               |
+filtered_changes=$(git diff --name-only $base $head   |
                        fgrep -vx .cirrus.yml          |
                        fgrep -vx .gitignore           |
                        fgrep -vx Makefile             |

contrib/cirrus/runner.sh

@@ -46,6 +46,8 @@ function _run_validate() {
 }
 
 function _run_unit() {
+    _bail_if_test_can_be_skipped test/goecho test/version
+
     # shellcheck disable=SC2154
     if [[ "$PODBIN_NAME" != "podman" ]]; then
         # shellcheck disable=SC2154
@@ -55,31 +57,45 @@ function _run_unit() {
 }
 
 function _run_apiv2() {
+    _bail_if_test_can_be_skipped test/apiv2
+
     source .venv/requests/bin/activate
     make localapiv2 |& logformatter
 }
 
 function _run_compose() {
+    _bail_if_test_can_be_skipped test/compose
+
     ./test/compose/test-compose |& logformatter
 }
 
 function _run_compose_v2() {
+    _bail_if_test_can_be_skipped test/compose
+
     ./test/compose/test-compose |& logformatter
 }
 
 function _run_int() {
+    _bail_if_test_can_be_skipped test/e2e
+
     dotest integration
 }
 
 function _run_sys() {
+    _bail_if_test_can_be_skipped test/system
+
     dotest system
 }
 
 function _run_upgrade_test() {
+    _bail_if_test_can_be_skipped test/upgrade
+
     bats test/upgrade |& logformatter
 }
 
 function _run_bud() {
+    _bail_if_test_can_be_skipped test/buildah-bud
+
     ./test/buildah-bud/run-buildah-bud-tests |& logformatter
 }
 
@@ -217,6 +233,9 @@ function _run_build() {
 }
 
 function _run_altbuild() {
+    # We can skip all these steps for test-only PRs, but not doc-only ones
+    _bail_if_test_can_be_skipped docs
+
     local -a arches
     local arch
     req_env_vars ALT_NAME
@@ -345,6 +364,54 @@ dotest() {
         |& logformatter
 }
 
+# Optimization: will exit if the only PR diffs are under docs/ or tests/
+# with the exception of any given arguments. E.g., don't run e2e or upgrade
+# or bud tests if the only PR changes are in test/system.
+function _bail_if_test_can_be_skipped() {
+    local head base diffs
+
+    # Cirrus sets these for PRs but not cron. In cron, we never want to skip.
+    for v in CIRRUS_CHANGE_IN_REPO DEST_BRANCH; do
+        if [[ -z "${!v}" ]]; then
+            msg "[ _cannot do selective skip: \$$v is undefined ]"
+            return 0
+        fi
+    done
+    # And if this one *is* defined, it means we're not in PR-land; don't skip.
+    if [[ -n "$CIRRUS_TAG" ]]; then
+        msg "[ _cannot do selective skip: \$CIRRUS_TAG is defined ]"
+        return 0
+    fi
+
+    head=$CIRRUS_CHANGE_IN_REPO
+    base=$(git merge-base $DEST_BRANCH $head)
+    diffs=$(git diff --name-only $base $head)
+
+    # If PR touches any files in an argument directory, we cannot skip
+    for subdir in "$@"; do
+        if egrep -q "^$subdir/" <<<"$diffs"; then
+            return 0
+        fi
+    done
+
+    # PR does not touch any files under our input directories. Now see
+    # if the PR touches files outside of the following directories, by
+    # filtering these out from the diff results.
+    for subdir in docs test; do
+        # || true needed because we're running with set -e
+        diffs=$(egrep -v "^$subdir/" <<<"$diffs" || true)
+    done
+
+    # If we still have diffs, they indicate files outside of docs & test.
+    # It is not safe to skip.
+    if [[ -n "$diffs" ]]; then
+        return 0
+    fi
+
+    msg "SKIPPING: This is a doc- and/or test-only PR with no changes under $*"
+    exit 0
+}
+
 # Nearly every task in .cirrus.yml makes use of this shell script
 # wrapped by /usr/bin/time to collect runtime statistics.  Because the
 # --output option is used to log stats to a file, every child-process