implement exponential backoff for curl retries in _cs_curl_retry function

lxc/install.func

@@ -45,23 +45,34 @@ _cs_core_url() {
   echo "${COMMUNITY_SCRIPTS_CORE_URL:-https://raw.githubusercontent.com/community-scripts/core/main}/${1:?func path}"
 }
 
-# Retry options for engine fetches. Repeated here rather than taken from
-# core/build.func, which stays on the host while this file is piped into the
-# container and has to stand alone. A container is at its most fragile right
-# after boot, before DNS is up; core/build.func carries the full reasoning.
 _CS_CURL_RETRY=(--retry 3 --retry-delay 1 --retry-connrefused --connect-timeout 10)
 
 # curl -S prints one line per failed attempt, so a download that recovers on the
-# second try would show the user two errors and then quietly work — during a
-# spinner, no less. Hold stderr back and release it only if every attempt failed.
+
 _cs_curl_retry() {
   local url="${1:?url}" err rc=0
+  local attempt=1 max_attempts=4 backoff=1 timeout=30
+
   err="$(mktemp 2>/dev/null)" || {
-    curl -fsSL "${_CS_CURL_RETRY[@]}" "$url"
+    curl -fsSL --connect-timeout 10 --max-time "$timeout" "$url"
     return
   }
-  curl -fsSL "${_CS_CURL_RETRY[@]}" "$url" 2>"$err" || rc=$?
-  ((rc != 0)) && cat "$err" >&2
+
+  while [[ $attempt -le $max_attempts ]]; do
+    if curl -fsSL --connect-timeout 10 --max-time "$timeout" "$url" 2>"$err"; then
+      rm -f "$err"
+      return 0
+    fi
+    rc=$?
+    ((attempt == max_attempts)) && break
+    sleep "$backoff"
+    backoff=$((backoff * 2))
+    ((backoff > 30)) && backoff=30
+    timeout=$((timeout * 2))
+    ((attempt++))
+  done
+
+  cat "$err" >&2
   rm -f "$err"
   return "$rc"
 }