rebuild when Alpine registry index is absent

scripts/apk/ci-package-cache.sh

@@ -30,16 +30,30 @@ registry_key_url() {
 }
 
 download_registry_metadata() {
-  local arch url key_dir
+  local arch url key_dir http_status
 
   require_registry_config INSTANCE_URL "${instance_url}"
   require_registry_config PACKAGE_OWNER "${owner}"
+  declare -gA registry_index_available=()
   mkdir -p "${registry_cache}"
   url="$(registry_url)"
   for arch in ${arches}; do
-    curl --fail --silent --show-error --location \
+    if http_status="$(curl --fail --silent --location \
       --output "${registry_cache}/${arch}.APKINDEX.tar.gz" \
-      "${url}/${arch}/APKINDEX.tar.gz"
+      --write-out '%{http_code}' \
+      "${url}/${arch}/APKINDEX.tar.gz")"; then
+      registry_index_available["${arch}"]=1
+      continue
+    fi
+    if [[ "${http_status}" == 404 ]]; then
+      registry_index_available["${arch}"]=0
+      printf 'Registry index is absent: %s/%s/APKINDEX.tar.gz; treating it as empty\n' \
+        "${url}" "${arch}" >&2
+      continue
+    fi
+    printf 'failed to download registry index (%s): %s/%s/APKINDEX.tar.gz\n' \
+      "${http_status:-curl error}" "${url}" "${arch}" >&2
+    return 1
   done
   key_dir="${registry_cache}/keys"
   mkdir -p "${key_dir}"
@@ -59,6 +73,7 @@ load_registry_index() {
   declare -gA registry_latest_versions=()
 
   for arch in ${arches}; do
+    [[ "${registry_index_available[${arch}]:-0}" == 1 ]] || continue
     while IFS=$'\t' read -r package_name package_version; do
       [[ -n "${package_name}" && -n "${package_version}" ]] || continue
       registry_key="${arch}|${package_name}"