automation: do not fail backport retries when nothing needs retrying

.github/scripts/retry-backports.sh

@@ -12,6 +12,8 @@
 # read, and GITHUB_REPOSITORY.
 #
 # Usage: retry-backports.sh [--stale]
+#
+# Nothing to retry is the usual case and exits 0.
 
 set -euo pipefail
 
@@ -29,7 +31,7 @@ R=$GITHUB_REPOSITORY
 			-q ".[] | select(.headRefName | startswith(\"$PREFIX\")) | select(.mergeable == \"CONFLICTING\") | .headRefName" |
 			sed "s|^$PREFIX||"
 	fi
-} | grep -xE '[0-9]+' | sort -un | while read -r n; do
+} | { grep -xE '[0-9]+' || true; } | sort -un | while read -r n; do
 	echo "Dispatching the backport of #$n"
 	gh workflow run kernel-nightly.yml --repo "$R" --ref "$TARGET" -f backport_pr="$n"
 done

.github/scripts/tests/retry-backports.test.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# Tests for .github/scripts/retry-backports.sh against a fake gh.
+#
+# The fake answers the two pull request listings from WAITING and STALE and
+# records every dispatch, so each case checks exactly which backports the
+# script started, and that having nothing to retry is a success.
+#
+# Run from anywhere: bash .github/scripts/tests/retry-backports.test.sh
+set -uo pipefail
+
+HERE=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+RETRY="$HERE/../retry-backports.sh"
+
+WORK=$(mktemp -d)
+trap 'rm -rf "$WORK"' EXIT
+mkdir "$WORK/bin"
+cat >"$WORK/bin/gh" <<'GH'
+#!/usr/bin/env bash
+case "$1 $2" in
+"pr list")
+	case " $* " in
+	*" --label backport-waiting "*) [ -z "$WAITING" ] || printf '%s\n' $WAITING ;;
+	*) [ -z "$STALE" ] || printf '%s\n' $STALE ;;
+	esac ;;
+"workflow run") echo "$*" >>"$DISPATCHED" ;;
+*) echo "unexpected: gh $*" >&2; exit 2 ;;
+esac
+GH
+chmod +x "$WORK/bin/gh"
+export PATH="$WORK/bin:$PATH" GITHUB_REPOSITORY=edera-dev/linux DISPATCHED="$WORK/dispatched"
+
+failures=0
+ok() { echo "ok   $1"; }
+bad() {
+	echo "FAIL $1" >&2
+	[ -z "${2:-}" ] || printf '     %s\n' "${2//$'\n'/$'\n'     }" >&2
+	failures=$((failures + 1))
+}
+
+# check <name> <args> <waiting> <stale> <expected pr numbers, space separated>
+check() {
+	local name=$1 args=$2 out got
+	: >"$DISPATCHED"
+	# shellcheck disable=SC2086 # $args is zero or one word
+	if ! out=$(WAITING=$3 STALE=$4 bash "$RETRY" $args 2>&1); then
+		bad "$name" "exited non-zero: $out"
+		return
+	fi
+	got=$(sed -E 's/.*backport_pr=([0-9]+).*/\1/' "$DISPATCHED" | xargs)
+	if [ "$got" = "$5" ]; then ok "$name"; else bad "$name" "dispatched '$got', expected '$5'"; fi
+}
+
+check "nothing waiting, no --stale" "" "" "" ""
+check "nothing waiting, nothing stale" --stale "" "" ""
+check "waiting pull requests are dispatched" "" "7 12" "" "7 12"
+check "stale backports are dispatched by number" --stale "" \
+	"backport/edera-6.18-lts/pr-23 backport/edera-6.18-lts/pr-9" "9 23"
+check "stale backports are ignored without --stale" "" "" "backport/edera-6.18-lts/pr-23" ""
+check "a pull request both waiting and stale goes once" --stale "23" \
+	"backport/edera-6.18-lts/pr-23" "23"
+
+if [ "$failures" -ne 0 ]; then
+	echo "$failures failure(s)" >&2
+	exit 1
+fi
+echo "all passed"

.github/workflows/automation-selftest.yml

@@ -64,5 +64,8 @@ jobs:
       - name: Backport tests
         run: bash .github/scripts/tests/backport.test.sh
 
+      - name: Backport retry tests
+        run: bash .github/scripts/tests/retry-backports.test.sh
+
       - name: Shellcheck
         run: shellcheck .github/scripts/*.sh .github/scripts/tests/*.sh