fix: prepare sshd runtime directory before validation

.github/workflows/bash-syntax.yml

@@ -59,3 +59,6 @@ jobs:
 
       - name: Test initial backup policy and upload retry
         run: bash tests/test-backup-initial-retry.sh
+
+      - name: Test step 3 socket activation runtime directory
+        run: bash tests/test-step3-runtime-directory.sh

ochenstarik-server-user-3.sh

@@ -73,6 +73,12 @@ is_valid_port() {
   (( 10#$port >= 1 && 10#$port <= 65535 ))
 }
 
+prepare_sshd_runtime_directory() {
+  # Socket-activated Ubuntu can have no active ssh.service after an upgrade.
+  # Its RuntimeDirectory then does not exist, but standalone sshd -t needs it.
+  install -d -o root -g root -m 755 /run/sshd
+}
+
 read_ip_mode() {
   local mode=both
   if [[ -e "$IP_FAMILY_CONFIG" ]]; then
@@ -517,6 +523,7 @@ UsePAM yes
 EOF
 chmod 644 "$SSHD_DROPIN"
 
+prepare_sshd_runtime_directory
 sshd -t || die "sshd syntax validation failed"
 # Capture the complete output once. With pipefail enabled, piping `sshd -T`
 # into an awk program that exits after the first match can terminate sshd with

tests/test-step3-runtime-directory.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.."
+source <(awk '/^\[\[ "\$EUID"/{exit} {print}' ochenstarik-server-user-3.sh)
+test_dir="$(mktemp -d)"
+trap 'rm -rf -- "$test_dir"' EXIT
+
+# Relocate just the privileged filesystem boundary; use the real install
+# binary to check first creation, idempotency and repair of unsafe modes.
+install() {
+  [[ "$*" == '-d -o root -g root -m 755 /run/sshd' ]]
+  /usr/bin/install -d -m 755 "$test_dir/sshd"
+}
+prepare_sshd_runtime_directory
+[[ -d "$test_dir/sshd" && "$(stat -c %a "$test_dir/sshd")" == 755 ]]
+chmod 777 "$test_dir/sshd"
+prepare_sshd_runtime_directory
+[[ "$(stat -c %a "$test_dir/sshd")" == 755 ]]
+
+# The runtime prerequisite must run before standalone configuration validation.
+prepare_line="$(grep -n '^prepare_sshd_runtime_directory$' ochenstarik-server-user-3.sh | cut -d: -f1)"
+validate_line="$(grep -n '^sshd -t ' ochenstarik-server-user-3.sh | cut -d: -f1)"
+((prepare_line < validate_line))
+printf 'Step 3 socket-activation runtime directory tests passed.\n'